I have recently aquired a 1981 SD. My first MB. My ac isn't working and I have ben trying to get to the bottom of this mystery. I'm looking for a wiring diagram and a componant layout for an 81SD. There are no markings for the relays in the fuse panel. The compressor clutch won't pick up on it's own, the pressure switch is good. There is a temperature regulator under the right side of the dash near the blower motor. there is another module close by. so where is the compressor relay? and where is the cutoff switch? if there is one on these. I picked up the compressor clutch with a jumper to the battery. the temp got to about 53 on the left vent. I can hear damper doors move when i vary the temperature on the temp wheel. I plan on following Mr. Patels troubleshooting step next week when i have a day off.

One of the things i have found is that the pin connectors tarnish a bit with age, and need to be polished with a scotch-brite pad. Found this to be the problem with the tach amp, and the cruise control amp. Also the pins on the ccu and temp regulator were tarnished, causing high resistance.

Also Has anyone tried alldata? Is there information indepth and accurate?
Thanks
Scott

Any 300SDL owners want to test out my CCU? I purchased it as a rebuilt and when I questioned the guy he about had a cow. I will pay for shipping to and from. And if we ever meet, I will buy the first round of beers.....Aya.
A

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1987 300SDL
187,000 miles

Sure........I'll do it. Send me a PM and we'll set it up. The '86 functions perfectly, so, any fault will be the CCU.

I have 350SD its A/C comes on at 1500 to 2000 RPM & it turns off below 1500 RPM but the blower keeps on working.Aux fan works fine when put a jumper.Engine temp is also good. I doubt it could be either Speed Sensor or A/C relay.Can some tell me what speed sensor really does can it be jumped to see if it is wrong.

What's the part number of the Klima relay?

Sixto
87 300D

The speed sensor compares the speed of the compressor to the speed of the engine. Its purpose is to turn off the compressor if it is running slower than the engine, so as to protect the belt.

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2012 E350
2006 Callaway SC560

There are two speed sensors involved in the compressor cut-off function. The compressor and the engine speed sensors. If the Klima relay detects a difference in the two speeds that exceeds 30% or so, then it cuts off the compressor, because it assumes that the compressor is seizing and if it is not cut-off, then the belt will break. So this is the function of the sensors. However, as far as I can remember, this test is only done when the car starts and if the discrepency is noticed, the compressor will be cut-out and then the only way for it to come back on is to turn off the car and turn it on again.

From your description, it looks like it is coming on after going off without shutting the car.

Is your charging system o.k. ? The alternator and battery are working normally ? If you jumper terminals 5 and 7 in the Klima relay socket (the pin diagram is in this thread somewhere), all the cut-off sensors will be bypassed, your compressor will come on as soon as the car starts. In this condition, check if there is still a RPM dependancy, if there is, it is likely to be in the charging system.

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Saumil S. Patel

The speed of the compressor is monitored constantly, not just at start up.

Oddly, mine will slip sometimes (loose belt) and shut down. But if I keep driving for five minutes or so, it will restart with no need to turn off the key.

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2012 E350
2006 Callaway SC560

A/c comes at 1500 to 2000 Rpms & when the Rpm drops below 1500 rpm air still comes from the vents but not cold. Please help me to know what could be wrong.I donot have any Relay Diagram to follow the instructions.

I did not know that the test is performed all the time, and all the time makes sense because the compressor could sieze any time, but once compressor is shutoff does the car not need to be turned off and then back on for it to work again ? Is it possible that your Klima relay has failed in some very desirable way, atleast for some ?

So then the problem may still lie in the speed sensors. You can test the speed sensor outputs at the Klima relay socket. somewhere in this thread I have the steps to test the sensor signals.

Or may be the relay itself, but check that shorting pins 5 and 7 gets rid of the problem.

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Saumil S. Patel

On page 6 of this thread, there is a layout of the Klima's socket.
Page 5 has the steps to test various cut-off mechanisms

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Saumil S. Patel

If it gets warm under 1500RPM, maybe it is simply undercharged.

The question is, IS THE COMPRESSOR STILL RUNNING when the air gets "not cold."

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2012 E350
2006 Callaway SC560

I too was surprised when it came back on without a restart. I've always heard it is not supposed to, but it always comes back after a few minutes. I've had to replace the relay before, and it still does the same. That's on a '90 124.

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2012 E350
2006 Callaway SC560

Brewtoo, so how sure are you that the speed sensors were causing the cut-off ? Because it does not make sense for the cut-off to be reversed without a car shutdown specially when the Klima's decision is based on the assumption that the belt is going to get ruined.

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Saumil S. Patel

Absolutely positive.

Good question, though.

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2012 E350
2006 Callaway SC560