Auxillary Fans coming on early - W124 - PeachParts Mercedes-Benz Forum

mbnz · #1 09-24-2006, 04:43 AM

I have had this problem for about 2 years now, but in the last month it has gotten worse.

When I start my car in the morning (engine cold), the auxillary fans are not on. After about a few minutes of idling, the temp goes up a bit and the aux fans go on. They stay on until the temp goes to about 80 degrees. With the fans always on, it takes a very long time for the temp to go up - so the fans are on for a very long time. Sometimes they do go off before reaching 80 degrees for a minute or two, but they do go on again.

This used to happen on occasion, but now it happens everytime.

Also, if I restart the car after it has been driven (engine still warm and temperature above 40 but below 80) the auxillary fans will go on right from start and stay on until it hits 80 degrees or so.

I know the aux fans aren't supposed to go on until about 90-100 degrees.

Finally, a new thing has happened a few times. When I re-start the car after it has been driven (warm), the idle will go up and up (above 1500 rpm to 2500 rpm) after that I just hit the gas to bring down the idle (sometimes I have to do it more than once).

Any ideas please? The noise of the fans is really annoying plus it is an extra drain on the charging system.

mbdoc · #2 09-25-2006, 08:16 AM

The aux fans are 2 speed..are they running at HIGH speed? If so then the blue 2 pole switch on the head below the air filter is the most likely problem.

mbnz · #3 09-25-2006, 05:58 PM

Yes,they go on at high speed.

I'll look into blue pole switch.

Do you think it could aslo be the coolant sensor?

Arthur Dalton · #4 09-25-2006, 07:58 PM

The blue sw is thwe coolant sensor.

Check the connector there for a tight connection b/c if it is loose the control will go into default and bring on the fan.....same as unplugging it.

milhan · #5 09-25-2006, 08:46 PM

I am having the same problem, with the same car. Weird hey? 90 300E. My fan will turn on first thing in the a.m. when the car is ice cold? I will also check the blue 2 pole switch

mbnz · #6 07-01-2007, 12:42 AM

I had the coolant sensor changed, but still same problem.

Any other suggestions?

btw, am is there any relay to reset or something when the coolant sensor was changed?

emerydc8 · #7 07-01-2007, 02:21 AM

Are you sure the fans are coming on at high speed--not low? If they were actually coming on at low (instead of high), then it sounds like the fans are doing what they are supposed to do.

mbnz · #8 07-01-2007, 04:25 AM

no, they are coming on at high speed, and they come on while the car is still cold (less than 80 degrees).

emerydc8 · #9 07-01-2007, 05:13 AM

Have you checked to see if anyone has intentionally bypassed the aux fan preresistor behind the left headlight? I read a post about people permanently bypassing the resistor to get high fans right away in hotter climates. The high fans would then be activated by the pressure switch on the receiver dryer. If your pressure in the receiver was still low, the fans would not come on right away after you started the engine. Also, I think a normal operation would be for the low fans to come on (or if the preresistor was bypassed--the high fans), then, when the pressure in the receiver dropped back down, the fans would shut off--all this taking place below the trip point for the coolant temp (105-107C) that would normally set off the high fans.

Check out Author Dalton's reply to GregS back in 2003.

http://www.peachparts.com/shopforum/showthread.php?t=67682&highlight=aux+fan+resistor+bypass

brewtoo · #10 07-01-2007, 09:19 AM

Could be a problem with the coolant sensor PLUG or WIRING HARNESS.

The function of the sensor is to keep the fans off when the engine is cool, not to turn them on when it is hot.

Arthur Dalton · #11 07-01-2007, 10:03 AM

Quote:

Originally Posted by brewtoo

Could be a problem with the coolant sensor PLUG or WIRING HARNESS.

The function of the sensor is to keep the fans off when the engine is cool, not to turn them on when it is hot.

Kinda' correct...
If one has an erratic or always ON high fan and wants to isolate the problem of ACC default due to open circuit or bad sensor, here is another simple test I came up with and use..
As most now know, you can get the high fan to come ON by simply putting the ACC into default by unplugging the blu sensor..this is just a simple/fast test for high Fan circuit integrity. However , when one has a fan that
always comes on or erratically comes on, it could be the sensor OR poor wires/connection between the ACC and sensor.
So, what I do in that case is to use a 1K resistor [ Radio Shack item/ .25 cents]. Bridge the resistor accross the unplugged harness . That shoud STOP the fan if all is well to the ACC. If that works, then the problem is the sensor or connection at the sensor.
This test simply verifies the integrity of the ACC circuit .I have one made up that plugs right into the harness. Mine also has a sw in-line so I can active the fan by open/close the sw....but you can just stick the ends in there [ bend them over for a good connection..
The 1K is below ACC default or cut-in spec , so that would be the same as a cold sensor............which will be NO fan.

** NOTE:
While we are at an understanding of this sensor/system, I will also say that, for those of you who have done a resistor/parellel bridging of the blu sensor to modify the cut-in spec of high fan to a lower temp trigger of the ACC, it should be noted that the Fan connector unplug/test for high fan verification will NO LONGER work if the bridging was done at the harness plug side....that is b/c, just as in the 1K resistor test, the bridging of the connector for temp modification will stop the ACC from seeing an open circuit when the harness is unplugged b/c the resistor is still in the circuit.. If the mod resistor is on the actual pin side of the blu sensor and the unplugged harness is now open, then the test will still work as stock..
This info is only for those who have chosen to modify the temp sensors values with a parellel Resistor for faster/sooner fan cut-in.....

mbnz · #12 07-02-2007, 08:20 PM

thanks guys,

i just want to add that since I changed the coolant sensor, i noticed the aux fans are coming on high when the engine is cold (like when i first start the car in the morning).

But when the cars warms up (60 degrees or so), then the turn off and stay off.

Any other thoughts......i will print out what you guys think and show my mechanic. thanks.

Arthur Dalton · #13 07-02-2007, 08:28 PM

>>

The entire system has basically all been covered ..