Mercedes Service CD Overview

Following are example screenshots from various Mercedes-Benz (MBUSA) service CD's. This information is provided only to illustrate the models included and contents of each service CD. These CD's are available for order directly from Mercedes (1-800-FOR-MERC).

Mercedes-Benz Driveability Evaluating Electronic Engine Controls

Steve Brotherton is co-owner of Continental Imports, a 22-bay shop in Gainesville, FL, that employs 18 people, 10 of whom are technicians. A Bosch Authorized Service Center since 1986, the shop specializes in Mercedes-Benz, BMW, Porsche, Volvo, Saab, Jaguar, Honda and Acura. Brotherton has been an ASE-certified Master Technician since 1973 and is a recertified L1 specialist. In addition, he is a Bosch Certified Master Tech, president of the Alachua County ASA Chapter and member of the board for Florida ASA. He holds a bachelors degree in Metallurgical Engineering from the University of Florida. Steve is also a regular contributor to MercedesShop's Shop Forum and currently moderates the Tech's Only forum.

When my wife's new ML320 had its "check engine" light come on with less than 15K on the speedometer, it was quite a job convincing her that she could drive the car until the dealer could fit her in 10 days later. I, of course, verified that the fault code indicated an O2 sensor problem and the risk was minimal to drive it around town another 10 days. I also evaluated the data stream to verify that the O2 sensor was functioning well enough so that the catalyst wouldn?t melt down in the meantime.

I have been working on cars for 30 years and now, more than ever, I see customers get in a panic over a small yellow light. These are the same people who often wouldn?t bring in their car when it was afflicted with other symptoms until it needed to be towed. This challenges technicians with the evaluation of a technical difficulty, with no external symptoms other than a small yellow light on the dash and whatever electronic self-diagnostics are available.In preparing to write this article, I talked to a number of technicians who answer Mercedes-Benz questions on the international Automotive Technicians? Network (iATN). They all said that the greatest number of questions being asked were still about CIS model cars. Most of these problems do have symptoms that are identifiable in the vehicle?s performance. While I use tools either listed as factory or cloned from such, I will demonstrate the simple ways to evaluate the electronic engine controls and make basic adjustments with common shop tools.Electronically, there are four windows to the KE-Jetronic System. Two of these windows involve self-diagnostic testing and fault codes. The other two involve dynamic monitoring of the two basic electronic servos. These are the electro-hydraulic actuator (EHA) and the idle control valve.

The first self-diagnostics were "real-time" only through fixed integrator duty cycles given on pin #3 of the diagnostic connector X11 (on the left fender). This value, that is normally read with Bosch?s Lambda closed-loop tester, KDJE-P600 (see Figure 5), can also be read with a good multimeter or scope on the duty-cycle setting.
Figure 5: Factory tool reading of duty cycle and EHA current.

Patterns 1 and 2 (see Figures 1 & 2) indicate fixed whole number duty cycles, indicating a fault.

Figure 1: ECT sensor disconnected.

Figure 2: O2 sensor cold or disconnected.

Pattern 3 (see Figure 3) shows a closed-loop active reading that changes continuously and indicates the basic mixture. As the mixture is changed by the 3mm air flow plate trim screw, the amount of correction to achieve lambda is indicated by the integrator value. The value of 38% indicates a rich reading. The total range of feedback control is from 0% to 100%. A properly adjusted engine will be in closed loop and have a integrator duty-cycle ranging plus or minus around 50%. This window gives real-time faults, average mixture settings, and a basic feel for the speed of lambda correction and O2 sensor speed.

Figure 3: An active, changing reading that, in this case, is slightly rich.

The second and probably the best overall view of this system is seen through the monitoring of the EHA current. The EHA is the final servo device for applying the corrections intended by the control system. All forms of enrichment are accomplished by increasing the differential pressure within the fuel distributor. The EHA acts as a controlled leak depending on current. Positive current flows cause enrichened mixtures by leaking down the lower chamber pressure (higher differential pressure). Negative current flows cause lean mixtures and, on deceleration, a negative 60ma causes fuel shut-off.

