Air Conditioning Clutch

[stope4]

Just installed a new rebuilt a6 compressor, expansion valve, new oil, new hoses, conversion to 134a freon, and all new seals. The original compressor was making noise, one hose was leaking and I didn't know what was in the system. Now that I have made the conversion, the compressor clutch does not shut entirely off when I turn the ac off from the switch. It starts to disengage but it makes a funny sound and after about three or four seconds disengages entirely. The funny sound is the clutch disengaging slowly. If I unplug the compressor, the clutch disengages immediately, the way it's suppose to. I replaced the diode thinking that was it but no cigar. Does anyone have an idea as to what can be causing this?

Oh yea, it's a 71 chevy, 402, original factory ac and I did not notice the other compressor doing this.

[Oldtruckfanatic]

I'm not sure how the clutch circuit works on your truck but you can hook up a test light to the power wire to the clutch and then turn off the switch, if the light takes a few seconds to go out then there is power backfeeding from somewhere. Without looking at the wiring diagram for your clutch power I can't tell you which component might be malfunctioning but it's a place to start. If the power comes straight from the switch then it could be the switch or a ground, If it goes through any pressure or vacuum switches you can check them one at a time and isolate whats not functioning correctly. As with all electrical diagnosis don't forget to check the grounds. Hope this helps.

[TugOwar]

OldTruckFanatic, I hope no one pisses you off around here. You know your stuff and don't mind answering questions, we need to keep you around for awhile.

[Oldtruckfanatic]

LOL thanks...There's always someone who knows more than I do about stuff but I like to help people out if I can. I'm not ashamed if I don't know more than the next guy, I learn by paying attention to other people too so It's pretty unlikely I'll get upset at anyone on this site it looks like a whole lot of good guys sharing info about thier stuff and I like it! Great bunch of people here. Thanks again!

[ChevyCrazy67]

I've seen this happen before with the pressure switch, Although I'm inclined to agree with otf start with a test light on your compressor wire and eliminate each componate from there. The way your describing it makes me want to think it is a back feed problem also.

[phlegm]

I agree with those guys, not because I know, but just because they sound way smarter the me.

:-)

[sneakysnake]

Heres a schematic, Sounds like you are still getting voltage to your clutch after you cut it off.Put a meter on it you may have the blower wired direct(no relay)and its back feeding the system(dc motor will become a generator)I know this from hooking up an electric radiator fan without a relay.LOL

[stope4]

I know the blower motor relay is connected cause I connected it myself. I'll check all the gound wires and see what I can find I'll also check the relays and see what I find. What about the antideiseling relay?. What's it for? Where can I find a new one?

[4x4Poet]

You might check the blower motor relay and any associated ground because the symptoms you described match the time it takes for the blower motor & wheel to stop moving.

[stope4]

Looking at the schematic above, there's two ground wires on the blower iteself, both go to the firewall. There's one near the blower motor relay. Which ones are for what?

[Oldtruckfanatic]

I believe the grounds you see on the two relays, the compressor and the side of the blower on top mean they are grounded through the attaching bolts. The one on the other side of the blower motor is a wire from the motor case to the firewall or heater core case.

[stope4]

I finally got around to doing a little checking. Replace the blower motor relay because it was cheap and easy. Lossened and reconnected all ground wires. Another symptom just discovered is the fan peridically continues to run after the car is turned off. After about 5 seconds the anti dieseling relay makes a clicking sound and then the fan turns off. I placed my hand on the relay and I could feel it at the same time. Does anyone know where I can get this part new? I also do not see any of the sponsors listing this for sale. Do I have to find a used one? The local parts store has no clue what this is.

[stope4]

What in thw world is the anti-dieseling relay for? Where can I get one?

[Oldtruckfanatic]

This is an edit...I think (still a think and not a know) that the anti dieseling relay has something to do with keeping the idle compensator from spiking the rpm just as you turn the truck off with the A.C. on due to the clutch disengaging and for a second not causing a drag on the motor. All this happens in a fraction of a second but if the idle went up enough in that split second the result would be dieseling. Someone may be able to correct me on this...Either way the only thing you need to know about that relay to get a replacement is if it's a normally closed or normally open switch or a grounding relay like the horn relay. My wife works in special orders for a good autoparts store so I can probably get you a part number tomorrow if you need it.

[stope4]

hey, that would be great. I took the relay out yesterday and there is a number on it. Will this help?

And, by the way, I agree this should be happening quickly but the relay is clicking off 5 seconds after I turn the key off.

[Oldtruckfanatic]

That 5 second thing sounds kinda suspect to me. you can post the part number or pm it to me but they usually don't help I will just have her look it up by the name it's amazing the stuff this place carrys for older vehicles they probably have it in stock so I can get you a delco number. If you have any trouble finding one I can have her get one and I can send it to you.

[stope4]

If you don't mind have her check. You can send me a pm. All I know is it's the anti dieseling relay for a 71 chevy pickup with ac.

[Oldtruckfanatic]

Will do...might even be able to get a number tonight If so I'll let you know.