Mercedes-Benz A/C Control Diagnostics

Steve Brotherton
ImportCar Link
June 2000

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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