A properly adjusted system will have current flows hovering at zero (for all systems but the early 190s, as they had only positive current movement and the middle was 8ma). The two readings in Figure 4 show a duty cycle reading of 35% and an EHA current of -3ma. Both of these readings indicate a slightly rich reading being corrected by the negative current on the EHA. Both of these readings are shown with standard electrical tools (see Figure 5 for the factory tool reading). We use an inexpensive multimeter with a fixed harness that easily plugs in between the EHA and the car?s harness. The time it takes to remove the air cleaner is the greatest labor involved. The wires are long enough so that the car can be driven while being monitored. The amount of current correction should be within 10% at various engine speeds. We usually check at idle and 2,000 rpm. Differences can indicate air leaks and, at least once, a bad EHA itself.
Figure 4: Duty cycle reading of 35% and an EHA current of -3ma.

Figure 5: Factory tool reading of cycle and EHA current.

A similar hook-up with an inexpensive ammeter can be connected to the idle control valve. The values change with this system on different vehicles, but monitoring current flow can show problems that cause the number one complaint I see ? intermittent stalling. The current usually runs from 600-700ma at idle. Adding A/C usually adds 50ma control. When the idle switch opens, the valve is held open at 800-900ma to prepare for sudden throttle release. The number one intermittent problem we see is caused by the over-voltage protection (OVP) relay. Watching the idle valve current allowed me to understand how the OVP can cause stalling.Since a disconnected idle valve limps home at a higher than normal rpm, I always found it hard to understand how removing the idle valve?s power could cause a stall. I finally saw it, though, while driving and monitoring the idle valve current. The normal current of 600-700ma dropped to 350-450ma and the idle dropped to about 450 rpm. A thump on the OVP relay caused the current, as well as the idle, to instantly jump to proper value. The current was being controlled not by the KE controller, but rather by the bad internal power feed connection of the OVP.

The OVP also totally fails or burns its fuse, killing all EHA current. This condition doesn?t affect a warm motor, as it should already be running closed loop at zero milliamps. But it causes most of the hard cold-starting problems. Watching the EHA current is a quick way to verify a good OVP relay. Since the ABS is also powered by the OVP relay (often by a separate circuit), a real quick diagnostic insight can be made by the combined symptoms of ABS light and hard cold starting.As these systems progressed, they were equipped with on-board self-diagnostics with memory and check engine lights. This "fourth window" is also accessible with regular shop electrical tools. In this case, I used a scope to verify the readings that I received from my impulse counter tool (see Figure 6) that is similar to the factory tool.
Figure 6: Impulse tool counter

This process of reading codes for these early systems works up to 1994 for all systems, and later on some systems. Basically, it will work up to OBD II systems, and applies to all systems, not just engine management systems. The code retrieval method is very simple. The communications take place over a single line on the appropriate diagnostic connector. On the car I used (a 1991 300TE), the connector (X11/4) is located next to the battery, next to the right side hood hinge.

The KE controller is accessed on data terminal #3. With the key on, engine off, self-diagnostics is activated by grounding the #3 pin for two to four seconds. Once this takes place, the control unit transmits the codes by itself, grounding the #3 terminal. (Remember that all systems work the same; all you need is a directory of which pins to activate and the code table to interpret.)

The impulse counter makes life easy by counting up some high numbers. In the scope pattern in Figure 7, I connected the scope negative lead on the battery positive and the positive lead in the #3 pin.
Figure 7: Code 7 reading.

I attached a jumper ground for three seconds and the control unit grounded the pin seven times, which could easily be counted. This was done again and code 13 was read, again and code 14 was read, and again and code 27 was read. If you are using the scope to read this, the time scale must be appropriate to get the whole thing, as every further activation moves on to the next code. They are listed one at a time until they are repeated. This can be done over and over. Each time the pin is grounded for two to four seconds, the next code is communicated by the control unit, grounding out the code number in "blinks." An LED can also be used to see a "blink" for counting. Once all codes are read, they can be erased by holding the jumper to the appropriate pin for six to eight seconds. After this is done, another activation with a two- to four-second ground will read the next code, and a further six- to eight-second ground pulse will erase it. This is continued until all codes have been read and erased. This will be shown by the final code of "one blink."

With the development of self-diagnostics in the late '80s, came the ability to receive fault codes, view actual data, activate elements and eventually change programming. The car mentioned above can be viewed and have its codes read by common shop tools. Reading actual data will require a factory scanner or a clone. Version coding and changing of adaptations has been out of reach without the factory tool HHT. Probably the most crucial place this has affected us has been in our body shop. All the air bag modules since 1995 need to be version coded, requiring a trip to the dealer. It is claimed that one aftermarket scanner?s newest version will now do version coding, reset adaptation values and even do some data graphing. I am eager to check this out as my plans were forming to spend the big bucks on the Mercedes-Benz tool, Star Diagnosis, which is now available in some form to independents.

