1992 400E Aux Fans; root cause help requested
 

As the temperatures have gotten warmer out here in Massachusetts over the past couple of weeks I have noticed that the aux fans were not kicking on at 105-107C
the temperature would continue to climb to 150-180 if I did not turn the heat on high.

Anyway, I have printed out and read nearly all the posts related to this problem I cannot find the answer to what I am encountering...so perhaps someone can help with advice...

-checked to see that aux fans work by disconnecting the Coolant Temperature
Sensor...Fans kicked on fine so I figured that I had a bad CTS.

I replaced the CTS but the problem does remain

-checked fuses -OK

-checked relay operation -OK ( I can hear it click on & off with ignition key in ON position)...No resistor in the high speed relay on my car that I can see.

-checked low speed resistor (under ABS control unit) -OK (But even if this was
bad it should not affect high speed fan operation when temps go up beyond 105C
right?)

-checked switch on top of receiver dryer -OK (Again, this is for low speed fan
so should not affect the high speed kick-on right?)

What have I overlooked?

<<aux fans were not kicking on at 105-107C
the temperature would continue to climb to 150-180 if I did not turn the heat on high.
>>

105C is above 180F..

What exactly do you mean???

Temps getting up near the redline
 

Arthur,
What I should have said was that temps continue to climb up towards the red-zone before I turn on the heat to bring 'em down.

My question is
Does the temp get past 105C without the high fan engaging??????????

You need to check the resistance values of your coolant temp. sensor. If the sensor is out of whack, it will not provide the correct resistance value and the relay will not activate the fan.

Does anyone have the proper temp / resistance values for reference?

Also - on my car (1995 E420) I have added a resistor in paralle to the coolant temp. sensor. it makes the fan activate at lower temperatures. The car NEVER gets above 98 degrees C anymore. Hopefully I'm extending the life of the engine and wiring harness by doing this. I don't mind replacing electric fans if they wear out - probably much cheaper than replacing an engine.

Hope this helps.

Troy
1995 E420 93k

Normal electric fan engagement is about 105C. If the A/C is on they will engage at low speed, periodically based on A/C system demand.

Duke

Yes, way past 105
 

Arthur,

It gets to up near 120C without any fan turn-on. There are two notches near 120C a white one and a red one...I usually kick on on the heater if it starts to get within range of the white one.

Troy,

A swapped in a brand new CTS...The chances that I have one CTS go bad and the new one also bad look slim but if I know what the temp resistance values should be I can check this out.

The fans are not turning on at low speed or high speed UNLESS I short the AC pressure switch on top of the drier OR I unplug the CTS respectively.

Does anyone have the correct part numbers for the fan relays?

OK....Can close this one...
 

Finally gave up on this and took it in to Hatch & Sons (Now called Heritage Motorwerks; they have been acquired by another company...that owns one of the big MB dealerships)....

My second experience with Hatch & Sons was good as the first one...root caused precisely to a bad CTS...the new one I installed was an aftermarket (Hamberg Tech) and it had not been turning the fans on at the trigger temp.)

Lesson learned...stick with MB parts for electrical stuff...