aux fan not working with AC - help 400E W124 - PeachParts Mercedes-Benz Forum

amg280 · #1 07-17-2006, 02:05 PM

Hey guys. My A/C just starting acting up. The system is charged full, no leaks. I found when stopped or when in park, the A/c compressor cycle on/off/on/off/on/off etc etc. I suspect the low/high pressure switch is shutting down the compressor due to excessive high pressure. While driving, however, it works fine. I noticed my Aux fans no longer come on. The fuse is not blown. The red switch on the receiver drier is working. When I short the red switch terminals, I can hear a click in the fuse box from a relay. I took the relay apart and it looked fine. Its not a Klima relay but more so a standard issue relay with two big relays inside it. I dont suspect the problem to be there, should I? I know the aux fans work because they were just on today when my temp guage ran hot.

I have no idea where to go from here? Can anyone help me?

mbdoc · #2 07-17-2006, 04:18 PM

Look at the ceramic ballast resistor..the lead-in wire from the relay will MANY times melt & then that is inop...

amg280 · #3 07-17-2006, 04:21 PM

Ok will do. This is the resistor that is in the general area of the receiver drier (drivers side front), correct?

Thanks for your fast response.

amg280 · #4 07-18-2006, 09:02 AM

Quote:

Originally Posted by M.B.DOC

Look at the ceramic ballast resistor..the lead-in wire from the relay will MANY times melt & then that is inop...

I looked at it. It looked fine. Would this wire still be suspect if the fans turn on by the coolant temp sensor? I can pull the sensor wire off on top of the engine (coolant sensor) and the fans kick on high speed. Are there two resistors, one for low speed (AC) and one for high speed (coolant)? If so, are they in the same junction box on the fenderwall? This really makes me mad that it has to wait till the hottest day of the year. Oh well thats murphys law. Any other advice doc? I could really use some.

david s poole · #5 07-18-2006, 10:02 AM

now that you have found the resistor take a test light and with ac on check for power on both sides of it.you will find that either there is no power on one side[replace resistor]or that the wires on one end are cooked[should be enough slack available to repair] good luck david poole owner european performance dallas tx.

chuck95e300d · #6 07-18-2006, 09:43 PM

Quote:

Originally Posted by amg280

Hey guys. My A/C just starting acting up. The system is charged full, no leaks. I found when stopped or when in park, the A/c compressor cycle on/off/on/off/on/off etc etc. I suspect the low/high pressure switch is shutting down the compressor due to excessive high pressure. While driving, however, it works fine. I noticed my Aux fans no longer come on. The fuse is not blown. The red switch on the receiver drier is working. When I short the red switch terminals, I can hear a click in the fuse box from a relay. I took the relay apart and it looked fine. Its not a Klima relay but more so a standard issue relay with two big relays inside it. I dont suspect the problem to be there, should I? I know the aux fans work because they were just on today when my temp guage ran hot.

I have no idea where to go from here? Can anyone help me?

A while back I had a problem where the AC would only blow cold air on the highway. In traffic I'd get warm air.
Did you check the fuse on the relay (can't remember if it is blue or green). To access this you need to remove some screws to get to the second half the fuse box. See post #6 on this thread:
95 E300D high engine temp only with ac on.
Replacing that fuse fixed my A/C for 2+ years.

However recently I had a similar problem to what you are describing with my car. I checked that fuse and it was fine. AC was blowing warm, compressor would engage, but the aux fan would not come on. I changed the ceramic ballast resistor for fun and it didn't help. I took the car into the shop. It was low on "freon." They added some and the air cond. worked again. They added dye but could not find a leak. After that the car was in the shop for a couple of weeks to work on a glow plug issue. I picked up the car tonight. Turned on the AC and everything was fine. Drove for about 10 minutes and AC started blow hot air again! I stopped by the auto parts store and bought a pressure gauge. Seemed to be plenty of "freon" in the system. Drove home and parked it. About an hour later I started the car and turned on the A/C...cold air again and the low temp fans operated as normal. So there is a gremlin in there somewhere.

My car is a 1995 E300D so I suspect the AC systems are the same...

amg280 · #7 07-19-2006, 03:10 PM

Thanks for all the responses guys. Im going to my mechanics later tonight and we are going to get to the bottom of this. From what I gathered, there is one resistor and two fuses (low and high speed fan). Since the high speed fan works, im guessing that the resistor is fine (i hope).

I will let you guys know my findings...

Arthur Dalton · #8 07-19-2006, 03:57 PM

< >>

The resistor has nothing to do with high fan.. it is only used to drop the voltage for low fan... so, a bad resistor results in no low fan, but high fan is fine.

amg280 · #9 07-19-2006, 04:03 PM

Quote:

Originally Posted by Arthur Dalton

< >>

The resistor has nothing to do with high fan.. it is only used to drop the voltage for low fan... so, a bad resistor results in no low fan, but high fan is fine.

That does make perfect sense, since on high speed, the fans are getting the full juice, so no "step-down" mechanism is needed. I will investigate that later. Anyone know how much a new resister is?

Arthur Dalton · #10 07-19-2006, 05:37 PM

http://catalog.eautopartscatalog.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1M30LAGSC1UQ11NWSK&year=1992&make=MB&model=400-E-001&category=G&part=Aux.+Fan+Resistor

amg280 · #11 07-19-2006, 07:27 PM

Well MB Doc was right on the money. The wire going to the resister looked like it caught fire and totally seperated from the resister terminal. Took about an hour to repair the connection (hard to get to). All is working now, and i got my cold A/C back in the city driving

Thanks to all you guys!