I recently had the pleasure to witness the testing of a 1998 GM product EVAP system with the Tech 2. The presenter showed how an OBD II EVAP monitor could be initiated using a "Service Bay Test" function. The tool also could manually do testing by activating the vent valve and purge valves. The Mercedes-Benz EVAP system is similar but has no similar way of doing a service bay test using any of the aftermarket scanners. I couldn?t find anyone who knew whether or not the Star Diagnosis tool could perform such tests. Without the ability to do those tests (especially if the manufacturer can), we need more than tools to figure them out at times. A recent case shows the latest tool: The internet. A case in point involves the iATN. A C280 came in with the "Check Engine" light on. The fault code indicated EGR low flow. We reset the light and checked that the vacuum was going to the EGR valve. The light was back on in a week. We replaced the EGR valve, thinking that it was intermittently sticking. Again, the light came back on in one week. While surfing iATN archives for a separate problem, I came upon the fix. It was a carboned-up port (see Figure 8).
Figure 9: Restriction at intake manifold side is difficult to access.

The value to the "archive fix" was that it pointed out that the restriction was at the intake manifold side. From Figure 9, one can see the location is difficult. From Figure 10, one can see that the blockage has been removed. Once there, it became evident that this wasn?t carbon from the EGR, but rather PCV vapors from the intake coating the hot pipe inlet.We have now added a job to our 30K services on the M104 motor. We open the steel line at the EGR valve and feed in a long section of speedometer cable. We spin the cable with a drill motor and run it through the whole tube into the intake manifold. This is a simple operation, yet it prevents having to unbury the actual fitting when it?s really plugged.
My thanks to the iATN and the unknown author.
Figure 8: Carboned up port.

Figure 10: Blockage removed from port.

Diesel Primer Pump Replacement on the 1985 300SD

I've gotten loads of free advice from this and other forums. Now it is my turn to share some guidance.
This past May I replaced my original leaking fuel primer pump. It is a simple painless operation that took about 30 minutes - 15 of which was spent taking the attached pictures.

The location of the existing fuel primer pump (next to the injector pump).


The updated replacement part - I paid about $30.00 from a dealer simply because I was on travel and did not want to wait to find it cheaper.

Use a hose clamp (I used vise grips) to close off the fuel line after the first stage filter (the clear one).

Using a screwdriver, loosen and remove the fuel hose.

Using either a 17mm or 19mm socket with an extension, remove the fuel line fitting.

Setting the fuel line fitting aside (on top of air filter)

Now you have room to access the old primer pump

Remove old fuel primer pump with a 24mm open end wrench (note pinched off fuel line at bottom of photo.

The new and the old parts compared. The new part came with an adapter that was not required in my application.

Clean up and ensure you have removed the old copper washer.


Install new fuel primer pump with a 17mm or 19mm open end wrench.

Replace the fuel line fitting, tighten, and enjoy the completed project!

Next I'll work on posting my project of adding rear headrests. I did this in under an hour with used brackets and leather headrests I got from out in L.A. for under $100 - shipped! If done properly it will be no different than if it was factory installed.

Cleaning the Hazard Switch

1) Remove the hazard light switch from the console.
2) Use finger nail polish to mark the top side in three locations for reassembly orientation.

3) Use a clean flat cloth covered surface to contain the work area.
4) Insert a small screw driver into the back side tab area.

5) Gently ease the back cover off, watch out for falling parts.
6) Use tweezers to remove the shuttle with caps and springs.


7) Use a cheap paper on wood type of finger nail file to clean the contact surfaces.

8) Set the button to the on position and insert the shuttle, carefully.
9) Install the back pin plate, be sure the back is oriented correctly and the bulb will enter the case hole.
10) Gently squeeze the back into place, check the side catches are fully seated.

If you loose a part, buy another switch.

