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  #1  
Old 08-10-2012, 08:46 PM
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Angry 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!!!!!
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  #2  
Old 08-10-2012, 09:05 PM
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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 - Anybody have this part? 003 545 6905

Sixto
87 300D^2
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  #3  
Old 08-10-2012, 09:27 PM
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That is a very good thought sending engine rpm to both. Can I do that with a jumper wire in the klima relay?
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  #4  
Old 08-10-2012, 09:43 PM
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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
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  #5  
Old 08-10-2012, 11:35 PM
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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.
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  #6  
Old 08-11-2012, 12:46 AM
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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
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  #7  
Old 08-11-2012, 01:15 AM
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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
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  #8  
Old 08-11-2012, 01:28 AM
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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
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  #9  
Old 08-11-2012, 02:26 AM
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Here's some reading on the 3-pole temp switch - Coolant temp sensor 1987 300D

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
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  #10  
Old 08-12-2012, 08:26 PM
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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.
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  #11  
Old 08-15-2012, 11:22 PM
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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.
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  #12  
Old 08-15-2012, 11:55 PM
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I check that AC voltage changes proportionately with rpm.

Sixto
87 300D
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  #13  
Old 08-28-2012, 09:54 PM
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Talking

Looks like a/c is still working properly. Thank you for your help Sixto.
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  #14  
Old 03-08-2016, 04:03 PM
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Quote:
Originally Posted by sixto View Post
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
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.
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  #15  
Old 03-08-2016, 04:04 PM
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New switch on order!
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