syoo8

I replaced the heater valve in my 86.5 yesterday which really helped my hot air through center vent problem.

But I am getting strange behavior from my HVAC controls and I am one of you would give me a diagnosis.

SQUARE BOX (All flaps open) position: air slightly warm.
Arrow down, arrow up and down: air cooler.
Arrow up: air slightly warm again.

Does this mean I have a vacuum problem, or is my HVAC head unit broken?

Thanks RListers for your help over the past couple of weeks!

StratfordShark

It would just be guessing at this stage without a thorough investigation.

It's actually a very logical system. Best thing is to get into the centre console then you can check all the actuaries (at the vac lines leaving the hvac manifold) and the solenoid switches.

Then move on to checking the cabin temp sensor, then the external sensor (latter more likely to fail, especially at the connector in the alt cooling hose) and temp control slider and microswitch.

Only after those checks - and setting motor - should failure of controller be considered.

blown 87

When in the square box position all the flaps should be closed IE you should only get air out the center vents and side vents.

If you place your hands on the AC tubes at the expansion valve with the AC on, one should be hot and one should be cold.
That will let you know the AC is doing at least part of its job.

If you have that I bet you have a vacuum issue, check the pods out as described above, then go on the the rest of it.

I just went through this on my 87, so good luck.

syoo8

Is polarity important with the external sensor in the alternator cooling hose? I've replaced the connectors with crimps (they were so badly gone) and the marks with my Sharpie were erased with a squirt of WD-40

syoo8

Oops answered my own question: thermistors are not polarity sensitive.

StratfordShark

Hi Scott,

Sounds like good progress on checking/fixing the temp sensor string.

If the connectors were not making contact then the sensor string would have been open circuit, forcing the system to full hot at all positions of controller except extreme left with microswitch closed.

As you correctly state there is no polarity issue. Any change with the new connectors?

Adrian

Mrmerlin

go to 928gt.com and on page 2 under tips and links follow wallys guide to diagnosing the HVAC system

Alan

Sounds like you may have flap position issues - check the flow:

As noted before the "Rectangular Box" symbol mode should have defrost & footwell vents closed, in the "Up-Arrow" mode footwell vents should be closed and in the "Down-Arrow mode" the defrost vents should be closed. Only in the "Up-Down" mode should all vents be open.

If these flaps aren't working well - likely the secondary mixing/bypass flap isn't either - This can affect temps at the console vents quite a bit.

Do check out the sensor loop - but with it failing OC you'd have much hotter temps

Alan

blown 87

Also verify that the heater control valve is fully closing and installed with the black side towards the engine.

If you have a big vacuum leak at the pods or elsewhere there will not be enough vacuum to fully close the heater control valve.

syoo8

Update, February 2, 2014

All of the symptoms in the initial post were alleviated when I replaced the HVAC diaphragms for the comb flap, footwell flap, and defrost flap.

Chalkboss

Excellent, thanks for the solution. Now you won't have to that pig of a job for many years! I still need to do my comb flap diaphragm.

syoo8

No problem Don!

What I would like to add is this: vacuum leaks anywhere in the HVAC system can produce strange results. Wally Plumley's guide and Dwayne's guide should be all you need to fix problems. They helped fix mine.

chitown928s4

ill be checking the sensor by the alternator tomorrow. I have same hot air issue . However having gone thru some trouble shooting I discovered that the PO has deleted 2 of the 5 valves underneath the radio where the vaccuum tubes go. Why would someone do that and would bypassing those cause hot air thru the system.

Bmw635

Funny timing. I spent last 3-4 days pulling, cleaning all the white carpeting inside then must have bump the orange vaccum hose off the HVAC manifold causing constant hot air today so I'll be pulling this manifold tomorrow to fix. Thx for this bump.