Washing part - 1

Washing should not be done in the direct sunlight. Shade or overcast is preferable as direct sunlight can cause waterspotting.
The first step to a good wash is a good pre-rinse. Using hose or diffuse pressure-washer spray, remove as much of the surface grime as possible with the water. When washing it is very important to work in a top to bottom pattern. This way you do not have to re-wash areas you have been over and you keep the wash-mitt cleaner.

To do things right, for the longevity of the car's finish, using 3 separate wash mitts is best. Each one has its distinct job and it will keep that job for its existence. (wash mitts are cheap, paint jobs are expensive!) Have one for the top painted surfaces and windows, have a second for the lower areas such as the body cladding and wheel-wells, and a third for the wheels and tires. By keeping separate mitts you don't grind last brake dust from the wheels into the nice shiny hood or smear tar/road grime into your fender.

That brings us to the next part of washing...soap. Please don't use dish-detergent or any household cleaners. At best it will only strip the surface of its wax and shine, at worst it will actually damage the finish. Any high quality car wash is good, I would personally recommend Zymol, 3M, or Meguiar's respective car washes.
The last element of a good washing?.the mitt. More people ruin finishes with sloppy handling or ill chosen mitts than perhaps any other mistake. Whatever you use make sure it has NO synthetic fibers in it. Even if it says 100% cotton burn test* the edges to make sure the thread used to sew it are not polyester/synthetic. A 100% cotton chenille mitt, a true natural sheepskin mitt, or a Boar's Hair brush are all good safe choices.

So now your car is out of direct sunlight, you have rinsed it thoroughly, you have your safe mitts/brushes ready for washing, a bucket of water with some high quality car wash in it (keep the water cool or luke-warm please), and you are washing your car top to bottom. When you have all that going on you are well on your way to preserving that "wow" finish everyone loves. Washing is as much preserving the finish as it is about cleaning it.

By-the-way, if your washing your car and you run across a glob of something that doesn't want to come off readily with the wash just let it go. Cleaning the paint is an entirely separate chapter. You are likely to do more damage grinding away at whatever it is than you are leaving it on there a bit longer till you can properly remove it with something specialized. As always, these are only recommendations for the person in their driveway.

*when burn testing the flame should just blacken or char the fabric, if you see any balling or ?gummy? look to it you have synthetic thread which is a major culprit of swirl marks.

Washing part - 2

In this segment of the detailing process I will address washing technique.
To sum up what we were doing from last have your car in the shade, using a 100% natural mitt/brush, have rinsed it thoroughly, are using cool-luke warm water in a bucket of high quality car wash (just a capful or so to a couple gallons as too much soap will devour wax) , and you have your 3 separate mitts at the ready. You also remember that you don't want to grind at anything that wont come up easily with the regular wash.


Cleaning the wheels and tires is another commonly abused area of the car. Just as household cleaners will ruin the look and possibly damage the finish of the paint, they will do exactly the same thing to your wheels! Virtually all wheels on Mercedes are painted and/or clearcoated. That finish is just as tender as the one on the hood, fenders, roof, etc. Don't do anything to it you wouldn't do to your hood! This tender finish is subject to some of the harshest conditions anywhere on the car. It gets coated with ultra-hot bits of dust from the brake pads that will actually eat into the paint given enough time and accumulation. (if it will eat into a metal brake rotor over time what will it do to your finish?)
Using a specialized and PH-balanced wheel cleaner (like Meguiars or P21S) is a must for someone looking to preserve the factory finish. Unfortunately household cleaners like 409 and Simple Green can destroy that delicate paint quicker than the brake dust. Either of those can be used on the tires, but only the tires. Remember that wheels cleaner is for wheels, tire cleaner is for tires. Tire cleaner can be damaging to wheels and wheel cleaner generally isn?t strong enough for tires.

While we are on the topic of tires I have noticed that certain silicone or petroleum distillate based tire dressings (Armor All or Son-of-a-gun) can be a real headache to clean up after next washing. A dressing like Lexol's Vinyl-X (for the wet look just apply and forget about it, for the matte/concours finish buff off with a clean rag after it has dried for a minute), Meguiar's Endurance or Mother's Back-to-Black gives all the look but doesn't turn your tire-mitt into a sludgeball.
In addition to washing the vehicle top-to-bottom you want to use linear strokes that run in the direction air travels over the car. This way if you accidentally cause swirls or light scratches they are far less noticeable than from a circular pattern. It is also important to never let the soap dry on the finish. Rinse frequently while washing only 1 section at a time. How big is a section? Well, that depends as much on you and the conditions as anything else. A section should be as much area as you can comfortably cover without any of the soap drying. (hood, trunk, top, etc)

Another point to mention is that when you have washed a section and are dipping the mitt back into the bucket give a few agitating shakes to dislodge any particulate that has worked its way into the nap of the wash-mitt. Better that it end up in the bottom of the bucket than sanding your finish?

