aux fan no low speed with a functional AC system - Mercedes-Benz Forum
 
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#1 (permalink) Old 07-02-2010, 09:03 AM
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aux fan no low speed with a functional AC system

Still in issue with above problem on my 94 E320. Question is based on attached electrical it would appear that the low speed should work if the AC does and the pressure switch must be working if the compressor engages? Will be looking for the resistor mentioned in the diagram, if anyone knows the location please advise thanks
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#2 (permalink) Old 07-02-2010, 10:49 AM
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Until at least 1990 MY the fan resistor was bolted to the left inner fender apron behind the headlamp assembly. Cracked and non-functional resistors were common on the first generation 124s but I think the design changed to fix that on later model years.

As for your diagnosis, I don't think it's valid. There are at least 2 pressure switches in the a/c system and the one you're looking at is for a/c fan activation only. So just because the compressor runs does not mean that the switch is working.

Also, as I recall, the a/c pressure had to get uncomfortably high to activate the fan. That may have gone away when MBZ went with 2 fans instead of one.

You should put an ohmmeter across the switch leads and see if you get continuity as the engine idles with the a/c compressor running.
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#3 (permalink) Old 07-03-2010, 10:48 AM
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Originally Posted by artsalfa View Post
Still in issue with above problem on my 94 E320. Question is based on attached electrical it would appear that the low speed should work if the AC does and the pressure switch must be working if the compressor engages? Will be looking for the resistor mentioned in the diagram, if anyone knows the location please advise thanks
The low & high speed aux fan relays are in the rear of the fuse box.

Check the fuses located on top of the relays.


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#4 (permalink) Old 07-03-2010, 11:52 AM
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I am currently going through this on my 300TE.

Disconnect the two connectors that go to the green aux fan switch/high pressure switch on the drier. Touch the two spade connectors together and if the fan turns on, then you have low refrigerant

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#5 (permalink) Old 03-07-2011, 07:33 PM
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one Stage one relay later...it runs and RUNS!

With hot weather rapidly approaching, I ordered the relay Saturday when I bought new cabin filter and oiled the blower. I had not been in a CarQuest in a long time but they were the only local shop besides MB that had filters and I pulled the relay and carried it there for a match. Price was same as I could get anywhere online although it was a URO, not OEM Hella.
Strange occurrence when I plugged in the new one, key and car were off but immediately heard a buzzing somewhere down low that did not go away. Car was warm as I had just used it to go PU the part. When I did start it, the aux fan took a minute to start, guess that pressure sensor was loading up but the strange thing was that when I turned the key to off and pulled it; the engine kept running? Couldn't believe it at first, thought the fan was ON and just loud but when I hit the accelerator, engine was definitely still running! Slipped the key back in and slid to the ON position then off , that did the trick. restarted and shut down fine a few seconds later. Hope this isn't the next gremlin starting, the intricacy's/complexity of these cars may be over the edge...makes me nervious, wonder if it would have shifted into gear w the key out?
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#6 (permalink) Old 03-08-2011, 07:45 AM
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If your engine is still running with the key removed, that sounds like a defective ignition switch which would be a insolated issue from your non-op aux fans. Since you have a '94 the AC fans should come (at low speed) shortly after engaging the compressor, although the time factor is affected by ambient temps as well.

As stated if your fans are non-op, replace the resistor behind the driver side headlight first. Also verify that your fans are still working at high speed. Easily tested by pulling the 2 prong sensor connector (blue ring sensor) on top of the thermostat housing.

I'd avoid URO parts whenever possible. The price is rock bottom, but so is the quality.

Last edited by sbaert; 03-08-2011 at 08:46 AM.
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