I am looking for some help in understanding an A/C problem I have with my E320 98 station wagon. A little over a year ago I paid quite a bit to have the A/C repaired. One of the items replaced was the dual auxiliary fans. I checked them by setting the climate control unit to test mode (key in position 2, depressed both Auto buttons 10+ seconds). Sometimes they turn on and sometimes I hear a humm and with a little spin help they will start. I have not seen them come on while the car is running. This could be the reason why my car cools at start but when faced in traffic it goes very hot. Any ideas to fix problem?

Just like almost all systems on that car! there are several computers involved in the operation of the engine cooling fan assembly.

Engine computer sends temp signal to the A/C control panel via the instrument cluster.
The instrument cluster also passes signal info from the A/C controller to the front SAM (signal activation module) via the EIS (electronic ignition switch). from there the front SAM sends a PWM signal to the fan control module.

Sorry if this doesn't help..BUT nothing is easy w/o computer to diagnosis.

Could be a programing issue with the A/C controller OR any other computer involved.

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i checked the voltage at the connector which rises and falls above 12 v. also once i got it to spin by turning the fan by hand. i shut off the ac from inside and the fans kept running. is this normal? thanks for any help

Your '98 has different parts controlling the auxiliary fan compared to my '96, but I believe that the method of operation is quite similar.

When the demand for the fan first appears, the controller pulses the power lead to get the fan turning. The motor will provide voltage to the controller during the "off" part of the pulse cycle, and the controller watches for this back feed. If the electrical signature is wrong, the controller aborts the start attempt, and tries again in a few seconds.

Because of this operation, a failing fan motor can cause the fan to fail to start unless you give it some help. Of course, so can a failing control module. Which is it? I doubt that there is any way to know for sure short of substitution.

thanks for your help and response. could you tell me where is the control module located? i bought some MB manual cds but they are not user friendly and Im having trouble identifying the control module. does anyone know of a good manual for these engines? thanks again. i appreciate the help

I believe that it is behind the left headlamp. Mine is in the underhood fusebox, but I think that your car is an updated design.

Matt L I am out of town for a few more days so I have not had a chance to check the location, anyhow I did look at a couple online parts and could not find a control module for my 98 E320. I was just going to go ahead and buy one if not to expensive. Do you think a straight 12v to the fan could exonerate the fan if works freely? Also beside dealer any suggestion where I can purchase control module. It is easier to find it if I know what i am looking for. Thanks for the help.

Matt L, thank you for the coaching. First the fan was inverted polarity; therefore like you said was not doing nothing. I put some gages on and the high pressure side went slightly above 400 psi (probably enough to to create a leak or maybe vent through some seals. Still the fan would not run on it's own but with my help they would work and drop the pressure dropped to above 220 psi.
I went ahead and bought a new fan. Like you said even though the old fan worked under 12 volts they would not work on command of the car. The new fan work like it is supposed to and now I can concentrate on a small leak I have in the evaporator. The temp in the car is consistent at 36 F with an outside temp of 70+. Thanks again.