polecat702

I'am sure this has been asked before, so here is a stupid question. I got hot air coming out the A/C vents all the time. Could this problem be caused by the notorious hot water vacuum valve in the heater supply line? Also I have read posts on this forum about a flapper valve located somewhere in the center console, or is it in the dash. I don't have much experance with these cars. I've only had my 87 S4 for about 4 months. I do have the shop books on a disk, I didn't want to just tear into the car with out an idea of where to start. Thanks, in advance for any help. Thanks again, Joe


87 928 S4
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j.kenzie@sbcglobal.net

Welcome.
I would suggest you try to figure out whether you have a refrigeration problem and/or an air distribution problem. Is your a/c compressor running? Is the output of the expansion valve cold? I doubt it. Do you have vacuum leaks in the a/c control system? Is your a/c head unit working?
Wally P has the best write-up on this system. Go to 928gt.com, go to tips then a/c system.
Good luck

Post number 911; I miss my old 78 911SC.

Alan

No - the symptoms you describe suggest a problem with the temperature sensor loop.

When you change the flap selections (lower slider) with the blower on full - do you get a change to the airflow central vent/defrost vent/ footwell vent? if so your vacuum at least mostly works.

The temp slider should give you somewhat cold air in the min temp position (there is a microswitch) even if the temp loop has failed (better if your AC actually works). Investigate to see if your external temp sensor is still there... its under the drivers side (LHD) headlamp. You can remove the headlamp cover & the headlamp to see it most easily (really quite quick & easy). Its in the hose that should be there to feed the alternator shround - it may well be gone... seems to be a part that commonly goes missing - for some odd reason.

report back what you find

Alan

tveltman

Max hot air all the time is usually heater valve. It may or may not leak, but you can ziptie it closed to see if the problem gets better (I am in the process of sorting this out now as well). It's probably worth it to replace the valve, there is a post around somewhere where someone bought a less crappy version that may hold up better and leak less, but I havent tried it to confirm. Heater flaps are adjusted under the instrument pod and next to the glove box (gotta remove the box first), but I'd choose the simple route first and close off the heater valve. You wont do any damage, and youll get a quick and free assessment of where the problem is likely to be.

If it isn't your hot water valve, other options include mixing flaps (as you said) or low refrigerant.

j.kenzie@sbcglobal.net

Sorry but hot air ALL the time probably means no refrigeration.
Even a bad heater valve will give you cold air for about 5 minutes until the engine heats up. Also, Alan, if he has hot air all the time, I'm sure he has tried sliding his temp bar to maximum cold and thus bypassed the temp control loop.

Alan

I disagree - the water valve is often open even with the AC on in active cooling mode. The only thing that allows airflow though the heater core is the mixing flaps. If the mixing flaps are closed, blower on high - the outflow air should not be much warmer than the ambient external temperature - certainly not hot. If the AC works the outflow air should in fact be much cooler than the external ambient. There seems to be a great misconception that heating to the cabin is primarily modulated by the hot water valve - this is simply not true.

The hot water valve must be open to heat the cabin - but the mixing flaps must also be at least partially open to have any real heating effect. If the sensor loop works correctly and the cabin is already hotter than the set temp the mixing flaps should be fully closed blocking all airflow through even a hot heater core...

Its certainly possible the hot water valve is also open - but that is not the primary cause of this symptom and may be quite normal if the sensor loop is calling for more heat due to malfunction or if the mixing flaps do not operate correctly (e.g. don't close properly).

You can observe the mixing flap actuation if you remove the LHS lower console side cover... see what happens depending on position of the temp slider ..

Alan

WallyP

Joe,

As has already been suggested - go to our website; click anywhere to enter; click on the + next to 928 Tips/Links; click on + next to Wally's World; click on The HVAC System.

This is a general tech paper on the HVAC system that will answer many of your questions.

StratfordShark

Fascinating thread and I'm beginning to get definitive understanding of how the system works.

But if the external temp sensor is being suggested as cause of control loop failure, is it equally likely that the cabin temp sensor is the culprit in telling the system the cabin is colder than ambient or would a faulty cabin air sensor give rise to other symptoms?

And what's the easiest way of distinguishing between an external v internal temp sensor problem?

Alan

It could be anything in the temp loop - but the external sensor seems to get messed with much more often - AND is much easier to get to... Usually the loop goes OC and this is the cue for unconstrained heating...

There are a barrage of tests you can do - but its good to start in the right places and to do the easiest and most likely diagnostics first. Getting to some of the test points is not easy... and possbly not needed

I agree tying the heater valve closed may be the most expedient way to get a more comfortable car - its not the fastest way to diagnose the problem though. Seems the AC probably needs to be fixed also - and its better for an AC shop to try this with the heater valve disabled under the current circumstances too.

Alan

j.kenzie@sbcglobal.net

Blow cold "canned air" into the internal or external sensor and see if the vent outlet air responds.

polecat702

Hi guys, Thanks for getting back so fast. My A/C works good, it was changed over to R134 by the PO. His business was failing, thats why he sold the car. He let some things go, and I'am trying to sort them out. When he changed to R134, he told me that the pressure switch was out and it cost more than he wanted to spend to replace it. He had them put a toggel switch in to pull the mag-clutch in to run the compressor. With that said: The car still has hot-air coming in with the system turned off, control levers pushed over to the left, and fan motor off. Hot- air still coming in through the center vent and door vents.
I have to close all the gates to choke off the heat. I'am sure the PO, mouse'd the system. Thanks again, Joe

j.kenzie@sbcglobal.net

"My a/c works good". "I have hot air through the vents all the time."
I understand. You live in the Arctic and it is only 35 F. outside.
Your system is fine!
Enjoy!

polecat702

Once again, thanks for all the help. Wally's site is great, I'am going to print a copy of the text so I can have it out in my garage while Go through the check list. We do have a really great auto A/C shop here in Vegas. I talked with them and was told to fix the pressure switch ASAP. The unit could slug the Compressor and destroy it, thats more money than I want to spend. I don't drive the car very much, I am trying to fix everything before Sharktoberfest. If its on this year. thanks, Joe


87 928 S4
Blk on Blk
auto
928 OC member
74 Pantera "L"
87 Monte Carlo Aero Coupe
84 SS El Camino
05 Tahoe, wife's ride

polecat702

Dave, What I ment to say was, it does cool down with the compressor running. Shut it off, and its like I turned on the heat. It DO GET HOT FAST. Thnx, Joe

j.kenzie@sbcglobal.net

Sorry.
You do not have an a/c problem.