Even after you are done with an individual area (washing and rinsing) it is a good idea to go over the car one last time in a top to bottom "finishing rinse".

*Note, if you are using a pressure washer or a commercial car wash with a pressure sprayer do not spray the radiator directly. Those things put off enough force to bend the cooling fins on the radiator and hence damage its heat shedding capacity. Then again, real pros generally try to avoid commercial "self-washes" and automated car washes entirely. In my opinion you should NEVER use those automated car washes on a car that you car about in the least. Company cars, off-roaders, and that ilk are what they are designed for.

Other small touches that make a difference would be using your headlight washers before you wash as they can become clogged over time and disuse. Also if you periodically wipe down the antenna while it is fully extended you remove the grease, grit, and grime that it accumulates. While all of this may seem extreme or overly time consuming, once you have made it routine and have a system of sorts established it saves your time and headache in the long run.


Unless your car has been washed and rinsed in only distilled water (which is not realistic) you will need to dry it to avoid water spots and mineral deposits. Plus when the surface is wet it is more attractive to dust. Also, if you plan to clean the paint or wax drying is a must. There are several schools of thought when it comes to drying. It is generally agreed that this is another process to be done in the shade.


We use synthetic chamois over the natural variants because they don?t rot, stink, get stiff, or leave oils on the paint. The synthetics hold just as much water and ring out more easily. If you?re a die-hard terry-cloth person then you may use them but drying a full sized car requires a big stack of towels. Our method is to use the synthetic chamois to get 90% of the water off on the first pass then pull the rest off with a 100% cotton terry towel.
Proper chamois technique is to have a bucket of clean water nearby. You rinse and ring before the first pass and often while going over the finish. Just as agitating the wash mitt in the bucket while washing dislodges any dirt and helps prevent swirls, so does the rinse&ring method while drying. As always if you drop the rag, mitt, or chamois STOP using it until it can be thoroughly laundered lest you grind in the grit you have spent countless hours removing. As with washing try to keep the drying strokes light in pressure and parallel to the direction of airflow over the vehicle.

Once major exterior surfaces are reasonably dried it is time to turn your attention to the doorjams, trunk sills, mirrors, and underhood. We recommend using a terrycloth rag/towel for those areas as they tend to hold dirt that can be better absorbed by the terry?s nap vs the ultra smooth synthetic chamois. Begin by opening all the doors, trunk, and hood. You may want to open and shut them briskly once or twice to dislodge as much water as possible.

Once the water is pulled up you may want to remove any dirt/grime in the area with a ?quick-detailer? such as Meguiar?s Quik Detailer or Final Inspection. These products can be wiped over a semi-dirty surface without causing swirls/scratches. They also feed the paint nutrients, extend the life of the wax, and enhance the look of the finish.
A visual inspection around the vehicle to make sure no more water has seeped out of a crevice never hurts. (wheel lug-nut holes, tail-light assemblies, mirrors, etc) If you were not cleaning the paint or re-waxing now would be the time to give the entire car a once over with the aforementioned Quik Detailer or Final Inspection.

Using a new, clean, 100% cotton terry lightly mist the finish and spread with the terry. Then flip the rag and lightly buff area with the dry side. The area covered should feel nearly frictionless with the rag. Repeat the process of mist, wipe with the wet side of terry, and lightly buff with ?dry? side over all the painted surfaces. If the Quik Detailer or Final Inspection streaks after buffing then it is an indicator you may need to re-wax.

