I think these steps should work where there is a seperate Klima socket as my 87, 260E and if the socket pin layout is similar. This one can find out from the parts vendor.

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Saumil S. Patel

That's very thorough!

Isn't there an evaporator temp switch between the PBU and the Klima to cut the compressor if the evaporator ices?

Sixto
87 300D

Evaporator, in-car, heater and outside temp sensor send their input to the push button unit, and then the push button unit sends the signal to the Klima unit accordingly.

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Saumil S. Patel

On the early W-126, this is the situation. The evap temp switch is between the PBU and the compressor relay.

The low pressure cutout switch is between the compressor relay and the compressor on the load side.

The SDL's are as described by Saumil.

I have noticed with my push button set to off. My defrost will come on by itself, blowing hot air(probably from engine compartment). I have no voltage on #5 and fuse LOOKS good.(did not test). Any thoughts on why the defrost is coming on by itself.
A

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1987 300SDL
187,000 miles

The automatic starting of blower motor and/or the opening of the monovalve appear to be a problem with your push-button unit sending an inappropriate signal, most likely a bad push button unit itself. But the problem of blower starting automatically is unrelated to the Klima relay and its control. Blower motor and monovalve for heat are controlled solely by the push-button unit. Klima relay is incharge of operating (more like cutting off in case of any problem because it sits on the same belt) the compressor.

You said, you dont have a voltage on Klima socket terminal 5 even when the car is on, then the Klima relay cannot operate and hence the a/c compressor can never be turned on. The fact that your blower motor is working (even if it is unwanted) tells us that there must be voltage on the fuse, so then the wire from the fuse to the Klima relay's socket terminal 5 is disconnected somewhere.

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Saumil S. Patel

Is the blower motor running when defrost engages on its own? If not, maybe having no power to the CCU sets the defrost flaps and monovalve to open (default position) and you feel warm air only when the car's moving.

Sixto
87 300D

p.s., you haven't answered my fuel line question

Yes, sixto has an excellent point. I assumed that the blower motor is running BUT what if the situation is exactly as sixto described, very much possible. You need to check voltage on the fuse # 7 (any of the two ends of the fuse) first. If there is no voltage there, then the wire from the battery (for diesel) or the ignition switch (for gasoline) to the fuse is broken somewhere. If there is voltage on both sides of the fuse then the wire from fuse to Klima socket terminal 5 is broken somewhere.

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Saumil S. Patel

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.

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