Intermittent A/C Clutch - A Relay, perhaps??? - PeachParts Mercedes-Benz Forum

speace · #1 04-08-2008, 06:59 PM

1992 129 500SL

A few weeks ago I began noticing that my A/C clutch was not engaging after starting the car. A quick restart would always restore operation. The problem has progressed and it now doesn't work more than it works...

I checked the A/C Compressor Clutch fuse.
The belt tensioner is good and there is no slippage.

What is left? The Climate control unit, some wiring and perhaps a relay or two?

Does this car have a Klima Relay?

I know my 92 400E had trouble with the wiring to a resistor related to a cooling fan that prevented the A/C clutch from engaging. Does any of that apply to this car?

mbdoc · #2 04-09-2008, 08:04 AM

The "MAS" relay engages the a/c compressor on that car.

Could be several problems that cause the compressor NOT to come on.

The a/c controller (pushbutton) sends ground signal to that relay thru the low pressure switch on the drier. Checking for that ground signal would be MY starting point.

speace · #3 04-09-2008, 08:13 AM

Quote:

Originally Posted by M.B.DOC

The "MAS" relay engages the a/c compressor on that car.

Could be several problems that cause the compressor NOT to come on.

The a/c controller (pushbutton) sends ground signal to that relay thru the low pressure switch on the drier. Checking for that ground signal would be MY starting point.

Thanks!

I see two switches at the drier. One appears to be a traditional A/C low-pressure switch (EDIT: S31) and is ON the drier. The other is green plastic and I think was on a fitting right AT the drier. Would the green one be a temperature switch? (EDIT: The Green one is a pressure switch [S32] that turns on the AUX FAN)

I will find a way to test at the pressure switch. It is good to know I am looking for a ground there from the Climate Controller.

Where is the MAS relay? I may want to examine it for bad solder joints.

speace · #4 04-09-2008, 04:21 PM

Hmm... I have 12v on the pins to the pressure switch on the drier. If I ground it there, the compressor and fans will engage.

If this connection's path to ground is directly from the pressure switch on the drier through the Climate Control Unit in the dash, then my CCU has gone bad on me AGAIN!

Is there any chance that there is anything else in the path that could be the problem? If not, then I guess I'd better R&R the CCU...

mbdoc · #5 04-09-2008, 05:07 PM

Before you go too far you need to look into the pushbutton for live data on its sensor inputs...that is done thru the pushbutton. Send me a e-mail & I will give you the instructions.

speace · #6 04-22-2008, 04:40 PM

Thanks for the information M.B. DOC.

Well, I still don't have a clear "fix."

I noticed that when I start the car I can hear the A/C Clutch and cooling fans kick in for about two seconds before dropping out.

I can ground the signal at the pressure switch on the drier and make it engage and the car will cool nicely.

Since the signal to the drier comes from the CCU in the dashboard, the problem would appear to be in the CCU. In examining the readings using the CCU tests supplied by M.B. Doc, there were no unusual readings that I could see.

I went to my shop and shared the symptoms and CCU readings with them.

Upon consulting the wiring diagrams we saw that the MAS controller sends some signals to the CCU that could possibly cause the CCU to de-select the A/C compressor engagement signal sent to the drier switch.

It was recommended that I use my LED reader to pull the codes from the MAS Controller to see if there was anything unusual there.

Upon reading the codes I found the following:

Code 11: A/C compressor engagement signal missing (87Z)
Code 13: Excessive A/C compressor clutch slippage

Once I cleared those codes it started working just fine! It COULD well be a coincidence that it started working after I cleared the last code from the MAS controller because there have previously been times that I could not make it fail while I was troubleshooting the problem...

Presuming the problem will soon return, I will re-read the MAS codes at that time...

speace · #7 04-23-2008, 11:27 PM

Indeed the problem returned. There are no error codes stored in the MAS Controller. I am convinced the CCU is the problem.

RConrad · #8 04-24-2008, 01:11 PM

I have a simular problem on a 97 SL 320 in reverse, I can't get the compressor clutch to release. What is the CCU and where is it located??
Thanks for any reply.. Ron

speace · #9 04-24-2008, 01:57 PM

RConrad,

The CCU is the Climate Control Unit in the dashboard with the control buttons that operate the A/C.

Your 97 has a different CCU than my 92, so advice I may give you may not be exactly right. The best thing to do is to start a new thread with your specifics and we can all chip in from there! I think there are some diagnostics you can engage by pressing buttons on your CCU that may give a clue.

It appears to me that the CCU style was changed for the 97 year model, (beginning 09/1995), but using my knowledge of the 92, you should be able to unplug one of the wires (or both) at the pressure switch on the top of the A/C Drier canister right behind the LF headlamp and disable the compressor.

