W124 300TD Aux Fan Always On, Relay, Temp Switch, AC Pressure? - Benzworld.org - Mercedes-Benz Discussion Forum

madeintaiwan · 03-26-2009, 08:33 AM

What could be the cause of my auxiliary fan constantly running? It runs as soon as I turn on the ignition. I sprayed down the coupler connection near the drivers side headlamp with electronic cleaner as the engine was getting a bit hot when driving long distances, so I cleaned it off, let it dry and plugged it back in. I also had a local mechanic replenish the 134a. Could he have over pressured the AC? Could the temp switch for the fan have gone?

Any insight is very appreciated.

Nate

jonbobshinigin · 03-26-2009, 10:25 AM

The little piece behind the drivers side headlight is called an Auxillary Fan Resistor. It was not coming on when it was supposed to on my 300td when I got it. Turned out the resistor was broken in half. Made by Beru. Explain how this began?

edcarls · 03-26-2009, 10:36 AM

If the aux fan turns on when you turn the ignition on, cold engine with the AC off, the temp sensor switch may be the culprit. To confirm, unplug the temp sensor switch and if the aux fan stops your temp sensor switch may be faulty. (If the Aux fan don't turn on with AC off, pressure switch may be faulty)

madeintaiwan · 03-26-2009, 11:09 AM

Right, great. Will go through those two marts and test and post and update.

Thanks edcarls.

NAte

jonbobshinigin · 03-31-2009, 04:53 PM

What did you find out?

arvsreyes · 03-31-2009, 06:19 PM

Hi, I have a w124 300td. What is the correct trigger for the aux fan to turn on? When I got the car, it was on all the time (once the key is turned on to ignition). I had it rewired to be triggered by the a/c. Should it also be triggered by the temp sensor?

Should it always be on as I live in a tropical country?

jonbobshinigin · 03-31-2009, 07:22 PM

It is supposed to turn on to different speeds at different temperatures. I think it turns on low at 95, medium at 105, and high at 115. I am pretty sure it is something like that. Also, it will turn on whenever your A/C is turned on. I am not sure which speed it runs at though. Hope this helps. Keep that engine cool whatever you do, heads are expensive!!

edcarls · 03-31-2009, 07:58 PM

Quote:

Originally Posted by arvsreyes

What is the correct trigger for the aux fan to turn on?

I'm not sure if this is the "correct trigger", but on my 94 E320 I observed the following:

The temp sensor switch turns on when the temp reach 110 and turns off at 90. (I'm not comfortable with 110 so I installed a resistor to make it turn ON at 100.)

The refrigerant pressure switch turns on when the pressure reach 45 psi and turns off at 25 psi. (I used the pressure gauge that comes with the 134a recharge kit.)

Base on the above sensor specs, the fan is ON when the temp is between 100-90 and when the refrigerant pressure is between 45-25 psi. So, on hot weather with AC ON, the fan may be constantly ON. (see matrix below)

Low temp + low pressure = Fan OFF
High temp + low pressure = Fan ON
Low Temp + high pressure = Fan ON
High temp + high pressure = Fan ON