I do still have the aux fan. It (appears to anyway) operate occasionally, especially when the engine is hot, and in hot ambient conditions. I have not yet checked voltage switching on the existing pressure switch, that is a task for this evening.

__________________
Lifetime owner
1977 240D

"new" owner
1988 300TE

My 87 had no low speed fan due to burned out resistor. Made a huge difference in both AC cooling and engine temp. I don't want to ever run under conditions that trigger the high speed function of the fan

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Hanno
'79 6.9 Sold (after 27 years)
'83 280SL, 5 spd.
'94 E320 Sdn. 5 spd conversion
'02 E320 Sdn.(on loan to mom!)
'87 300E (5 spd. conversion) Sold
'05 E500 Wagon

Exactly...and that is what will happen if the low fan circuit is not operating correctly..
Common, misdiagnosed problem...alway check low fan circuit on a/c and engine over-heat problems ..specially when it take 2 seconds to verify the low fan circuit with a simple paper clip.

Have you tried jumping out the high pressure cutoff switch yet?? Have you ohmed out the clutch coil when the compressor fails yet?? You already know that it fails, why don't you check these items?? As Arthur says, jumping out the high pressure switch only takes a few seconds, and ohming the coil only takes a few more...

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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

As Ricrard states, all/any of his conditions can cause your problem.. the main reason I am leaning in the aux fan / overcharge possible is b/c you do not seem to have the problem until you loose airflow from a non-moving vehicle/traffric/idle condition .. that points me towards conderser airflow vs. blockage drier/lines/etc. .your comp. cut-out seems to be the RESULTS of high pressure run away due to lack of airflow...but, the test wll tell the tale.
If the fan circuit verification test passes, then you may also want to look for debries between the cond and radiator...but fan jumper sw test is still top of my list..you mention seeing fan at hot engine, but I think you are mis-interputing that as FAN OK, not realizing that coolant high fan has NOTHING to do with a/c LOW FAN operation..two completely different circuits that just share the same fan motor. LOW FAN jumper test must be done to verify the electrical intergrity of that circuit. If no low fan with that test, you could have a problem as simple as a fuse or burnt wire at the resistor.

Ahh, but that requires the multimeter which I have not had unpacked before today. That is in fact my plan for this evening.

__________________
Lifetime owner
1977 240D

"new" owner
1988 300TE

Ok
Just be aware that the aux fan can be triggered by both engine coolant temp [ high fan] and the high pressure sw at the drier [ low fan] . There are 2 switches at the drier .The one I posted for you is a dual cut-out sw [ as you can see in the part discription] . That cuts out if the refrigerant charge is too low so that the comp will not come on w/o the system having a charge, but it also cuts-out if the high pressure gets too high ..so, this is one of the saftey switches to protect the compressor..the other comp cut-out one is a coolant temp sw that cuts the comp out if the engine coolant gets too high, and then there is the speed one in case the belt/clutch slips .
There is another sw at the drier [ the one on the side with the pigtail wire harness] that controls the low fan. If you jumper that switch w/key ON, you should see low fan.
This sw has nothing to do with the comp circuit, it only triggers the fan, BUT, if that fan does not come on when the pressures start to get high, then the sw on top of the drier will reach cut-in pressure due to no airflow from fan, resulting in a comp cut-out. So, you can see that the aux fan is needed to keep the high side pressure in check so it never reachedes the comp cut-out spec of the top sw.
Best to use gauges to see what the high side pressures actually are , but if the comp cuts-out and you see that the reason for it is b/c of the sw I posted has opened , you then know you have too much high side pressure..from an over-charge or a NO FAN condition
Just two easy things to check that would fit your condition/diagnosis.
Verify fan circuit w/jumper test..and check high pressure comp cut-out switch w/ohm meter/ continuity test when condition exist.

Ok, didn't get anywhere NEAR as much checked out as I wanted to before the severe thunderstorms rolled in last evening, but I did check the pressue switch (S31/1 - Pressue Switch, refrigerant compressor according to the wiring diagram, but it is the component that was linked above) - which is showing a complete circuit both in operation and at failure (no change between states). However, S32 (Pressue Switch, Auxillary fan) changed from a complete circuit at operation to open circuit at fialure. This is NOT the picture linked above, but the wiring diagram says it is the one that runs the Aux Fan. The Aux fan ceased operating at that point as well (having previously run the entire time the engine had been running).

I did not get a chance to measure coil resistance (or even locate where to do that) before the thunderstorm came up. Maybe today...

__________________
Lifetime owner
1977 240D

"new" owner
1988 300TE

It's normal for the electric fan to turn off shortly after the compressor stops. The fan switch is triggered by high side pressure. No compressor, no high side pressure. The fan should not shut off at exactly the same moment the compressor stops, a period of time (few seconds to a few minutes, depending on about a million factors) is required for the high side pressure to drift down.

Easiest location to check the clutch coil resistance is by pulling the Klima relay. I think you want to check continuity & resistance between pin 7 & ground - but double check that pin number on your wiring diagram.

The coil should show about 5 ohms resistance.

- JimY

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, 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.