A/C compressor not engaging
 

Looking for some trouble shooting help.
This is in my '79 240D
A/C was blowing ice cubes a couple of weeks ago. Since then , I installed a new voltage regulator - that is the only change.

I have no reason to think that there was any refrigerant leak - the system is dry everywhere, including the compressor.

I swapped the fuse. I checked the connection at the compressor (tight and has not moved). I shorted the pressure cut-off switch, still did not engage, and I replaced the cube relays with known working ones from my other 240D.

What do I do next?

Have you checked for power at the connection? Does the blower fan work?

If you put 12 volts directly to the a/c clutch. does it operate?

The Blower works, and turning the AC switch on increases the air flow (I think that has to do with vaccuum flaps closing when the AC is turned on).

I will check for voltage at the connection, then see what happens if I hot waire 12V straight to the compressor and report back.

Thanks!

Have you looked to see if the compressor clutch is engaged and is spinning while the AC is on? I would start there. If it is spinning and no cold air is coming out then it is an air flow (vent) problem.

If it is not spinning it is either low on refrigerant, not getting power to the compressor or the belt is not causing the compressor pulley to spin.

For low refrigerant you will need a set of gauges to hook up to the ports. If you have no gauges, to see if it is completely leaked out you can take off one of the caps and just press the valve with a screwdriver (or other cylindrical object) to see if refrigerant shoots out. (Just a quick press will do) I forget the pressure at which the switch will cut the compressor out but you could still have too low a charge which will not allow the compressor to engage.

To check the power, turn on the car and press the ac compressor switch. Pull the wire connection at the compressor and put a multimeter on it. It should have voltage.

Checking the spinning is easy. Look to see if the belt is spinning the compressor while running. :D

The problem should arise. If all checks out then most likely your refrigerant charge is too low.

OK, finally got under the hood...
With the AC turned on - (BTW this is a MCC system)
There is no voltage at the compressor. Jumping the pressure switch has no effect.

There is also no Voltage at the fuse with the AC on (but the plug unplugged from the compressor). I checked for continuity across the fuse that is in the slot, and it looks about like all the other fuses, so the fuse seems to be ok.

I dont have a great way to get power directly to the compressor. The oil cooler hoses on this one are run pretty close to the clutch, so I would have to get the lead in blind :eek:

Could it be a bad switch? How would I know?

?bump?

Thought I would add that I have a bad remanufactured starter in the car....I will be doing a warranty replacement on it this week. Not sure if the bad solenoid on it could be at all related?

Just to clarify, your system is the Servo system, correct? You have a switch to turn the compressor on and off? I would start by checking the switch. If that is not working then it won't supply power to the circuit. Pull it out (fairly easy without removing the whole plate) and check for voltage with it on by separating the pin harness from the switch just a tad and stick the multimeter pins in there. It would be easy to run a jumper to the correct lead at the harness to see if the compressor engages (of course the car would need to be on).

No, no servo. This is the MCC system, standard on all 204D'sI think...

This picture is from JimmyL's 240D.

I will try to pull the switch this weekend. Seems like the problem must be upstream of the pressure switch and fuse, so has to be the switch itself, right?

I'm not familiarly with that system but if you have a switch to engage the compressor, start there as it does seem most likely to be the culprit. It could be as simple as a loose connection.

Going backwards (or upstream) from the clutch, per the FSM wiring diagram for the '79 AC, there's the low pressure switch, then a relay then the switch on the console.

The relay is controlled by the starter - when the starter is engaged the relay is supposed to open to make it easier for the engine to turn over since you're not also driving the compressor. This is also linked into the NSS on auto trannies.

So yes, your starter issue MIGHT be contributing to the compressor issue. Find the relay - I think in the ice cube tray - and see if that is closing (or opening, I guess?) when the starter is NOT engaged.

Since the blower is working, you might be able to see if there's +12v @ pin 30 in the relay receptacle. If so, then it's either a bad relay (unlikely since you checked) or the starter is disengaging that relay.

I just did a warranty swap on the starter it did have a bad solenoid.

I am crossing my fingers that I will have cool air for my morning commute (though it is oddly cool this week in San Antonio!)

Thanks for your input. I'll report back.

OK, so still no power to the compressor.

My next step is to hotwire the compressor to ensure it engages. I expect it will.

After that, I guess I would want to try and bypass the relay to see if the starter is disengaging it. Like I said, I have swapped both ice cube relays for known good ones with no effect.

The other thing is that I am also not getting any voltage across the fuse that feeds the compressor. If the starter was disengaging the relay, wouldn't I still see voltage at the fuse? Not sure how the circuit works here.....