blandis

Have been working on my 88's AC some over the weekend. Rear AC is cold front is not. So system should have a decent charge.

First check was the heater valve. It seems to be operating as it should. Just replaced this summer when I did my intake refresh. Would it hurt to zip tie it closed to make sure that is not my issue?

Second I wedged the flap closed against the firewall on the passanger side (above fuse panel). Should now only be drawing air from inside the car and no fresh air from outside.

Checked setting motor and only operates when switch is moved to defrost setting. Arm moves up all the way up when switched on and back down when you turn off defrost.

When defrost is on air is always hot regardless of what the temp is set at. All other setting air is warm, not hot, like its drawing air from outside. If sensor in alternator cooling hose were bad would the air always be hot regardless of setting?

I assume my vacuum actuators need replaced. I can hear the "click" when I move the switch from floor, vent, mix, defrost but the air still comes out center vents all the time. Have yet to put mighty vac on anything to test them.

Before I go any further, just want to know what my next steps should be. At this point just want cold AC.

Thanks in advance for your help!

JoeTaylor

Is your expansion valve plugged up on the front AC unit?

The back AC works, thus the compressor, rear AC expansion valve are all working.

dr bob

Air should come from the center vent regardless of mode switch position. IIRC, the defrost and floor vent settings can cause the comb flap to close, restricting center vent air.

Warm in front cold in back is most typically a heater valve problem, so you are on the right track. I'll guess it's close enough to summer now that you won't need heat until October, so it's a safe bet to close the heater valve with a zip tie. Keeping the heater valve closed this way should negate almost anything in the temp string, but it does not lock the temp setting motor. FWIW, you should be able to hear/watch the temp setting motor move with the mode slider in position 1, just to the right of the 'off' position. Move th etemp slider back and forth and the motor should change direction as you pass ambient temps going up and down.


Grab Wally's HVAC write-up from the 928 Specialists website if you haven't already. Troubleshooting tips in there for the flaps and doors and heater valve are invaluable as you work through this.

Other possible problems include a failed or partially obstructed expansion valve in front. You can finger-test the valve with AC on and the plastic rain shield removed to expose the expansion valve. Valve should be cold on the small line downsteam of the valve itself (towards rear of car). It should also be at least cool on the large return line either side of the valve. If not cold, there's quite possibly a restriction in the expansion valve. Diagnosis is via the guage set and a couple thermometers, high press too high and low press too low for plugged, vice-versa for stuck open. If stuck open the rear unit would not be very cold though. Test the front system with the rear system off completely.

WallyP

It is not unknown for the heater valve (coolant valve) to fail internally, so that coolant flows even though the lever arm is in the closed position. It is also possible for the valve to be installed backwards, so that coolant flow simply pushes the valve flap off of the seat, allowing coolant flow.

The temp control flaps inside the HVAC box can be maladjusted so that full temp control is not possible.

It is normal for there to be airflow from the dash and armrest vents anytime that they are open.

It is normal for the temp to be full hot on defrost. To the Germans, defrost means full blast heat and air flow. You get some flow to the windshield all the time, so that should prevent fogging up.

blandis

So heater valve could be bad even though arm is moving.

Best way to bypass that, to eliminate the heater valve as my issue? Seems to be the easiest thing to eliminate. I think it is installed correctly. Pic is below.

Thanks Bob and Wally. Did not know the German translation to Defrost is HOT.

Tom in Austin

My U&()#$&*(FY*(^# plastic heater valve arm broke after I tried to unwire it last fall! The thing is so frickin flimsy it wouldn't even hold up through one wire and unwire cycle!

I'm going to do the petcock deal so I can shut the coolant flow off completely in the summer. All the air that goes through the evaporator has to first go through the heater core, so it's really working against itself when the core fills with hot coolant after you turn off the engine.

Randy V

If the heater valve is installed backwards it will not properly seal and thus allow a small amount of hot water into the heater core.

blandis

Going to get napa 660-1413 to eliminate the heater valve as a potential problem.

dr bob

Try the front AC test before the car warms up. If the heater valve is leaking cold water, it won't affect the AC temp. If cold then and warms as the engine coolant heats up, you've discovered the problem.

joejoe

I found that if charge is a bit low the rear will still be very cold and the front will not cool well (just 2 pennies here)

SeanR

Take a look in here and see what is there.

If the evaporator is clogged, you won't get much in the way of cool air.

blandis

Sean
Brad and I cleaned it 6 months ago or so. It was caked with the black crud in your pic. How it pulled any air through it still shocks me. And that f'in boot is the hardest part of that job.

SeanR

Coolio, I thought you did but figured I'd toss that out there anyway. How many weeks of schruul do you all have left?

FredR

Has anyone come up with a reliable method to clean the evaporator matrix without disassembly?

Regards

Fred R

SQLGuy

BG has an A/C evaporator cleaning process some of their dealers do. I was thinking of trying it. Maybe Greg can comment on it as he does some other BG work?

http://www.bgfindashop.com/bgservices/climate.htm