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Problem: I recently bought a 1982 928. When working with the HVAC controls, it doesn't seem to matter what setting the control unit is on, I get roasting heat.
So before we jump to conclusions and say it's the heater valve, let me tell you what I do know:
- Heater valve vacuum pod holds pressure for more than 60 seconds when applied
-Heater valve is correctly oriented
- Vacuum to entire system holds pressure
- Checkvalve by brake booster is working
- Temp Sensor behind driver side wheel cover is no where to be found. Two yellow wires were just zipped tied to the fender. If this is the problem now what? The alternator does have the cooling cap but no hose.
Can someone also post a picture of their period correct center console air vent with cover removed. It looks I am missing the lever arm and something to connect the vacuum pod to the flap.
the hot water valve diaphragm may hold pressure, but the VALVE can leak over time..replace it.
If the dash cabin air temp sensor dies, it will blow high fan hot air in all positions except 'off'.
It causes the system to only have full cold or full heat.
Would you mind posting a picture of you HVAC head unit? In 82 they still had the older manual one and the automatic one was an option.
A failure that is less common is the diodes on the HVAC motor going bad and causing the motor to stop functioning at all.
Another odd failure that happened to me recently is the vacuum actuators attached to the front of the HVAC box can leak vacuum. Check them with a mityvac when no power is applied.
I don't have a pic available right now, but upon disassembly of the center console of the '79 parts car a few weeks ago, I noticed the vacuum line was pinched above the top nipple. , the vacuum pod is fine, I also found cracked hard vacuum lines in various places., so before you send the MAF out for a rebuild, check the simple , but unobvious parts.
I'll edit with a pic later today.
ps; another possibility could be a failed o ring inside the heater valve, I had this issue with my '85, the valve actuated fine, but the sealing o ring inside had failed.
Ah Ha, found some pics... Yellow vacuum line was pinched
Also the black vacuum line receiver had a couple of cracked rubber connections, would be a troublesome issue for the system to hold vacuum and operate properly.
...no mafs were harmed in this photo...
Last edited by OTR18WHEELER; May 2, 2017 at 06:44 AM.
If there's no external sensor and the wires to it are disconnected, then you will always have system demanding full heat once moved off override micro switch at full cold position.
The heater mixing flap motor is effectively seeing very high (infinite!) resistance, so it 'thinks' full hot is selected and it's cold outside and in the cabin. You need to restore the ext temp sensor - it is housed in a metal tube which joins both sections of the alternator cooling hose. Without the hose the system will still work but more sluggishly, as airflow to alt quickly registers outside air temp changes for the (missing) external sensor.
This may not be only problem, but nothing will work till fixed!
If there's no external sensor and the wires to it are disconnected, then you will always have system demanding full heat once moved off override micro switch at full cold position.
The heater mixing flap motor is effectively seeing very high (infinite!) resistance, so it 'thinks' full hot is selected and it's cold outside and in the cabin. You need to restore the ext temp sensor - it is housed in a metal tube which joins both sections of the alternator cooling hose. Without the hose the system will still work but more sluggishly, as airflow to alt quickly registers outside air temp changes for the (missing) external sensor.
This may not be only problem, but nothing will work till fixed!
edit: The last 928 I had, none of the heater valves I installed would fully close under vacuum. They are a normally open valve, and under accel literally 100% of them would loose enough vac that they would then bypass some amount of hot water. So, finally I got tired of it, and put an inline shut off in the delivery side of the heater hose. During Apr to Oct I just leave the shut off valve closed, so there is no chance of circulation. When it started to get cool, I would open the manual valve and let the vac operated heater valve just leak like it always will.
I am going to add the temp sensor back into the mix to help eliminate that variable. If that's not it, I will definitely return back to the heater valve.
edit: The last 928 I had, none of the heater valves I installed would fully close under vacuum. They are a normally open valve, and under accel literally 100% of them would loose enough vac that they would then bypass some amount of hot water. So, finally I got tired of it, and put an inline shut off in the delivery side of the heater hose. During Apr to Oct I just leave the shut off valve closed, so there is no chance of circulation. When it started to get cool, I would open the manual valve and let the vac operated heater valve just leak like it always will.
Well I added the alternator cooling, temp sensor and no change. Although, the random turning on the control unit stopped. I also noticed I wasn't getting blazing heat anymore. Next step - I tie-wrapped the the heater valve and got cool air. Now I noticed that my AC is not kicking on...so off to a new adventure...
Last edited by checkmate1996; May 22, 2017 at 02:09 PM.
Locate the wire to the AC pump clutch. It comes up into the harness near the oil dipstick. Make sure you have the wire for the AC pump clutch.
Jumper 12V from the jump post to that wire connection(engine off), and listen for the pump clutch to pick. If you hear that, then you can go through a bunch of troubleshooting on freeze switch, pressure switch, button on the dash, and most important the relay in the HVAC head which is a high failure item. But - before ALL that, apply 12V to that wire connector above the AC piump to insure your AC clutch is working.
Locate the wire to the AC pump clutch. It comes up into the harness near the oil dipstick. Make sure you have the wire for the AC pump clutch.
Jumper 12V from the jump post to that wire connection(engine off), and listen for the pump clutch to pick. If you hear that, then you can go through a bunch of troubleshooting on freeze switch, pressure switch, button on the dash, and most important the relay in the HVAC head which is a high failure item. But - before ALL that, apply 12V to that wire connector above the AC piump to insure your AC clutch is working.
Great suggestion! Will do!
On the 82 there is no "AC Button" that normally sits where the central warning display is. On mine, when you slide the switch control unit to 'AUTO' it is suppose to turn on...
Locate the wire to the AC pump clutch. It comes up into the harness near the oil dipstick. Make sure you have the wire for the AC pump clutch.
Jumper 12V from the jump post to that wire connection(engine off), and listen for the pump clutch to pick. If you hear that, then you can go through a bunch of troubleshooting on freeze switch, pressure switch, button on the dash, and most important the relay in the HVAC head which is a high failure item. But - before ALL that, apply 12V to that wire connector above the AC piump to insure your AC clutch is working.
I applied 12v and heard alot of "clicking". I presume the AC clutch is working.
IIRC - 78-79 is all vacuum driven whereas 80 and up and is mechanical plus vacuum. The center console vent is also different.
I don't know what you mean - all years use vacuum actuators for all airflow selection and water valve - except for the cable driven systems on very early cars w/o AC.
Without a fully operational temp sensor loop (inside + outside sensors) you will always get full heat (most of the temp slider) or full cold when in max cooling - this max cooling will only work if the micro-switch (end of the slider) is adjusted and functioning correctly. This is easy to check in the head unit - you can see all the parts.
I don't know what you mean - all years use vacuum actuators for all airflow selection and water valve - except for the cable driven systems on very early cars w/o AC.
Without a fully operational temp sensor loop (inside + outside sensors) you will always get full heat (most of the temp slider) or full cold when in max cooling - this max cooling will only work if the micro-switch (end of the slider) is adjusted and functioning correctly. This is easy to check in the head unit - you can see all the parts.
Alan
Yeah I was referring to the early 78/79 where they were cable driven and all the vacuum lines went directly into the HVAC unit.
I bought a used sensor and hooked it up with a new alternator hose from roger. I 'think' that is working now... I really need to dig into the head unit next..
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82 HVAC Help - All Heat Issue
