The auxilliary fan does work if you jump the fan itself. The fan does not come on as the engine is running regardless of engine temp. The auxilliary fan resistor is original iin this vehicle and it is cracked along with wires seized to the resistor. Do you feel that I should change the resistor and that is the problem with the fan not working?

The resistor either works or it doesn't, I wouldn't care what it looks like. Check it, if its bad replace it. BTW, though the relay doesn't have anything to do with fan operation due to engine temp. It only is the mechanism for reduced speed on the low blower speed used when the AC pressure is high.

Of course if it worked properly for the AC the car probably wouldn't get hot.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

The aux fan comes on for two reasons. The first is a low fan speed due to the air conditioning pressure. Look at the a/c receiver dryer and locate the switch which turns the fan on. Remove the clips and jump the wires with the ignition switch on. This sends power through the resistor and should turn on the fan. If the fan turns on, then the switch on the receiver dryer is bad. If the switch does not turn on, check the resistor. I owned an 89 300e and the resistor looked good when it was really bad. Heat will break the porcelain down. I would replace the resistor just as a preventative maintenance measure.
The second reason the fan turns on is for high engine temperature via a temp switch on the engine head. This puts the fan in high speed and bypasses the resistor. I believe it should cause the fan to turn on at around 107C.

Unfortunately the advice about the drier switch will be wrong 90% or more of the time. I almost never see a bad switch. The problem invaribly is that the AC is running at a reduced pressure either due to low charge or other mechanical issue.

The low speed fan doesn't come on till a high side pressure of about 295psi occurs. On a full system and a hot day that occurs before the engine is hot, but on a cool day it may never occur and if low on charge it won't occur either.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

Here is the schematic..you can see how the relay brings the R15 dropping resistor into the circuit to accomplish low fan . You can also see how the other relay [ coolant temp sensed] by-passes the resistor so the same fan motor gets full battery/system voltage, resulting in high fan operation.
The confusion on these fan circuits is the fact that there are two different circuits , but they both SHARE the same fan motor/motors...
This shows that design and makes it quite simple.

http://members.aol.com/ajdalton7/auxfan.jpg

ok guys just tell me where s25/5 is located on the 88300e in question here??
larry perkins

ok i got it,the low speed aux fan is controlled by the red sw at the dryer,the high speed is controlled by the sensor(blue 3rd one from rear of engine)) on head and there is no s25/5 on the 88 or 89 300e.
larry perkins

as you say , the aux fan will run if you jump it,so why dont you disconnect the blue sensor temporary and see if the fan starts up?
if not then look at fuse#d and start from there, those fuse holders are crap and need help.
cm why dont you try the same test?
larry perkins