As I understand it, you are saying that S31 does not change states when the failure occurs, but shortly after the failure occurs the aux fan shuts down. Now the component to check is S25/5, the coolant temperature switch. Check the output "b" line, the brown/violet wire to see if it has changed states. It feeds a signal to the compressor cutout control unit if the engine temperature has exceeded 115 degrees centigrade that shuts off the compressor. If it is ok, your compressor cutout control unit is faulty or your ETR switch is signaling that the evaporator temperature is to cold, which will also turn off the compressor. You can monitor the voltage at S31 to check the ETR switch output. Another, but remote possibility is that the signal from the idle control unit is going bad. The compressor is also cutout at engine speeds above 5750 rpm and turned back on at speeds below 5550 rpm. You can monitor this signal at the compressor cutout control unit, pin 4.

__________________
Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C

Right. S31/1 never changes state from engine off and key out of the ignition all the way through the test and well beyond failure.

S32 changes state immediately at failure (I'll have to test again to see if it switches before or after the compressor switches off), and does not change state back.

If the weather cooperates, I'll test some more this evening - though from the look of the sky, that is not likely.

__________________
Lifetime owner
1977 240D

"new" owner
1988 300TE

So, That takes the aux fan out of the diagnosis..
..also look at the wires at the comp closely..sometimes the speed sensor/coil wires get funky ...
How did you confirm s31 ???
and... Are you still workimg w/R12??

I attached to the leads coming out of S31/1 and S32 (they are adjacent connections) and monitored their continuity during the test.

The system was converted to R134a some time ago, I only wish I were running with R12.

__________________
Lifetime owner
1977 240D

"new" owner
1988 300TE

OK .. the reason I asked was b/c once the comp shuts down , the high side pressure drops and could , in fact , close the high side sw due to the drop in pressure ..one wants to monitor the sw while the comp cuts out to make sure the sw has not reached that cut-out point at the time of occurance..it is possible to have the sw cut the comp and then you go and do a cont test on the sw and find good cont. only b/c the high side has dropped pressure in that time frame and you assume the sw never reached cut-out .
Using 134a with the orig pressure sw can run into such a problem b/c of the higher pressure on 134a ...your condition seems to be right at that high pressure , high heat , same running time , so that all has to be considered .. I agree the problem seems to be in collant temp or ETR cut-outs , but just want to make sure you were aware of the possibility of testing the pressure sw after comp cut-out could fool you..
Your refrig charge has to be decent b/c you do have aux fan trigger w/comp is running and it goes off when high side drops..if s31 were to reach cut-out , overcharge would still be a viable diagnosis..
One more item I would check would be the fuses .. intermittent cut-oot problems can be as simple as a poor connection at the relay feed fuse..you have the schematic, so check out what ones are in that circuit and give them a twist in their holder ..Benz fuses are notorious for making intermittent contact at the bullit ends and a simple twist will cure it..not saying that is your problem, just something that does happen ..

Ok, measured the Coil Resistance today - it was about 8.5 ohms.

Some additional poking around turned up a broken component that I cannot identify (pic follows). It is located by components of the AC system, but I do not know if it is a part of it or not. It appears to be a ceramic coated resistor, but the ceramic coating has broken, and there is evident rust on the inside. In both pics, the bottom of the picture is towards the front of the car. Is this part of the AC system or something else, and how quickly should I replace it?

Attached Thumbnails

   

__________________
Lifetime owner
1977 240D

"new" owner
1988 300TE

The broken thingy is the resistor for the low speed operation of the electric radiator fan. Low speed is triggered by the high pressure switch on the a/c drier - the red or green one. When the high side pressure in the a/c system reaches the setpoint, the electric fan is triggered on low speed. This mostly happens when stopped or driving at low speeds. You can test the low speed operation of the fan by disconnecting the two spade lugs from the red/green switch on the drier and shorting them together. Fan should run, assuming ignition is in the run position. This really oughta be working. Without it the high side pressure stresses the a/c compressor. You also stay more comfortable - more air moving across the condensor means more cooling inside the car.

The coil resistance sounds too high. It would only be drawing ~1.6 amps, which is about half what I recall. I suggest you run the a/c to failure then retest the coil resistance. I wouldn't be surprised if it has gone infinite at that point.

