I think 5 & 7 are the socket numbers where the Klima relay plugs in, 15 & 87 are the pin numbers on the Klima relay itself. If memory serves (usually doesn't in my case...) socket 5 is +12v and socket 7 is the compressor coil.

Got a volt/ohm meter? Check the continuity through the compressor coil to ground, both when cold and the a/c is working, and also when hot and it stops working. I'm starting to think the coil is going open when hot.

Of course, these symptoms also fit an excess compressor clutch gap, which you haven't checked yet.

I don't think it's the compressor-engine speed comparator circuit because the compressor never comes back on after a restart. If it cuts out once it'll do it again, but frequently runs for a minute or two first.

To be thorough I'd also check to see if the CC head unit is calling for compressor operation when it cuts out. I forget which pin this is on the Klima socket; it's on the wiring diagram. Should see +12v on this pin when a/c is called. Easy to test by switching between a/c and econ.

- JimY

Pin #10 (on the KLIMA socket) is the activation signal from the PBU, I can't from the diagram if its a +12V signal or a switched ground.

I know the ping 10 activation signal is ground on a diesel 124 - BTDT. I've been told it's +12v on a gas 124. The two cars use different part numbers for the CC head unit, so I tend to believe this information.

- JimY

Ok, another round of testing/feedback results:

Ambient Air Temp 66°F
Ambient Humidity: 88%

AC runs as normal, bringing vent temps down to about 45°F and cabin humidity down to about 25% before cutting out at about 20 minutes. Stop and go traffic (more stop than go).

Ambient Air Temp: 85°F
Ambient Humidity: 36%

AC runs not quite as cool, vent temps only reach down to about 54°F before cutting out. Humidity stays between 30% and 40% until system failure at 17 minutes. Mostly stopped traffic. AC cut out just as I got off the surface roads and onto the high speed road.

Ambient Air Temp: 65°F
Ambient Humidity: 45%

AC runs not quite as cool at first, but gets well and truly cold (43°F) after about 15 minutes. Humidity never gets below 40%, and the AC runs until the 36 minute mark, then cuts out. Stop and go traffic. This is the first time since I've been doing detailed monitoring that it has run for more than 20 minutes and still cut out (I have had it run without failure for an hour or more, the only reason it stopped was I turned off the engine when I reached my destination).

I have unburried my multimeter, and will be running checks on all the relay connections this evening.

__________________
Lifetime owner
1977 240D

"new" owner
1988 300TE

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=23I0RAMEE23W0MC6KZ&year=1988&make=MB&model=300-E-002&category=R&part=A%2FC+Pressure+Switch

Is the aux fan coming on low speed??
IF NO , check this part...you may have too much high side pressure from no fan, cutting out compressor.
Or, if this sw is cutting the comp out, you could also have an over-charge.
You should have aux fan operation with either condition.
Do you have fan?

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

__________________
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

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.

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...

__________________
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

The failure symptoms can be quite deceiving on some of these A/C problems - I had a problem on my '82 380SL where the thing would blow warm to hot air even with the AC set to max cool (knob at cold end detent), but once the car was moving it would cool down pretty well. The problem turned out to be that the auxiliary water pump wasn't working! The auxiliary water pump is necessary in order for the monovalve to operate correctly, ie, it works on water pressure, like the solenoid valve in your washing machine works... a small primary valve is operated by the solenoid, which moves the larger diaphram using the water pressure... in my case, I eliminated both the auxiliary water pump and the monovalve and replaced them with a vacuum operated water valve operated by an electrically operated vacuum solenoid, similar to the other solenoids in the system that operate the flap valves. Now it works great all the time- PLUS, the car runs much cooler, as it isn't trying to both heat air and cool air at the same time.
I'm sure you will make progress once you perform the two simple tests...

__________________
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

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