is your refrigerant switched to 134 type? if so the drier switch may have not been functioning properly and the person may have just jumped it. I know thats what i did on my 300SD. this way the fan stays on regardless and i have a much less chance of harming my a/c system which is always on in florida. just a thought
R

It sounds like you are only having this running warm situation when the A/C is on and sitting in traffic, (is that correct?)

Your aux fan should be running almost continuously under those circumstances, due to the switch in the A/C drier turning the aux fan on (low speed). So with that in mind, and if your aux fan is running, it should not even matter whether the engine temp. switch is operational, as the aux fan is already running with the A/C switch.

It sounds like your viscus fan coupling is weak, which replacing it will help with air flow. Also I have seen many times, dirt clogging the fins of the A/C condenser and radiator, severely restricting air flow more noticeable at idle speeds with A/C on. I use a long air blower with a 90deg angle tip and go between the radiator and condenser, blowing out the debris. The thermostat is always a source of problems like this also, if it is not opening completely it can cause problems. A problem I run into occasionally is the aux fan running backwards, the wires are reversed in the plug. The fan should turn clockwise to push air through the rad/condenser.

Let us know what you find:)

HELLO FELLOWS,
My '87 300 TDT has an aux fan that will not come on. If I unplug the two wire switch (blue) the fan does not come on at high speed. If I hot wire the fan at the fan connection, the fan works good. Why is the high speed not coming on?
I took a test light and noticed that there is less than 12 volts going to the temp sensor on top of engine (blue). Could that be the problem?

Have you checked the aux fan fuse? Replace it even if it looks good, verify with your test light that you have power to the fuse and through the fuse.

It is also possible that the high speed aux fan relay is defective.

MB Medic, Question About "Blue" Switch
 

MB Medic,

I have been following this thread with interest since I have a 1991 350SD that has been heating up with the A/C on and heavy load (driving up a long hill at highway speeds or lower) or sitting idle in traffic. At other times it runs at its normal 90 or so degrees C. I noticed only one fan was running when I lifted the hood while it was hot.

So, when I saw the comment here to disconnect the blue switch leads and the fan(s) would run in fast speed, I gave it a try and only one ran. The car was cold, and not running, I only had the key in the ignition turned all the way on. I gave the driver's side fan, which was not running, a tweak with my hand and it started. I stopped and started the fan (attached and unattached the plug on the blue switch) and it no longer needed any assistance to start. I have concluded the temperature switch is ok from this data, for the moment. I will let the car heat up and check the switch with a multimeter in the morning.

I will give the car a test drive tomorrow to, but I was wondering what the logic was for this circuit with a normally closed position for the temperature sensor switch. Is the relay energzed so when it fails the fans come on and stay on? I noted this was different than my 190E 2.3-16, which has a normally open switch in the temperature sensing position, and that car has "used" two of these sensors in 199,000 miles.

Thanks for the information. Jim

This switch is a thermister type. As it heats up, the resistance drops. When it drops to the correct ohms , the fan relay is energized.

Arthur, that is what I thought, based on my experience with the 190E 2.3-16 and reading the wiring diagrams. But in this case when you disconnect the switch, the resistance goes to infinity, and the fans turn on. I checked the resistance on the "blue switch" with the engine cold, and it is a "dead short" meaning zero resistance. I will check it out today and when it is hot I will make another resistance measurement and let you know what the value is when the fans come on. Jim