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.

__________________
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

on the blue temp sensor on the head(3 from rear) even though it is a varable value from about 3k ohm down to about 300 when hot,if you unplug it (open ckt) the aux fan should run at high speed,so this test does bypass the low speed ckt for a test of the high speed. of course if it dont run then you have to trouble shoot.
larry perkins

[QUOTE=larry perkins;1505261]on the blue temp sensor on the head(3 from rear) even though it is a varable value from about 3k ohm down to about 300 when hot,if you unplug it (open ckt) the aux fan should run at high speed,so this test does bypass the low speed ckt for a test of the high speed. of course if it dont run then you have to trouble shoot.>>>

You have the test correct, but it does not by-pass low speed circuit..has nothing to do w/low speed circuit.
The high fan on the thermister type sensors is controlled by the A/C Control panel circuitry..which defaults if it sees an open circuit in the thermistor or wiring leading to/from the sensor...that is an engine coolant temp saftey design in the CP circuitry in case the sensor goes out, the engine still has high Fan..all is you are doing when you do the high fan test by unplugging the termistor is you are making the Panel see an open sensor.wiring and causing the CP to go into fan default. Nothing to do w/low fans circuit.

One can also change the cut-in of the termistor by bridging it with a parallel
resistor to change its Total R factor by a few degrees . Many do this simple mod just to have the High Fan come in at around 100C, vs the factory spec of 105/107... this works well for temperate climate loactioned cars

my point was that one could test the aux fan to see if it would function,and it seems reasonable that one should know this is a easy way to get the temp down in case of an emergency.
i firmly believe that heat kills and any way to get to a service is good info.
larry perkins

I'm having the same problem in my 88 300E. Could the fault be with the Climate Control Panel itself, or can I rule that out before testing circuits?

__________________
1992 400E current
1988 300E sold
1973 280C totalled
1973 280SEL never got it running
1971 250C my first love but rusted out

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

That is a well know test and I am in agreement with you on that..my point was you used circuitry by-pass info including low fan , so that would confuse the point that these are two independant circuits, except for the fan itself.

..and with the electrical circuits design of the later thermistor , it is an all new ball-game..the blue sensor does not trigger a hign fan , it sends a temp temp input to the CP , which then controls high fan relay along with the compressor high temp saftey from the same thermistor input . The early sw was just a on/off switched ground without these features.

Do as LP has posted ..that will test the integrity of the high fan circuit and the fan motor itself. If still no fan , I would start with testing the feed fuses
The test for LOW fan [ a/c], is to jumper the pigtail leads at the high pressure sw..this will also test the fan motor along with the low fan circuit..

I did try that, Larry and nothing happened. My compressor never worked until I disconnected the aux water pump, and there is no ground to the heater valve(grounded by wire to battery). When I jumped the wires on the blue thermistor, the compressor stops, but that's it. I am thinking that my aux water pump fried my control panel and that if I get another one all of this may work again. Does that make sense or am I off in left field?

__________________
1992 400E current
1988 300E sold
1973 280C totalled
1973 280SEL never got it running
1971 250C my first love but rusted out