I've got an '88 300CE and I've been troubleshooting some issues with my air conditioner all summer. The Auxillary Fan does not turn on at all. I've checked and replaced all relevant fuses and relays. From my research, I am thinking the next few things need to be checked are the following. I am wondering if anyone has experience with these:

1) Resistor: down in the engine bay under the driver's side headlight is a resistor. I've read this is a common failure point because it's more exposed than, say, the relays. However, I've also read that this is pretty robust and doesn't always fail. Does anyone know of any way to test this (i.e. what resistance should I be looking for)? It's not expensive to get a replacement but I'd rather not if my current resistor is fine.

2) Wiring after relay to resistor: I don't think this is a failure point, but I'd like to know if there is a way to test if power is even getting to the resistor when the A/C is on. I assume I can do this with a test light at the point where the resistor connects to the non-fan side.

3) Test the fan itself: it's possible the fan motor is dead/defective. Anyone know of a way to jump this, i.e. hook this directly up to the battery to see if it runs?

4) Would the blower motor fan resistor/regulator affect the operation of the auxillary fan at all? I've read that this is a common failure point with the blower motor, and I wouldn't expect this to be linked at all to the operation of the auxillary fan. Still just want to get a second opinion.

If there's anything else I should consider, like a temp sensor, please let me know.

 

To test the temp sensor you can measure the resistance across it at cold and hot. Hot, it will be a couple hundred ohms, hot is will be in the 2-3Kohm range.
Read the spec's on it.

A quick way to check if the system (including relays and the controller) works is unplug it while the ignition is on. The aux fan's should turn on at high.
This is because the sensor is connected to a window comparator. An open circuit forces the aux fan to turn on.

If it does not turn on, your temp sensor is likely not the culprit, there is something else faulty in the system.

 

well he is half right---as of 88 all 124 had different first and second stages of the aux fan---the CE in 88 the first stage fan actuation was via the refrig pressure switch(on at 20bar and off at 15bar via the K9 relay and series resister R15------the second stage is via the coolant temp sensor and actuated via the control unit
you would know all this if you had a blink tester that will tell you all the fault locations----------------if any of this does not make sense see a good mechanic

 

I should correct this:
Hot, it will be a couple hundred ohms, hot is will be in the 2-3Kohm range.

To say:
Hot, it will be a couple hundred ohms, cold will be in the 2-3Kohm range.

 

Sure seems like the pressure switch that turns on low speed for that fan is in the top of a receiver/dryer for A/C.