MC Luc

Guys - no heat or cold working because it was confirmed that the HCV was stuck open - no vacuum. The problem is that we cannot find a leak anywhere in the system. Is there any common things to look for, or actually, uncommon things to look for, that we might not be thinking of? This is at my Indy's shop, I have not had time to tackle myself, but he is frustrated. Thanks for any help!

PaulD_944S2

The diaphragm in the heater control valve often goes bad and hides the vacuum leak. Vacuum test the HCV and see if it's good/bad.

MC Luc

PaulD - yes, we tested it, and I have a replacement, too. Unfortunately not the HCV.

Tom M'Guinn

In the factory manual, starting on 87-61, there is a troubleshooting guide, which for your issue seems to focus on the three interior temp sensors inside the under-dash hvac unit (which you can test via an accessible plug), and on the control solenoids that regulate the vacuum lines. I don't see mention of the HVAC control box in the dash, but that part is known to go bad too so I'd put it on the list. If you don't have access to the manual, I'm sure one of use could post the relevant pages...

MC Luc

Tom - would love the pages from the manual, that would be very helpful.

MC Luc

Anyone, can I get these pages out of the manual, car is still in the shop and we are stumped. Thanks!!

Tom M'Guinn

The pages are out of order, but may help. For nonprofit education us only of course.

steven74

Humboldtgrin

Archer911

I have a similar problem on my 86 Turbo but the heat is stuck on. Will dig into it tomorrow and if I make progress will let you know.

Jfrahm

Check and see if you can verify that the heater valve is closing. I had one that still leaked enough hot water to have heat in the cabin even though the valve appeared to be shut. Replaced valve with the metal Audi 5000 part, put a ziptie on the vacuum line and it was fixed.

Archer911

No matter what I set the controls to the valve doesn't move at all. Wide open all the time.

Thanks for the tip though and the Audi reference.

Jfrahm

OK if you end up replacing the control unit, look for a later one as they improved them over time. Until you get to 968 they all seem otherwise the same, the 968 has a detent pot for the temperature.
Maybe also try checking to see if vacuum to the heater valve will close it, maybe the valve diaphragm is leaking.

Archer911

The heater control valve definitely holds vacuum under test.

I'm going to do the sensor test as mentioned by Tom that resides on 87-61 but haven't yet determined what the manual means by "Remove control switch and pull plug A."

Anyone know?

Thanks