hvac system

[honerboys]

anyone on here know exactly how this system works - last summer i repaired the a/c system and it does indeed blow cold air - i measured the temp at the vent and it was down to about 40 degrees - the problem is the car never seems to get comfortable - if i hold my hand up to the center vent i can feel some warm air coming out - this mostly is at a stop light when idling - it seems to be mostly towards the bottom of the vent - so i replaced the coolant control valve by the firewall with a new one but i still can feel some warm air - also if i have the fan speed set low it randomly comes on full for a few seconds before returning to it's set speed - i figure this is the automatic temp setting "working" - so does this auto system open and close that coolant valve to modulate the air coming out of the vent - and how is that tied to the sensor in the dash - is it all done with vacuum or is there a mechanical component too - could it be caused by a vacuum leak? - i saw all this stuff when i had the dash out and it all looked intact but obviously something is not working as it should - all the vacuum operated doors open and direct air where they should - i had the windows tinted yesterday and maybe that will help but it just doesn't seem to operate correctly

[Dmitry at Pelican Parts]

Hello - check out the link below to one of our site's DIY tech articles, it's a great guide to how the system works. If you have any other questions, feel free to post a Comment just below the article and someone will get back to you with more details.

Pelican Technical Article: Porsche Air Conditioning Frequently Asked Questions


-Dmitry

[honerboys]

thanks dmitry - almost all of those articles and comments were for 911's - not much on there for 928's - and those that were for 928's didn't have my problem - so does anyone on this forum have any answers - thanks

[Tampa928s]

Your heater control valve may be stuck closed.
It operates by vacuum and defaults to hot.
Your blend door on the center vent controls heat from the heater core, make sure it's working and not disconnected.
Above your Ce panel is a fresh air door make sure it closes when the air is running.
These are the most common to fail and induce heat into the cabin.

[harborman]

I also have problems with heat coming out when it's turned off. I replaced the heater control valve and it still does it.

[honerboys]

hey tampa - thanks for the input - is that control valve supposed to completely shut off the flow of coolant - like COMPLETELY - or does it still allow a little bit of coolant through - when i removed the old one it did operate but i could blow air through it when it was closed but not much - so should the output hose from that be cool when it's closed - what actually tells that valve to close? - electronics in the head unit controlling vacuum? - and what does the temp sensor in dash send a signal to (head unit?) - the fresh air door closes as it should - the blend door could be my culprit as i had to do a repair on it's linkage when dash was out but it seems to operate - the warm air seems to be just at the bottom of the center vent - so maybe it's not actually closing all the way and leaking some warm air at bottom? - should the fan be coming on in short bursts like it does? - a lot of questions i know, but i can't seem to find a description of how all these pieces interact with each other - so if anyone knows i'd appreciate some input - thanks

[Tampa928s]

https://www.928gt.com/T-WallyHVAC.aspx

You will get some leakage via the valve if your arm is not closed when air is set to cold then use a mighty vac and check. Vacuum comes from the controller.
Blower should not cycle check your connections and take time to read this link.

[Tampa928s]

https://www.928gt.com/T-WallyHVAC.aspx

You will get some leakage via the valve if your arm is not closed when air is set to cold then use a mighty vac and check. Vacuum comes from the controller.

You need to indicate you're year on your signature, depends on year but outside and inside sensors come into play.

Blower should not cycle check your connections and take time to read this link.

[Linderpat]

As Tampa points out, Wally's write up is the definitive 928 hvac bible. Here's the link again for good measure: https://www.928gt.com/T-WallyHVAC.aspx

[honerboys]

wow.. that is exactly what i'm looking for..thank you for the link - and thank you wally for the write-up - now i can test this system and see what i got going on - i did see where wally said the auto system does not control the fan speed so i wonder why mine turns on and off (seemingly at random) - the fan control switch works the fan at every speed correctly but if it's set to "0" it will just come on for 5-10 seconds then stop then repeat that every 5 minutes or so - i was thinking that was the head unit trying to get the temp to the set range but now...??? - thanks again

[Linderpat]

you have the magic blower syndrome! Do a search here and on rennlist, and you'll find lots of good info about what causes it and how to fix it.

[honerboys]

thanks linderpat - i'm going to look at the resistor pack today - i replaced that early on in the restoration of this car - but i did replace it with the pack from a 79 so it wasn't new - i'm going to try the bend the coils away from the bi-metal strip method - probably look at the center comb flap while i'm out there

[Danglerb]

The 928 cooling system, especially when converted to 134 is a bit marginal. Fixing it no small task.

[honerboys]

hey danglerb - while we're on the subject - this car has been converted to r-134 - but recently i discovered some available r-12 cans - so can this car be un-converted back to r-12 - all the lines were replaced with the newer r-134 type - are they compatible with r-12 also - i did slightly move the coils on the resistor pack and drove the car a bit - so far the mystery fan operation has not happened so maybe that's fixed - the window tint job should help a bit too - it seems my other cars a/c is much colder and they are v-6's - i expected more from a v-8 - years ago my brother had a 5.0 mustang and the a/c in that could get downright uncomfortably cold on a summer day - like a meat locker - what makes these cars so anemic?

[honerboys]

oops... forgot to mention - i removed the center vent and the comb flap seems to be operating correctly - there were 4 holes in the "duct" on the lower left side, 3 round, 1 oblong - about 1/4 inch diameter - like maybe something was supposed to be mounted there? - the right side has the vent door switch installed - could heated air be coming up through these holes? - i taped them over to see but haven't really driven car enough to see if that makes a difference in the warm air coming out of that vent at the bottom

[JK McDonald]

Hey Honerboy - One other point concerning the Factory Style Heater Control Valve is that some seem to seal off better when water is flowing in One Direction better than the other. Before installing the new valve - select closed and then blow air through the valve from both ends. I've been told that the flap inside is at an angle so when the water pump pressure is felt on the correct side of the flap it pushes it farther toward the sealed direction.

Also you might consider installing an additional Generic Manual Shut Off Valve in series with the factory Heater Control Valve. I control the ADDED Manual Heater Valve with a short "T Handle" cable that sticks up by the Radiator Reservoir Tank.

The reason for adding the Manual Valve is because in the heat of the summer - the factory Heater Control Valve often creeps open when you turn off the motor and your HVAC vacuum lines slowly leak off. Since the Factory Heater Control Valve now falls to the open condition - your AC must first cool off the hot water in the internally exposed heater core each time you fire up your car before you begin to feel any cold air. In Texas this can be a problem.

Each fall I simply select the added Manual Heater Control Valve to "OPEN" which allows the factory heater water circuitry to operate normally.

Good Luck, Michael

[Mrmerlin]

also in plain sight is the inlet written in German

[Mrmerlin]

[JK McDonald]

Hey Stan's the Man - Your right. The higher quality Heater Control Valves do designate which way the water should properly flow to help avoid any seepage when closed but the Chinese brand of knoc-ka-noff parts usually don't have "Fahrvergnügen" written anywhere on them.

A friend had tried several of the lower grade Chinese Heater Control Valves off Ebay for his daughter's car that constantly seeped hot water passed the internal valve in the "OFF Position" no matter the direction they were installed.

After a lap or two laying on top of his motor while swapping out a couple of duds, I had later decided to add the manual Generic Heater Control Valve on Miss Purdy just as a back up.

Michael

[Mrmerlin]

Mike please tell your friend to stop shopping for bargain parts,
fix it right the first time then you can forget it.
Roger has quality parts with the printing on the valve about 18.00 each,
plus swap out the short hose when you put in a new valve