w126 A/C diagnostics - where do I start? - PeachParts Mercedes-Benz Forum

BrettS · #1 07-18-2007, 06:13 PM

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

Larry Delor · #2 07-18-2007, 06:25 PM

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?

BrettS · #3 07-18-2007, 06:51 PM

Quote:

Originally Posted by Larry Delor

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?

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

Larry Delor · #4 07-20-2007, 03:10 PM

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.

saumil · #5 07-20-2007, 03:31 PM

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.

BrettS · #6 07-20-2007, 06:43 PM

Quote:

Originally Posted by saumil

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.

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

BrettS · #7 07-20-2007, 06:46 PM

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

saumil · #8 07-20-2007, 07:13 PM

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

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

saumil · #9 07-20-2007, 07:15 PM

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.

BrettS · #10 07-20-2007, 07:15 PM

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

saumil · #11 07-20-2007, 07:17 PM

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.

saumil · #12 07-21-2007, 02:47 PM

You should get a low resistance between pins 9 and 11. So yes either the sensor is open or you have a broken wire somewhere.

Now since we are finding car based differences, you have to make sure that your compressor has a speed sensor and that the wires of the sensor arrive at the pins 9 and 11. To double check, verify connectivity between sensor connections at the compressor and pins 9 and 11.

This finding fits very well with your symptoms. And if you do find that the compressor speed sensor is broken, I am not sure what you can do about it. You have to call the dealer and see if they can just replace the sensor for you, it is a serviceable part but may be so old that it is not available anymore. Otherwise, will need a new compressor.

BrettS · #13 07-21-2007, 03:59 PM

Quote:

Originally Posted by saumil

You should get a low resistance between pins 9 and 11. So yes either the sensor is open or you have a broken wire somewhere.

Now since we are finding car based differences, you have to make sure that your compressor has a speed sensor and that the wires of the sensor arrive at the pins 9 and 11. To double check, verify connectivity between sensor connections at the compressor and pins 9 and 11.

This finding fits very well with your symptoms. And if you do find that the compressor speed sensor is broken, I am not sure what you can do about it. You have to call the dealer and see if they can just replace the sensor for you, it is a serviceable part but may be so old that it is not available anymore. Otherwise, will need a new compressor.

I've found that the speed sensor is located on the back of the compressor. My usual supplier has them (approx. US$70). It has white and red wires leading from it to a harness on the top-front of the compressor. On top of those is another set of wires - blue (was dirty, might have been green), red and white. The white and red went to slots 9 and 11 (can't recall which was which). But (this seemed odd to me) each (red and white) showed essentially no resistance to its own slot (which is how I determined slot ownership), and only limited resistance to the slot of the other - perhaps that is how it is supposed to work?). I don't know where the blue/green wire goes. There is also a black wire in the mix, which appears to be a ground.

But here is an odd thing. As I was trying to check these for continuity, my results kept changing. Then I plugged the Klima back in, and the A/C worked for a minute or two. Then I checked continuity again (showed fine). I plugged the Klima back in, and the A/C started working and didn't stop this time (several minutes of A/C - what luxury!)

So now I'm thinking that I have a bad/dirty connection or a weak wire down at, or around, that harness on the top-front of the compressor. Testing there (nudging wires and such) seems to change my results and the run/not run condition of the compressor. Either that, or perhaps an intermittent problem in the sensor (belt tension seems fine, so I'm ruling out that the sensor is actually doing its job properly).

I'm going to go out now and clean the contacts, make sure the connections between the wires and the harness look good, etc., and see if that fixes the problem.

-Brett

BrettS · #14 07-21-2007, 04:04 PM

Quote:

Originally Posted by saumil

You should get a low resistance between pins 9 and 11.

I forgot to mention, that now I am getting low resistance between 9 and 11 (was infinite when the compressor did not engage). It would seem to me that these new (low resistance) results appear to be consistent with the red/white resistance measurements I mentioned above.

-Brett

BrettS · #15 07-21-2007, 04:38 PM

I cleaned the contacts and all around the harness. I found there was a blue wire on the bottom too. The connections looked sound, so I limited my actions to cleaning.

It seems to have worked. I gave it a test drive a couple miles away and back, and I was actually getting cold with the A/C on full.

-Brett