That resistance was at failure. I plan on replacing the resistor (would that be a dealer-specific part, or would I likely be able to pick it up from the numerous auto-parts stores in this area?) today, and doing a logic-circuit test (read the voltage in real-time on as many of the control lines as possible) to determine what is failing when.

__________________
Lifetime owner
1977 240D

"new" owner
1988 300TE

i have been following this thread and trying to develop a convenient way to troubleshoot.
here is my question,has anyone tried to eliminate both the compressor speed signal and the engine speed signal for the purpose of determining if the compressor would run in the auto mode?
has anyone got access to the internal drawings of the kilma relay?
in most cases ac signals are rectified and converted to a pure dc value for the comparator circuits. it sure would be convenient to eliminate this possibility when troubleshooting the system.
by the way some one asked about the signal level at n22 pin 1 from s31/1 and it is high when compressor is off and pulled to ground when on.
also i am working on a 88 300te,86 300e and both have the same problem,at times wont cycle back on when in the auto mode but will run continously in the manual mode.
larry perkins lou ky

Seems like a whole lot of diagnosis work here....has anything been replaced? I would work under the assumption, since the AC does work - all be it sporadically, that it is probably a broken solder connection in the CCU in the dash (the push button unit). My 87 560SL was doing the same thing your car is doing. Compressor would come on and then shut down, the fan would come on and then stop for no reason. It was maddening!
Replaced the CCU with a rebuilt one from Benz Barn our of Georgia for $135.....everything has worked great since - even got the vents to work again (was so afraid it was a heater box thing!).
Even if that part does not solve your problem it will take a big known problem component out of the diagnosis and get you some new shiney buttons!
Good luck!

I have not had any time in the last week or so to do any more diagnostics - though I did take some down time during my Rescue Shift to replace my aux-fan series resistor (which was quite well mangled. If I get a chance, I'll post pics.

The only change I've noticed in the last few days is that I do not even get 15 minutes of cooling now (down from the 20-25 I was getting a few weeks ago) - this tells me I'm probably chasing a freon leak. I've not been able to see any visible dye tracer, but I'll be hunting it with a black-light when I get a chance this week.

__________________
Lifetime owner
1977 240D

"new" owner
1988 300TE

i tell u that fellow is good,got mine to my steam cleaner,washed that sucker real good around the a/c compressor,ran a 1/4 tank of fuel out yesterday and the ac cycled at any position i put it.
i really didnt want to post so fast as i will be driving this week in 94f temp but it sure looks like the signal from the ac speed sensor is getting clipped down to a level that wont operate the klima relay,with the crud becomming more conductive as the engine heats up(shorts across the output wires)
iin reference to my earlier subject on how to troubleshoot the system and bypass the compressor speed sensor,i wanted to be able to keep the relay in place for the elimination process. i know about the output contacts and all that.
ctm i bet a car wash will do the trick also if you dont have a steam cleaner. let us know.
larry perkins lou ky

Let me see if I understand you correctly: You are saying that I should take my engine and get it steam-cleaned, with a specific focus on and around the a/c compressor, and that should fix the problem?

__________________
Lifetime owner
1977 240D

"new" owner
1988 300TE

Considering the information you have given earlier in this thread, what makes you think you have a freon leak? Has the weather been warmer in the last few days? If it has, it would indicate you are getting to a high pressure cutout sooner, which could be the result of an overcharge! The way to tell if you have a low freon charge is to connect a freon gage set and verify pressures. I'm beginning to think you should just take it to a qualified AC tech and get it fixed, as both your reasoning and diagnostic techniques don't seem to be getting you to a fix. Summer may be over before you arrive at the problem.

__________________
Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C

It hasn't been warmer, it has infact been cooler, and yet I've been getting shorter and shorter running time.

I am, at this point, planning on taking it to an AC tech to get it fixed, but I'm waiting for cash-flow from my move and job-change to catch up (gotta love burocratic paperwork). Should be in a couple weeks, assuming nothing else strange comes up (Like having to replace major components of my wife's car because a previous mechanic couldn't do a simple job correctly!). My car is driveable, the A/C is (at the moment) a luxury item that I do not NEED to get back and forth to work with. Once everything that I NEED to have functioning taken care of, I'll start paying for the things I'd LIKE to have. That said, at my own pace, I expect summer will be over before I arrive at the source of the problem. So be it.

__________________
Lifetime owner
1977 240D

"new" owner
1988 300TE