1987 300sdl a/c compressor not engaging
 

My wifes 300sdl has had an intermittent problem with the compressor not engaging. It would be working fine until say stopping for fuel and then would not work upon restarting. Finally it stopped working at all and I think it is a good thing. It has allowed me to find the problem but not the cure. I tested the limit switches and they are good. I can unplug the klima relay and jump it to force the compressor to run (250psi on the high side and 40psi on the low side). I then ohmed out the speed sensor on the compressor at 880ohms OUCH!!!!! Just a bit out of spec. I am looking for information to by-pass the speed sensor. PLEASE HELP!!!!!

I don't know how to bypass the compressor rpm sensor other than possibly sending the engine rpm sensor as both inputs. That assumes engine and compressor turn at the same rpm.

Until you sort it out, you might bypass the Klima relay using the notes in this thread - http://www.peachparts.com/shopforum/diesel-discussion/95748-anybody-have-part-003-545-6905-a-2.html

Sixto
87 300D^2

That is a very good thought sending engine rpm to both. Can I do that with a jumper wire in the klima relay?

I meant that conceptually. You might fry the Klima circuit if you don't isolate the rpm inputs. If a new Klima relay is necessary, I believe from the 1990 model year Klima relays no longer have the delay in engaging the compressor after starting the engine.

Sixto
87 300D^2

There has to be a way to trick the klima. Would the compressor run if the klima recieved a signal that the engine rpm and the compressor rpm were zero? If so would the tachometer still function? There has to be a better way than a 400 dollar compressor.

I was looking at another thread with the same problem. They said to unplug the single pole on the thermostat sending unit. I popped it loose and compressor engaged right away. I have no idea why but I am glad to have some cold air. A couple of unanswered questions still...

Does this mean the sensor is bad? How to test?

Should the auxiliary fan turn on when compressor is running? mine doesn't

The 3-pole switch serves 2 functions - to turn on the fans at high speed at one setpoint and cut the AC compressor at a higher setpoint. If the coolant isnt over 120*C, you've already determined the temp switch to be bad. Did the temp switch get wet recently? They don't like to get wet.

The aux fan low speed is triggered by the temp switch on the dryer. Disconnect and jump the connectors on pigtails to check the low speed function. Disconnect and jump the 2-pin connector on the temp switch to test the high speed function. The fan doesn't necessarily come on because the compressor is engaged.

Sixto
87 300D^2

ahhh haaa the part in question is a fan switch. Duhh

Will a bad fan switch cause the compressor not to run?

Sorry for all the questions and post. I have only had this car for a year or so and it is our first Benz. Alot to learn still I am sure.


There are not any stupid questions only stupid people that ask questions :rolleyes:

Here's some reading on the 3-pole temp switch - http://www.peachparts.com/shopforum/2986381-post2.html

There's a low speed fan switch in the refrigerant circuit and a high speed fan switch in the coolant circuit. It's difficult to test the low speed fan switch since it's impractical to measure refrigerant temperature - I don't know if an IR temperature reading of the exterior of the low speed switch is representative of refrigerant temperature, and the switch is exposed to the refrigerant system - you can't replace it without discharging the refrigerant.

Sixto
87 300D^2

Temp snd pressure switch test good. I have ordered a replacement fan switch. When installed I will post if my speed sensor on the compressor ohms out normal.

Installed new aux fan switch and compressor engages correctly. Ohmed compressor speed sensor at 668. I thought it would have been much lower. Is it tested with engine off? That is how I have been checking it.

I check that AC voltage changes proportionately with rpm.

Sixto
87 300D

Looks like a/c is still working properly. Thank you for your help Sixto.

LOL I just washed the engine in my SDL and the aux fan is stuck on high. I unplug the switch and it stops. And my A/C clutch won't turn on.

New switch on order!

I had an '87 300D that used to turn on the fan when I washed the engine, but it would dry out and work fine later.

New switch installed, aux fan is now acting normal.
BUT- still can't get the A/C compressor clutch to turn on. I jumped the low pressure switch at the dryer to see if I can fool it to turn on just to make sure electrically the thing works -- nothing. Fuses are are good, including the long strip one by the inner fire wall.
Any other ideas???

FYI my tach works good and I already tried a klima relay too.

What do you mean by tried the Klina relay? Look for a Klima relay pin diagram then send power directly to the compressor. For kicks, replace the Klima relay with a generic relay just to test.

Sixto
83 300SD

I already have an extra klima relay and replaced it, no change.
I jumped pin 5 & 7 in the klima socket- the compressor clutch comes on and the compressor turns freely.
So now where do I go??
And thanks for the replies!

Then one of the Klima inputs is flaky. Do you get an AC (as in alternating current) signal from the compressor rpm sender? Fit an ice cube relay until you figure it out... or don't :)

Sixto
83 300SD

Thanks for the clues!
I hope I can figure it out and fix it without having to add the ice cube relay and switch - trying to keep it original.

Well I have spent some time around the good old " 12-pole connector" for the klima realy.
I think I messed up a test procedure- in the repair manual it mentions setting a meter to (V ~ )
Is this a symbol for AC and not DC current?

So I tried 2 "know good" klima relays and 2 push buttons.
Then did every voltage, alternating current, ohoms and RPM tests in the book.
Did some deeper digging, removed the connector for the evaporator sensor. I was not sure what way the 2 pin thing worked so if its bad I just unplugged it. Nope, still no A/C clutch. I jumped it, still no good.
Now I'm getting real worried that something rotten may be going on with the EDS - electronic diesel computer.
I take a shot and buy a new Klima relay... It WORKS!