Porsche Repair: 87 928S4 climate control, vacuum lines, sweaty head


Question
Dave;  I have a few questions I would really appreciate your time trying to answer.

1)  My climate system blows hot air only, even when AC is on, and regardless of the temp setting.  I can feel that the AC is working, but there is more hot air coming out of the dash board vents than cold air.  When the temp setting is at 65F the air blows very hot.  When the setting is at 85F the air blows very very hot. This has been an long term problem which has gotten worse over time.  When the problem started, I would have to have the Temp setting all the way to 65F to get ambient air, at 66F it would blow hot.

I believe that the valve sending hot water from the engine to the heater core is not shutting the water off when I turn the heater off.  There is an exposed portion of the hose forward of the bulkhead that I can verify is hot when the heater is off.  Do you agree with my valve theory and if so, where is it and how do I repair/replace it?
I just checked the temp control module and it apparently is working OK.  I think a mechanic who worked on the system 8 years ago disconnected one of the temperature sensors which may explain some of the other symptoms.  Is it true that there is a temp sensor near a front wheel well?  Where are these sensors and how can I make sure they are connected and working properly?

Is there anything else, like relays?, vacuum lines? etc.?

Thanks very much!

Tony

Answer
Tony,

I'm the resident 356 expert but as you know on hot days my A/C is to roll down the windows and stick my sweaty head out into the air.  And as for 928's...my wife can tell you I've lusted after these beauties for a long time, but I've never seen one that isn't doin' 85 and whizzing past me!

The good news is my daughter owned a '91 944 S2...and coincidentally she never got the A/C to work well.  We did find a way to stop the heat from blowing all the time, but I know you actually want cold air now and then.  The 944 and 928 are certainly not the same, but they do come from the same family and era, so I hope my 944 knowledge will transfer over.  So here's what I'll give you:

1.  the trick to stopping hot air in the summer
2.  a test on the #1 reason for the lack of cold air
3.  thoughts on the hot water valve idea
4.  ideas about the temp sensors

1.  You can pull the A/C relay, stop the hot air and not affect anything else.

2.  Open the doors and hood, turn on the A/C, then:

2a.  Make sure you or a buddy "see/hear" the change that occurs with the A/C compressor just before the A/C is turned on and just after.  Inside the pulley for the compressor is a clutch that allows the pulley to rotate  without affecting the compressor when the A/C is off.  When it's on the clutch activates the comprtessor and it starts doing it's thing.  This change is noticeable if you watch for it.  All you want check for is to make sure the compressor IS being activated.

2b.  Locate the sight glass for the A/C system, most likely near a port where freon could be added.  While the A/C is on check in the sight glass...you should see Nothing!  The freon in the system is clear and if there's nothing in the line, or a few bubbles, that's good.  If you see something cloudy or a stream of bubbles, you need freon added.

3.  I assume the hot hose you've found is on the engine side of the firewall (the bulkhead?).  The hose you should be concerned with is the heater hose (close to 5/8") that exits from the water pump.  This carries water to the heater core.  If you find a valve in between the water pump and the heater core, that's your valve...then you want to make sure the hose that carries on into the heater core is not hot when the heater is not on.  Make sure the heat you feel is not from regular engine compartment heat.  Turn on the heater and see if you feel water rushing through the hose (without burning yourself)...and when the heater is turned offf, the water will become a trickle.

If there's no valve before the hose passes through the firewall and into the heater core, then the valve is just as inaccessible as the heater core and you don't want to search it out until the core is leaking and you have to replace it.

4.  There are at least 3 temp senors on your car.  One measures outside air, one measures cockpit air, and one measures the temp of the air just before it is released into the cockpit.  If these three aren't in place, your A/C is probably quite confused!  Unfortunately, my reference material doesn't say where these are.  In the cockpit it's usually behind a tiny little grill and it'll be dashboard height and up.

My favorite reference book series (Haynes) doesn't cover the 928 unfortunately, but maybe you could find a Chilton or similar reference source for the car.  One other idea is to check for internet 928 groups...some would have resources listed and others would be forums to chat about problems like these.  If you'd like, you could use the follow-up feature on this site and give me an update along the way...I'd like to know what actually causes this little nightmare.

I hope you get that beauty back on the road as quickly and as cooly as you can!

Dave