Aux fans are not coming on; how to test?
Thread Starter
Newbie
Joined: Mar 2008
Posts: 4
Likes: 0
1997 E420 AMG Sport
Aux fans are not coming on; how to test?
Car is a 1997 E420, 140,000 miles. Recently replaced a leaking radiator, and installed a new thermostat. Temperature gauge was sitting rock steady at 80 degrees Centigrade after replacement. However, we got warmer here in So. California in the last couple of weeks, and now the temp gauge will creep up to 90+ C or even higher when the A/C is on and the car is stuck in slow moving traffic. It appears that the auxilliary fans are not working. I would think they should spin when the A/C is switched on, but they don't.
When replacing the radiator, there was no temp switch attached. What controls the aux fans? Is there a temp switch somewhere on the engine? If there is, how do you test that it is working? Also, I would think the fans should run when the coolant water gets to a certain temperature number, even without the A/C on. Where should I look and what should I test? Please advise.
Regards,
Dave
Woodland Hills, Ca
When replacing the radiator, there was no temp switch attached. What controls the aux fans? Is there a temp switch somewhere on the engine? If there is, how do you test that it is working? Also, I would think the fans should run when the coolant water gets to a certain temperature number, even without the A/C on. Where should I look and what should I test? Please advise.
Regards,
Dave
Woodland Hills, Ca
Member
Joined: May 2006
Posts: 197
Likes: 0
1997 E320
First of all check your fuses.
If you have a multi meter check unplug the fan and check resistance between the + and - terminals of the fan (not the terminal coming from the cars electronics). If the fan is good you will read a low resistance something between 1 to 10 ohms or something like that.
other wise
unplug the fans and connect them to 12v from the battery, but very carefully. Watch your fingers and don't short anything out. you can use the body metal for negative and the positive jumper post in the hood.
The auxiliary fan is controlled from the cars computer. it starts slow at I think around 97 Celcius and speeds up as temp rises. there are several coolant temp sensors in this car my guess would be it takes reading from the one next to the thermostat. if it doesn't feel the fan is connected then it cuts voltage to the terminal.
Another way you can test your fans is with engine running hold down the left and right auto buttons on the climate control for about 30 seconds. this will kick the fans into full speed. to turn it back just hold them down for 30 seconds.
It is very likely the fan is dead. I had to replace mine last year for $200 from an online store. You only need the motor fan not the whole assembly. and probably a new belt to drive the other fan.
If you have a multi meter check unplug the fan and check resistance between the + and - terminals of the fan (not the terminal coming from the cars electronics). If the fan is good you will read a low resistance something between 1 to 10 ohms or something like that.
other wise
unplug the fans and connect them to 12v from the battery, but very carefully. Watch your fingers and don't short anything out. you can use the body metal for negative and the positive jumper post in the hood.
The auxiliary fan is controlled from the cars computer. it starts slow at I think around 97 Celcius and speeds up as temp rises. there are several coolant temp sensors in this car my guess would be it takes reading from the one next to the thermostat. if it doesn't feel the fan is connected then it cuts voltage to the terminal.
Another way you can test your fans is with engine running hold down the left and right auto buttons on the climate control for about 30 seconds. this will kick the fans into full speed. to turn it back just hold them down for 30 seconds.
It is very likely the fan is dead. I had to replace mine last year for $200 from an online store. You only need the motor fan not the whole assembly. and probably a new belt to drive the other fan.
Junior Member
Joined: Sep 2010
Posts: 20
Likes: 0
From: SF Bay Area
1998 E300 TD
Hi, I have the same problem. I first changed the thermstat to no avail. The first overheat (120C) burst the radiator. Noticed the aux fan only barely turning when engine is hot and A/C on.
Tried the recommended test within this thread just now. It worked but the aux fan just cycled between very low and med speed. It never got up to the high speed I used to know.
What gives? Thanks
Tried the recommended test within this thread just now. It worked but the aux fan just cycled between very low and med speed. It never got up to the high speed I used to know.
What gives? Thanks
Junior Member
Joined: Sep 2010
Posts: 20
Likes: 0
From: SF Bay Area
1998 E300 TD
The test triggered both aux fans to rotate, gradually increased in RPM, then stopped; and this cycle repeats. I was expecting the test will drive the fans to go full speed and stays there.
Senior Member
Joined: Feb 2017
Posts: 392
Likes: 9
From: Australia
W202 C43 1999
This is the same post in the W124 section------just follow the directions---you can turn the key to the second position which energizesall the circuits, but does not start the car-----then press---AT THE SAME TIME the two AC auto buttons----do the fans go wooosh!!
In the W124 section you said you already tested and they worked----whats up!!!
In the W124 section you said you already tested and they worked----whats up!!!
Such as that fitted in the 1999 C43
