My car's been doing this for the past couple of months. The heater and air conditioner only work when I'm accelerating or in motion. When I come to a stop neither work. I did a quick search and it seems like this could be a vacuum issue? Anybody have any ideas on things to check and how to resolve this?

Thanks

 

Go to 938 Specalists at www.928gt.com, then to tips and links on the left hand side column, then click on Wally's World and you will find the HVAC system, the best and most concise description to solve your problem/s.

I sounds that you have a grade 1 vacuum leak from the brake booster vacuum connection to the non return valve, to the splitter to the HVAC system, to the hot water valve and other points. It appears that the leak is greater than the vacuum being generated at idle, however, when running the vacuum is sufficient to overcome the leak and the system operates.

In Wally's write up he also takes you through a logical trouble shooting procedure.

Tails 1990 928S4 Auto

 

just to add to that, if you have a vacuum leak on the hvac circuit this could mean the heater valve is open and thus overcoming the cooling of the AC. I guess it was designed that way for cold climates because the default is heating rather than cooking. You might be lucky and just have a bad check valve or heater actuator, both of which are easy and cheap to fix. Leaks elsewhere are also cheap to fix If you do it yourself) but require more time to solve.

 

Yes. This IS a vacuum leak. There are 5 Vacuum 'pods' that go bad (they only go bad because the rubber diaphram gets old and brittle and doesn't seal. IIRC, Roger(ROG100) has the rubber diagphrm that can be replaced on a couple of them) . These pods control the flaps that open and shut heater control valve, footwell flaps, defrost vents and center air flow vents.

Your next step is to get a hand held vacuum pump and open the hood and find your 4-way vacuum connector (next to the brake booster) and start testing each line. It shouldn't take that many pumps to hold vacuum, if it does you have a bad leak. Each line should hold pressue without the engine on for atleast two minutes. If you see the pressure going down at a slow pace and not holding, then you have found your slow leak. I recently had to replace all of my pods on my 79 so don't be surprised if you see more issues as you diagnose.

Good luck.

 

wouldn't the fan continue to operate even with a vacuum leak? So if the fan stops when the car stops wouldn't that indicate some sort of electrical issue.

 

Yes the fan operates but the flaps are closed or open (depending) due to the lack of vacuum...........preventing the air to flow properly...

 

I noticed the other day that cold air is slowly coming out of the vent, but the fan isn't working so it doesn't blow out and just stays inside the vent. Even when I get the car up to speed it doesn't seem like the fan works. I don't know much about this stuff, but is it possible that the problem is only with the fan? Or is any problem with the fan being caused by the vacuum issue?

Thanks

 

You have identified a problem with the fan, fix the obvious problems first, and from what you have posted the fan is a problem.
Did the fan stop working all at once, or did you lose one speed first?

 

Airspeed dependent HVAC control == windows down That's the way I like to drive, even in the winter. Wear a coat, pull off the rear muffler, and enjoy the ride!

 

Move to Georgia, you will change that attitude in a hurry, all it will take is a few days of 100 plus temps and 95% humidity.

 

I remember even when everything worked, it would usually take about 5 minutes once I started the car before the fan would turn on. As far as I can remember the fan stopped working all at once.

How easy would something like that be to replace myself?

 

With the ignition switch on, lower lever in heat or A/C position, turn fan to highest speed. If the fan will blow in the highest speed, but no other, the problem is probably in the resistor pack (under the hood in the HVAC plenum).

If the fan will not operate in any speed, move the lever to the "DEFROST" position. If the fan blows, the problem is probably a faulty blower relay (Relay XXI).

If the fan will not blow in DEFROST, the problem may be Fuse #17 or its holder. Check the fuse, and carefully clean both ends of the fuse and both ends of the fuse holder.

 

Wally is thinking the same thing I am.

 

The fan doesn't work with the defroster. I changed fuse #17 with a new one and the holder looks good, but the fan still doesn't work. Any other ideas? Could it still be the relay?

Thanks

 

Any more advice? I still haven't heard whether or not this could still be the relay.

Thanks