If the compressor disengages by doing that, the CCU may have been keeping it engaged.
If it doesn't disengage, I would look towards the MAS controller.

On my 92, the signal continues from the pressure switch to the MAS Controller which is a glorified Solid-State Relay that, among other things, sends the voltage to the clutch on the compressor. I don't know it's location, but as I think about it, from my experience servicing other electronics, Solid State Relays usually go bad in the always-conducting state...

The only other thing that comes to mind is that the clutch could be mechanically frozen. That doesn't sound likely, though. You should be able to follow the wire from the clutch to a connector that you can unplug that would prevent the clutch from electronically engaging as a test.

My bet would be on your MAS Controller, but I don't see a listing for MAS diagnostics for the 97 model so I must presume that function has been moved to another device...

I hope my rambling has given you some ideas, to try, but again, it would be best that you start a new thread on this so as not to confuse the readers of this thread.

SPeace

speace · #10 05-01-2008, 04:41 PM

UPDATE:

Well, the problem is still unresolved and my level of frustration is rising...

Being confident that the CCU was the culprit, I sent it to ProgRama to be repaired. They returned it with "No problem found."

When I first start the car, the A/C will engage for 2 or 3 seconds and then it is over... Turning the A/C off and on at the CCU does not generate another 2 seconds if operations. Restarting the car will cause the compressor to work each time for 2 or 3 seconds...

The codes 11 & 13 (mentioned above) were successfully cleared from the MAS controller several days ago. Today I checked the MAS codes and the 11 code (A/C Compressor engagement signal missing (87Z)) has returned. I cleared it and it did not immediately return.

Occasionally, while driving, the A/C will engage and start cooling. When this happens, it usually continues to function for the remainder of the trip.

So, what do I do now? Replace the MAS Controller?
Where is the thing? Perhaps I will inspect it for cold solder joints.

deanyel · #11 05-01-2008, 06:01 PM

Sorry for not reading the whole thread but it sounds to me like your code 11 is your definitive clue here. Have you checked the AC speed sensor on the back of the compressor? They do fail and sometimes they just have bad connections. If the compressor RPM signal isn't going out it can't be compared to the TD signal and the signal to engage the compressor will be blocked.

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=1W50YN0EQ2CY12105R&year=1992&make=MB&model=500-SL-001&category=R&part=A%2FC+Speed+Sensor

speace · #12 05-01-2008, 06:14 PM

Deanyel,

I agree with you that it seems that the code 11 is a likely clue, but if the speed sensor were not sensing the engagement of the clutch, wouldn't it render code 13? (Excessive A/C compressor clutch slippage)

How difficult is it to replace the speed sensor? It is probably the original device, and the car is getting up in years...

I want to be careful not to just replace things because I suspect them. I can see many, many $$$ flowing out the window as I pursue this problem.

deanyel · #13 05-01-2008, 06:33 PM

Not sure, but it's possible that the clutch slippage fault requires getting both signals and finding a disparity between the two, as opposed to just not getting one of the signals and being unable to do the comparison. Not specifically familiar with your car but shouldn't be hard to replace the speed signal sensor, normally doesn't require removal of the compressor, but the first step would be to have a look at it, to see if the connections look good and wires not bare etc. One of things that I've done, and I'm sure this in primitive and unelegant, is to run a 12v wire off the fuse box (proper amps) to the compressor power line to see if the clutch engages, stays engaged and cools. If so you know you don't have a compressor problem, clutch engagement problem, clutch gap/shim problem etc. This confirms that you have an electrical problem - that the signal is simply not getting through the morass of tests in the system.

speace · #14 05-01-2008, 06:40 PM

Good points...

I can ground the signal at either side of the pressure switch at the drier and reliably engage the clutch, resulting in good cooling...

I'm leaning towards the MAS module or the speed sensor at this time.

I'm hoping someone that has seen this several times will jump in here and rescue me before I spend needless money on parts! LOL!

speace · #15 05-28-2008, 08:36 AM

Well, back to the A/C clutch sensor...
Problem resolved!

deanyel hit it again with the wiring! I measured the resistance between pins 5 & 6 of the MAS controller socket and found that it measured a dead short rather than the proper 600 ohms. When I accessed the plug on the compressor to test the wires back to the MAS controller, I noticed that the wire from the plug that reaches to the sensor on the back of the compressor was very rigid. Then it hit me! The wire had "petrified" and the insulation had broken down. There were bare wires showing at the plug. The harness that goes back to the MAS controller seems to be fine!

I opted to replace the compressor and receiver/drier since the sensor would have cost half of a rebuilt compressor, and the labor would be about the same. As far as I know, at nearly 200k miles, the car still had the original compressor and a refresh might be due any time!

I'm glad that's over!