Polishing, Cleaning and Waxing

Ah, the eternal battle with swirl marks! j/k 3M, Zymol, and Meguiars are all top quality products. I would have to give the nod to 3M or especially Meguiars. Zymol looks great but is a colossal pain in the butt to use. You can get comparable results with the others with much less effort. Unless you detail for exercise I think the other two are a better-bet all around.
Assuming you have washed and dried the finish as prescribed earlier you are ready to clean the paint and/or de-swirl. For this job I would go with the Meguiar?s professional line known as "Mirror Glaze" that comes in a tan bottle. Of course, all these steps should be done in the shade or indoors under BRIGHT lighting. (I have a several 500w Halogen spotlights)

I would start with...Meguiars #9 Swirl Remover. You can do it by hand but an orbital buffer with 100% cotton pad can speed things up if you are careful. Apply the #9 to the finish and buff vigorously. Then remove with linear strokes of a 100%(including threads/seams) terrycloth.
Following that I would examine the paint visually and under sunlight or intense light to see if you have eliminated the swirls. Remember, swirls are scratches in the paint. Therefore to remove them completely they have to be buffed out by removing paint. If you have not removed the marks or not removed them to a satisfactory degree you can repeat the previous steps with the #9 again. I always prefer to make multiple steps of something less harsh than one step of something harsher.

Assuming you have buffed the swirls out to your satisfaction the next step would be to polish. Using Meguiar?s #7 Show Car Glaze, apply and buff with a clean 100% cotton covered foam applicator pad or 100% cotton orbital buffer pad. Then remove with a separate, clean, 100% cotton terry. For ease of removal it may be helpful to work one section at a time. (hood, half the roof, trunklid, etc) If the paint feels "baby's butt smooth" go ahead and wax.


If you feel a bit of roughness on the surface of your paint you can try a high quality paint cleaning clay. For clay use, spray the area being cleaned with a surface lubricant like Meguiar's #34 Final Inspection or Quik Detailer then rub the clay back and forth over the lubed area. Keep plenty of lube on the finish and turn/knead the clay periodically. The clay is great for cleaning things ON your paint. The polish and swirl removers clean things IN your paint.

To protect your hard work you then apply either Meguiar's #26 wax or you can also try the new Meguiar's Gold Class wax for clearcoats. I have not had as much experience with the new wax but, I have used it with good results thus far. Beware, it is for clearcoats only. Simply apply the wax with a cotton covered foam applicator then remove with 100% cotton terry.
If you have completed the above steps you should have one helluva shine going. If the finish of your car didn't respond to the above steps you are in need of heavy duty refinishing. As always, if you drop a rag or applicator during ANY phase of this discard it immedately and replace with a clean one. I hope this helps out with the swirl and paint cleaning issue.

Clear Title or NO Sale!

I am tired of seeing all these title issues:

  1. Never buy without a CLEAR title.
  2. Never trust the seller. Copy the VIN from the car yourself, get a xerox copy both sides of title, be sure the vehicle VIN and title VIN are a perfect match, and check it with the local DMV.
  3. Have the local DMV explain the correct way to fill out the title.
  4. Have a notary witness the transaction; it is worth having an extra witness to the transaction.
  5. Transfer the title to your name, same day if possible, until it is transferred, a crooked previous owner can apply for a lost title and sell it out from under you, or claim theft.
  6. Lock the title in a fire safe or safety deposit box.
  7. Get the vehicle off their property the same day if possible, I have been charged storage of $100 per day by a crook, he found out the true value of vehicle after the sale, and tried to get it back by seizing it for storage fees.

For some reason; diesel owners in general seem to be more old school type honest and trustworthy.
Please don't trust anyone on titles.
I have been burned enough times to never allow it again.

Banjo Bolt & Fitting on the 1985 W126 300SD

This is to help people new to Mercedes Benz diesels.

Banjo bolt location on the 1985 W126 300SD:
Rear of intake manifold.

What is it?
A hollow bolt with a hole in the side to give passage.

What does it do?
Pass intake manifold pressure to the ALDA.

Why does it need cleaning?
Oil sludge, carbon and other gunk builds up and plugs the bolt and line.

What happens if the bolt or line are plugged or broken?
Massive power loss.

Do NOT over tighten the Banjo bolt; you will strip the threads from the manifold.

Follow the line to the ALDA, and remove the Hex banjo bolt from the ALDA, you should clean this bolt and banjo fitting at the same time.


Mercedes-Benz A/C Control Diagnostics

Air conditioning was once just a refrigeration issue. The electrical controls were merely a mechanical thermostatic switch, a simple relay and maybe a low-pressure compressor protection switch. As with other automotive systems, the development of "electronic control systems" has allowed manufacturers like Mercedes-Benz to go to extreme lengths to improve temperature control, airflow and systems protection.

