My '89 300SEL's air conditioning stopped working. The compressor is not engaging at all. I also noticed that the aux. fan is not running (even at high engine temps). I don't know if the two things are related. In the cabin, the vents are getting airflow, as if the system is trying to work, but they are blowing warm air (doesn't seem to be heated, just ambient air temperature). Fuses all seem to be fine.

I don't have R12 equipment, so I want to rule our electrical trouble before moving on to coolant.

What would be a reasonable list of things to check to rule out (or find) electrical troubles with the A/C system?

Thanks.
-Brett

First and foremost, would be fuses. Check them close and carefully (hairline cracks often go un-noticed at first glance)
Does the aux. fan come one once the engine temp is high enough for it to come one?
Does the compressor come on even momentarily? (at the instant you push the A/C button).
Did the system cool well before it stopped working, or was there some degredation prior?

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It is a truism that almost any sect, cult, or religion will legislate its creed into law if it acquires the political power to do so. Robert A. Heinlein


09 Jetta TDI
1985 300D

Thanks Larry.
I just checked a couple more things.

The aux. fan wouldn't come on at all, regardless of engine temperature. But I just unplugged the temperature sensor and it came right on. So I assume it is a bad sensor. Sound reasonable?

I was incorrect in stating that the compressor does not engage at all. It engages for about 3-5 seconds and then stops. Then it takes a restart of the engine to get the compressor to engage again.
Could this be anything other than low refrigerant levels?

I don't know if it cooled well before. I bought this car in inoperable condition around September last year. It had been donated to charity and sold at an auction (where I bought it) so I don't have any information from the previous owner. I'm just now getting it on the road.
-Brett

I would suspect low freon - just because of its chequered past, and because it does attempt to start compressing, and then gives up (most likely because of low pressure). Your system does use R12 and if I were you, I would stick with it. Before you have somebody add freon, check it for leaks - just in case the previous owner added dye to it - if you have a leak, hopefully it will show.

__________________
It is a truism that almost any sect, cult, or religion will legislate its creed into law if it acquires the political power to do so. Robert A. Heinlein


09 Jetta TDI
1985 300D

Coming on for 3-5 secs, and then needing a restart to do the same again means either the compressor is not turning at the same rpm as the engine, i.e. you have a loose or a slipping belt OR your compressor's speed sensor is faulty OR your engine speed signal is not reaching the Klima relay. The speed sensor signals can be tested. Does your car have a Klima relay similar to what I have on my 87, 260E ? If yes, then check this post, I have posted a step by step method to check the compressor cut off.

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Saumil S. Patel

HI Saumil.
Mine does have a Klima relay (next to the fuel pump relay). The link to your step-by-step guide didn't come through. Would you mind posting it again?
Thanks.
-Brett

I have a short in the circuit supplied by fuse 15. Is it in any way tied to the air conditioning system in an '89 300SEL? The fuse cover lists several things for 15 (seat memory, etc.) but nothing that seemed A/C related.
Thanks.
-Brett

Oops !! I forgot to paste the link, here is it:

http://www.peachparts.com/shopforum/showthread.php?t=158591&page=5

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Saumil S. Patel

I dont think Fuse 15 short would cause these symptoms. Your symptoms are classic symptoms of the compressor cutting out because of speed mismatch. You should be able to check both the signals (compressor as well as engine speed) at the Klima relay socket.

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Saumil S. Patel

Looking through old posts I found that jumping the slots for pins 15 and 87 will bypass the safety features controlled/monitored by the Klima relay. I did this and the compressor clutch engaged and cool/cold air started coming out of the vents.

How do I now determine which of the shutdown mechanisms is my problem?

-Brett

So it is one of the sensors for sure. I think you can jump to steps near 40 or so to check the speed sensors.

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Saumil S. Patel

Thanks Saumil.
I'm reading through the thread (to which you provided the link) right now .
-Brett

Saumil, I started through the excellent instructions you provided, but I didn't have to go very far down the list. I unplugged the low-pressure switch and checked for continuity across the switch (rather than jump the wires). The switch showed perfect continuity. Figuring I needed to go on down the checklist, I removed my meter and reattached the wires to the switch. When I did that, the compressor clutch engaged and stayed that way. (The car was running at the time.) Apparently the problem was just a bad connection at the low-pressure switch.
Thanks for all the help.
-Brett

I just ran the car down the road and back. I got about a mile down the road when the A/C stopped working again.

I pulled over and checked the connections on the low-pressure switch. All looked fine.

I drove back home and performed the same continuity check as before. All looked fine. I reconnected the wires, expecting the compressor clutch to engage, but it did not. It is back to the 3-5 second, only on startup thing again.

This mystifies me.

Also (perhaps related?) the aux. fan will not come on at all now (even with the engine temp. sensor unplugged).

I'm going to wait until tomorrow to look at this again.

Any thoughts on my weird temporary fix?

-Brett

I was just working through Saumil's checklist again. I got down to about #40 (or so) and found slot #4 to have 12v. According to Saumil's list, this indicates a bad throttle cut-off switch. This actually makes sense (in a weird electrical problems sort of way) given the symptoms I experienced yesterday.
The A/C cut stopped working after heavy throttle, then simply did not come back on again (I realized this only in retrospect).

But now, where is that throttle cut-off switch?

-Brett