a/c compressor not kicking in - PeachParts Mercedes-Benz Forum

mdisav · #1 01-08-2007, 08:45 PM

I have an 87 300TD and blower works as do the proper climate control setting but the compressor is not kicking in.

So far I have replaced the OVP and Klima and checked the strip fuse along with all the other fuses which are good. I have also replaced the climate control unit itself which what I believe to be a good one.

Any thoughts on what else it may be? I do not have 12 volts at the pressure switch. So when I bypass the pressure switch the compressor still does not kick on.

Knightrider966 · #2 01-08-2007, 08:47 PM

My first question would be.... "Are you sure there is refrigerant in the system?" The compressor should turn on unless the electromagnet is burned out. Also, can you turn the compressor clutch by hand fairly easily?

Hit Man X · #3 01-08-2007, 08:50 PM

Bad alternator will hurt you. So will a Speed Sensor. CCU could be bad also.

I have a thread on here "SDL + A/C = No Go" or something to that tune, there is lots of info there.

babymog · #4 01-08-2007, 09:43 PM

The '87 should have a low pressure cutout, and a high pressure cutout. Both switches are on the filter/dryer.

mdisav · #5 01-08-2007, 09:46 PM

The tach is not working up to par. It seems to register half the value. Maybe the speed sensor is bad.

Am I right in saying one wire to the pressure switch should have ground. I was thinking it should have 12 volts but I think that was wrong.

So I do have ground at one wire at the pressure switch.

What should the three wires that plug to the compressor register? One 12v, one ground and one????

Shorebilly · #6 01-09-2007, 07:24 AM

Quote:

Originally Posted by mdisav

The tach is not working up to par. It seems to register half the value. Maybe the speed sensor is bad.

Am I right in saying one wire to the pressure switch should have ground. I was thinking it should have 12 volts but I think that was wrong.

So I do have ground at one wire at the pressure switch.

What should the three wires that plug to the compressor register? One 12v, one ground and one????

There are 2 switches on the receiver/drier......the one on top is the pressure switch, and it must have power....because the power passes thru that switch to the AC clutch....safety thing.....(I have my AC clutch disabled by removing one of the wires here)

The other a little lower down is a temperature switch that operates the auxiliary fan.....

The power to the pressure switch/AC compressor clutch.....comes from a relay/contactor just in front of the power brake diagphragm.....

The relay/contactor receives it's electrical input from the temperature wheel......

This is how the '85 300D AC works.....I didn't notice that you had an '87 until after I had typed and posted this....

SB

dannym · #7 01-09-2007, 08:55 AM

It's probably the electrical connector at the compressor.
Try jumping 12v directly to the compressor.

mdisav · #8 01-09-2007, 10:12 AM

It has three wires going to it. I am not sure which terminals to jump? If it had two I am sure it would not matter if power and ground were switched.

Shorebilly · #9 01-09-2007, 03:32 PM

Quote:

Originally Posted by mdisav

Any thoughts on what else it may be? I do not have 12 volts at the pressure switch. So when I bypass the pressure switch the compressor still does not kick on.

I don't have a Service Manual for heating and AC in a W124.....but I looked at a parts list......they probably work somewhat similarly......if you don't have 12V at the pressure switch on the Receiver/Drier......your AC clutch will not engage......

The purpose if that pressure switch is to cut out the compressor if the refrigerant discharge head pressure gets too high.......

My car has a rather large relay under a cover just in front of the power brake booster.....under that cover is a black plastic relay with "Siemens Klima" printed on it.....the power that goes thru the HP cutout switch to the AC compressor clutch comes from the relay....when it is energized by the Klima Control/temperature wheel....

SB

babymog · #10 01-09-2007, 05:32 PM

I believe that the pressure switch will cut out with the pressure too high or too low. Check for 12v at the switch, if 12v then check the other side of the switch. You should be able to short across either switch to check operation, one will turn the fan on (via the speed resistor) and the other switch should engage the compressor clutch.

mdisav · #11 01-09-2007, 05:38 PM

I have no power at the pressure switch and jumping the switch does not engage the compressor.

Shorebilly · #12 01-09-2007, 06:03 PM

Quote:

Originally Posted by mdisav

I have no power at the pressure switch and jumping the switch does not engage the compressor.

Have you located the relay that I spoke about....that supplies power to the pressure switch "when energized" ??

You could run a wire from the positive terminal of your battery to the pressure switch......one of the leads goes directly to the compressor clutch.....and if the clutch engages....you will know that all is good from that point.....

SB

mdisav · #13 01-09-2007, 06:17 PM

I do not have the ice cube relays. I have just a klima and an ovp. I also have three wires going to my compressor instead of two. I just want to be sure before I jump things. I do not know if I am suppose to have 12v or ground at the pressure switch. From reading Hitman's post it looks like ground from the pbu engages the clutch. I think I will do some jumping from the klima and see what happens.

pmckechnie · #14 01-12-2007, 05:43 PM

mdisav
I believe you have found your problem. I hope you have enough wire coming out of the clutch coil to rehook mthem. If now the clutch pully will have to come off and it usually takes some special pullers to get them apart. good luck

mdisav · #15 01-12-2007, 09:22 PM

The wires are severed in a way I can not even see where to attach them behind the clutch/pulley assemby. I wish I could put one to ground and the other to 12v to see if the clutch activates.

I have the pressure switch bridged right now so I believe it does not need to have any freon in it for me to just hear the clutch kick in.

I will see how much freon is in the sytem once I get it up and running.

Can I get the clutch assembly off while the compressor is still connected in the car? How do I go about doing this so that I can attach the wires to their correct locations?