I searched the previous posts about this but it brings up much more serious problems than mine. On the winter schedule is to find out what keeps the AC compressor engaged on my '89 with rear AC, at any position other than off. Everything else in the AC system works pretty much normal, but if all I want is just heat, or just outside air, the AC compressor is engaged.
The bearing was replaced and the clutch shims done so it works good, and the owners manual alludes to the 'Cold mixing with hot all the time'. but should the compressor be ON at all positions other than OFF? It seems a little ridiculous when all I need is heat or vent air. If this isn't normal, I do suspect the AC relay behind the control head. Thanks

 

no the compressor should not run all the time.........is the a/c button stuck? also, the air direction lever will activate the compressor when direct at the windshield (defrost)

 

Not stuck, There seems to be colder output when the AC button is pressed. The compressor kicks on at the first position (Side and center vents)

 

ok.......the hvac controller can be rebuilt.......IIRC look at 928INT site

theres also a great write up by Dwayne:

 

Yes, Dwayne has saved me countless hours on TB/WP, Axle rebuilds etc. I planned on replacing lightbulbs for the *****, and working on the temp slider control because it is very coarse in the temp adjustment. The relay for the AC is back there and then I can meter it. Thank you.

 

search on Dwayne HVAC.........link copy doesn't seem to work

 

Are you sure that the compressor is running all the time?
The clutch pulley rotates all the time and then the compressor pulley locks in when the AC switch is pushed in the dash.
The only way to check if the compressor is running is to put gauges on the system.
You have probably done this, but just anted to make sure.

 

Yeah, its cold air, and i can see the face of the clutch engaged and spinning. I also see a voltage drop slightly on the dash voltmeter. This winter i'm going to take the hvac head apart and check the sliders, relay etc. I'm pretty good with the electrical.

 

Unless the A/C button is not depressed, at any slider position other than 'O' the compressor should be running (on a '90+ with the circular arrows instead of 'O', the compressor will run even at this position). If it is cold out, eventually the anti-freeze switch should cut it off and it should cycle. The HVAC system is supposed to mix the cold air from the evaporator with warm air coming from the heater core via flaps in the HVAC box. These flaps have foam attached to them that disintegrates after a decade or two and they no longer seal like they should, causing air to bleed by them and possibly messing up the temperature of the air mix.

If your system is sufficiently undercharged or overcharged with refrigerant, the evaporator will never get cold enough to trigger the anti-freeze switch and the compressor will run all the time.

Dan
'91 928GT S/C 475hp/460lb.ft

 

Thanks Dan, this is what I've been seeing, in disbelief at the design because if I want Heat, the compressor runs. I don't need to mix it with cold air. So its a bit under charged but still blows cold when I need it. So, then what does the AC button do differently, when I press it, if the A/C is on all the time anyway?.

 

The compressor should run only when the AC button is depressed; or when the condition lever is on DEFROST. Anything else seems to indicate that the wiring has been "modified" or there is a problem inside the HVAC control head. The AC compressor relay in the control head much more commonly fails "open" than "closed".

 

OK Thanks I will be in there soon and will study the wiring diagrams before digging in.

 

Did you check to see if there is an actual 12v charge on the clutch wire of the AC compressor clutch? or if the AC clutch is just stuck, warped or frozen?

 

A good point - not a common problem, but a very quick and easy check.

Look behind the oil dipstick tube for the single black wire that powers the compressor clutch coil, and find the connector on that wire. Disconnect it, and crank the engine. If the center of the compressor turns, the clutch is stuck/seized, and the problem is strictly in the clutch.

 

Thanks for the advice, but the funny thing is the AC works ok. Clutch engages and disengages as needed, just when in any slider position other than off, the clutch engages, then I adjust temp ok. There is no problem with the compressor itself. I even just replaced the main bearing and cleaned the rust and installed proper spacers for the clutch face. So based on Wally's post #11, i need to check what is it in the slider that is engaging the clutch, at any position other than defrost. Thank you all. I'll be doing this after thanksgiving and will follow up here. Every other issue I have had or needed to perform, like TB/WP, Shocks, Ride height, dash LED's, fuel lines etc, have been expertly laid out for me here by the fine gentlemen of this forum.