Diagnostics now require deciding which of following four separate systems or combinations are malfunctioning when complaints are checked.

Of course, refrigeration efficiency still plays a major part. The compressor control protection system is the number one intermittent problem with lack of cooling complaints. The control of the heater valve is often more than just an open or closed issue and the automatic control of air distribution opens many possibilities for poor temperature control.


Refrigeration on Mercedes-Benz has been a simple cycling system with metering expansion valve and receiver-drier for all models until at least 1996 when variable displacement systems were introduced. The problems are similar to most systems with a couple exceptions. The Nippondenso compressors experience the same "Black Death" as has been seen on certain domestic cars. The problem with these compressors seems to be that the pistons are coated with Teflon and during failure, the resulting mixture of burned oil, Teflon, aluminum and refrigerant forms a coating throughout the system. This coating is resilient during flushing and becomes mobile again with heat.

Another problem often related to the first is that the evaporators on many models have multiple-tube flow patterns. During flushing, the flush liquid is hard to remove from the lower passages once the upper passages are clear. When we flush these evaporators, we use a low boiling point, volatile flush liquid and use plenty of dry nitrogen to vent the system after the flush is gone. Over oiling and residual flush are the causes of many poor performing systems.


The greatest challenge to a diagnostic technician comes from the intermittent problems involved with the compressor control system. This system, once used only for evaporator temperature control and low refrigerant compressor protection, is now used for multiple protection, efficiency and engine power considerations. The compressor can be disengaged to protect the belt system and the engine temperature, and possibly even the electrical system from low voltage and the idle from going too low. It is disengaged to control evaporator temperature and is engaged to dehumidify the windows in defrost no matter where the temperature is set. The compressor is disengaged on diesels to give more power in full throttle.

The biggest challenge in the diagnostic process is the extreme intermittent nature of failures in this system. The system consists of two controllers: The pushbutton controller (PBC), located in the console; and the compressor control relay under the hood (called Base Module, Klima or MAS, depending on the model and year -see photos).

These two controllers have very different mission statements. The pushbutton controller is concerned with temperature control as it relates to occupant comfort. It controls the compressor clutch for evaporator temperature and humidity during defrost conditions. It also sends a control signal to the compressor control relay whose mission is to control the compressor to meet the engine's needs.

The compressor control relay monitors the engine speed vs. compressor speed to protect the belt system from a locked compressor. It disengages to protect an overheating engine and to provide extra power in diesel models, and may possibly disengage to protect a low or erratic idling motor or an under-voltage system.

The first place to start in this diagnosis is to decide which controller is disengaging during the event. The easy access is at the low/high pressure switch on the drier. The signal from the pushbutton controller passes through the switch to the compressor control relay. This is a ground signal in most, but not all, cases. Verify the signal when the system works, then monitor the signal to determine which controller drops out.

Pushbutton control problems usually are just that. In early units, the controllers had internal connection problems, bad solder joints and poor connections between the temperature wheel and switches to the PC board. Occasionally, there are evaporator sensor problems and control often goes astray when the aspirator pump won?t pull air over the ambient air temperature sensor due to pump or tubing problems.

The inside controls have self-diagnostics on later versions starting in 1990. These diagnostics can be read with an impulse counter or LED on models up to 1994. The code is read by initiating self-diagnostics, which is done by grounding the appropriate system pin on the diagnostic socket for two-four seconds. The code can be read by monitoring the pin with an LED powered by the battery.

The diagnostic pin will be grounded for a number of flashes to transmit the code (Code 4 = four flashes, etc.). After reading all codes, they can be erased by rereading and then grounding of the diagnostic pin for six to eight seconds. Each code is to be read and erased until Code 1 is reached. Code 1 indicates no faults. This method of self-diagnostics can be read on any system (with self-diagnostics) up to when the systems went digital beginning in 1995, depending on the system.

The real tough ones are under the hood. Once it has been verified that the PBC is doing its job, a few observations should be made. First start with the belt tension and tensioner. A belt that flops is going to cause enough disruption to fail the comparator test in the compressor controller. A system that starts and then stops and never restarts until an ignition cycle is likely to have failed this test, although low voltage has also been claimed to cause disengagement that requires an ignition cycle.

