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  #1  
Old 08-16-2005, 07:18 AM
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'92 500E A/C problem

(This is cross-posted in Hot Rods)

All,

My 500E A/C is inoperable. Since the compressor was not engaging, I went to my local garage today to sort it out. First we topped off the R134a; very shortly after beginning, the compressor engaged but the electric fans were still not turning. And the air does not blow cold.

Ideas?

Thanks in advance

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  #2  
Old 08-16-2005, 08:29 AM
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Hopefully your system has been converted to R134a - in 1992 they were still using freon in MB air conditioners.

Is the auxiliary fan resistor shot? This is what allows low-speed fan operation when the A/C is engaged. It sits behind the driver's side headlight in the engine bay, down low. I had to replace mine in the E420. about 20 bucks and 1-2 hours to do (I hope your fix is this simple and cheap, they rarely are). Could be more thn 2 hours in the even more cramped 500e engine bay, depending on your skill and tool collection.

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1995 E420 97k
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  #3  
Old 08-16-2005, 09:14 AM
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I'd bet the fuse on the low speed aux fan relay (stage I) is blown. Its a 15A fuse but I believe MB Doc or Arthur Dalton were recommending replacement with a 20A fuse per updated MB specifications. The relay is located just behind the fuse box (toward the windsheild) you'll have to remove about 6 phillips screws to get the cover off and the relay is blue in color.

About the A/C...what are the low/high side pressures, ambient temp, and vent temperature?
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Old 08-16-2005, 09:52 AM
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Good call skier! You will need to remove the cover assembly from your fuse box so you can expose the relays located behind the fuse panel. 6 phillips head screws, 2 are located under the fuse box lid. A 20A fuse will withstand the 16A to 18A surge when the fans begin to operate, once their spinning the current usually drops below 12A (good fans/circuit).
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  #5  
Old 08-16-2005, 01:14 PM
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Quote:
Originally Posted by y2kimmel
Hopefully your system has been converted to R134a - in 1992 they were still using freon in MB air conditioners.
1992 was the first year for r134a in most W124's: 500E's, 400E's, and the diesel models.

300E's (300E 3.2 and 300E 2.8) got it in 1993.
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  #6  
Old 08-16-2005, 11:11 PM
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Update

Well, I checked for continuity across the ceramic resistor behind the headlight: seems fine.

Pulled the relay cover off, pulled each module & re-installed them. None had a visible fuse.

Fired the car up, and neither the fans operated nor did the compressor engage as it had yesterday. Further thoughts?

Thanks again
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2014 E63S Estate
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  #7  
Old 08-17-2005, 09:43 AM
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after looking on the fast lane it appears that the 500e has one auxillary fan relay (the 300e/e320 has one relay for each stage).

SO...I'd start at the fans...do they run at all (ever?)

If not..check to see if they work by unplugging their connector and apply 12V to them.

Now, you said that your compressor isn't engaging again. Is it possible that your A/C system has a leak? Was it checked for leaks when you added more refidgerant?
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  #8  
Old 08-17-2005, 10:47 AM
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Good Morning Michael

On the AC problem double check the harness connections at the compressor!

I believe if you disconnect the blue two pole switch located at the forward part of the intake manifold (beneath removable black cover) your aux fans should engage at high speed. If you bridge (connect the two wires together) the red/green switch located on the receiver/drier the fans should engage at low speed. On the 124.034/036 models the fan relay is a double size relay (located in fuse panel assembly) and it is absent of fuses (I am viewing the diagram at this moment). There is a fuse located outside the main box. Fuse 16 powers up the fan relay, the external fuse powers up the fans through the relay. The relay utilizes a ground signal via the pressure switch (when closed) at socket 4 of the relay harness connector and a ground signal from the AC pushbuttons to socket 2 of the relay harness connector when conditions are met.

The relay harness connector will have two 6-pole connector assemblies (x1 and x2), some of the sockets may not have electrical connectors. The relay will have the following markings on the plastic housing surrounding the pins:

87b-output for Fan High

87a-output Fan Low

15(85)-input from fuse 7

30-input from external fuse

Relay basiscs:

85-power for relay/solenoid

86-ground for relay/solenoid

87(etc)-output to device

30-power for device

Hopefully this information will help you resolve your fan problem, if not let us know.

Last edited by MrCjames; 08-17-2005 at 10:53 AM.
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  #9  
Old 08-17-2005, 05:43 PM
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Update

Okay, I pulled the temp switch plug: no fans (with A/C on, engine running; no compressor engagement).

Pulled the wires near the rec'r/drier-when bridged, a/c compressor engaged but no fans.

Add'l thoughts?

Oh there was dye in the refridgerant, but no readily apparent leaks
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2014 E63S Estate
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1995 E500
1986 Porsche 944 turbo
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  #10  
Old 08-17-2005, 06:23 PM
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You can bridge the low pressure switch on the rcvr/drier (blue/red wires), if that is the switch you bridged and the comp engaged than you have low refrigerant or a faulty switch. (AC comp function)

Use the information in my previous post, it will guide you through the sockets located under the double size relay which is in the rear section of your fuse panel assembly. When you jump either one of the 87 circuits to a battery source than the fans should operate at their respective speed. When you jump the other 87 circuit to the battery then the fans should operate on the opposite speed. PLEASE , use a FUSED lead when bridging your fans to a direct power source.

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