The low voltage problem has been described as a problem during start-up where the voltage drops below a threshold (probably around 9v) and the current load is reduced by disengaging the clutch. I haven?t verified this, but it is an internet fix response. Other reports, including one I can verify, say that a low and rapidly changing idle can set off this comparator.

I experienced this situation on a 1995 Mercedes-Benz E300 D. The problem with the car stemmed from the over-voltage protection relay. This notorious relay plays no part in the compressor circuit but does power the idle controller that converts the induced crank sensor signal from AC to a pulsed DC engine speed signal. This signal is used by the tach, the A/C compressor controller and the EGR controller, which also controls the variable manifold length servos. While the speed signal never varied, the idle controller was losing control when the relay faulted. During this time, if the compressor engaged at idle, the speed would take a big hit and the compressor would disengage.

The A/C clutch should be checked for both gap and an oil-free state. Power steering fluid, engine oil or A/C shaft seal leaks can oil down the clutch until it slips enough at idle to set off the comparator. Try some brake clean on the clutch if it?s oiled and look for signs of overheating.

If no mechanical problems turn up after a visual inspection of the belt system, then begin electrical testing. The testing takes different approaches depending upon the year and model. The 124 and 126 models up to 1990 (and some later diesels) use the Klima relay. The V8s don?t use the compressor speed signal and speed comparison, as they have a separate belt for the air conditioning.

To test, we remove the relay and bridge the power to the output to engage the compressor. With it turning, the compressor speed signal should be an AC voltage with a peak-to-peak voltage difference of less than two volts (see waveform at right ? less than 1v was coming from this functioning system). The engine speed signal is converted from an inductive crank sensor AC voltage to a pulsed DC signal by the ignition controller (gas models). See accompanying waveforms for before and after signals (top left and bottom right, respectively).

The diesels use either the EGR controller or the idle controller ISC to do the conversion. On diesels, the controller disengages when the full throttle micro-switch is closed (pin #4 Klima). This signal should be battery voltage until full throttle. I have had a bad controller that was pulled internally a little less than 5v and would disengage.

Clutch coil current should also be inductively checked off the jumper. The amperage should be less than 5a and is usually around 3a. If a controller is bad all the time, definitely check this current before replacement. Unfortunately, this is also intermittent and will burn the controller just like a fuse. If a new controller comes back, I would definitely suspect the clutch coil.

Many unnecessarily condemn the compressor speed sensor, as a very small signal is monitored. The photo shows the end of the shaft that is read by the sensor. This particular compressor locked up. The sensor shows the arc that the shaft segments follow, as the shaft was hitting the sensor before it locked up and caused the damage. Notice that the sensor is off center from the shaft and normally it is flat at the bottom. The only shaft speed sensor problems we have seen have been from rebuilt compressors where the distance from the sensor to the segments was too great.


The air distribution system is complicated and getting progressively worse, but usually it only causes the air to go to the wrong places. Due to safety concerns, most vacuum systems default to the defrost position. This causes reduced air flow to the center ducts and often fogs the window. One serious duct temperature problem (10-15 degrees at times) can come from the recirculate door. This door also defaults open so when the vacuum servos fail, the system pulls in too much outside air.


The last factor involved in Mercedes-Benz climate control is the heater water valve. From simple vacuum-controlled, on/off water valves to multiple pulse width-controlled water valves, each model can be different. Early models used auxiliary water pumps to help when temperatures got too cold. Systems starting with the 124 chassis have used the auxiliary water pump to maintain a uniform heater core temperature. Failure of this pump causes a couple of nasty problems. The first one is straightforward, if you catch it before it damages more than one pushbutton controller. The pump locks and the current flow burns out the controller ? a good thing to check out if you are replacing the controller.

The second problem caused by the pump (or lack of pump) is more subtle. It shows itself as sporadic, sudden bursts of hot air, which are produced when a car that is mixing heat and cold to achieve control sits at a light. Due to a no pump and poor engine flow condition attributed to the slow speed, the water valve is opened to full to try and raise the heater core temp. When the car starts moving, the water flow grows rapidly and the heater core goes full hot before the valve can control it.

The heater valve on the early VDO climate controls had a removable capsule that produced a pattern failure in the heating mode. One common problem with the capsule was that it worked initially, but lost heat as the car was driven for a while. The valve is held closed by the electrical circuit that pulses in the middle ranges to control heater core temperature. Bad connectors have caused full heat on many systems.

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