Pekman

FredR

First thing to check is the passive vacuum system. This takes vacuum from the side of the brake booster nozzle and uses a storage vessel located in the drivers side rear of the front wheel arch. Check this is holding vacuum. If it is, then check that vacuum is getting to the heater valve and it is holding vacuum. The heater valve is located at the back of the inlet manifold on the passenger side. When the car is parked this valve is open. When running the a/c this valve is closed. If this valve is open then the hot air will cancel out any cold air from the a/c system.

There is an excellent aide for troubleshooting the HVAC system written by Wally Plumley. I seem to remember it is available on the website 928GT.com [928 Specialists].

Speedtoys

Like, no fan control or anything? Or just never cool

dr bob

Hi Peter, and welcome to Rennlist!

The guidance shared above is great. Here's some details that may help you further:

-- The failed positions of the vacuum system components include full heat, as the heater control valve fails in the open position. The heater control valve is a vacuum-operated sliding gate valve at the rear of the right-side (passenger side on our left-hand-drive cars) cylinder head. Access the valve by removing the full air cleaner housing at the rear of the engine bay. The valve has black and white plastic housing, and a small diaphragm actuator with a plastic hose and rubber connector. With the temperature slider in the console at the full left (cold) position, the heater control valve should close with the engine running. Vacuum will pull the arm on the little gate towards the actuator to close the valve. The valves are a notorious failure item, either with the actuator failing or the sliding gate inside leaking. Once you go through the trouble of removing the air cleaner housing for a look, it makes sense to just replace the valve and the short section of hose that connects to the cylinder head. Replacement options include a Mercedes-Benz valve that's been more reliable than any of the aftermarket replacements. In the US, our reliable 928 parts suppliers have that valve available, as well as a new Porsche valve. Whether it makes sense to have the valve shipped in from here is up to you, as I know that import duties and VAT can easily exceed the costs of these parts locally.

-- There are several other vacuum-operated flaps and air diverters in the system, including one that controls the comb flap in the center of the dash. The comb flap should be at full-open position when the system is operating in any but heating or defrost modes. You can look through the center vent to see the comb flap move when the engine is running. Replacement can be a challenge, as access requires the center console out of the way, and some delicate blind surgery through the glovebox to get the vacuum actuator linkage apart.

-- Leaks in any actuator can cause all the other actuators to go to failure positions. A small hand vacuum pump is a key diagnostic tool in finding failed actuators in the system. the factory workshop manuals have a very good troubleshooting method for the actuators and the solenoid valves that control them.

jpitman2

If your heat control goves you cool when on full left of the adjustment, but goes full hot when moved off that setting, check the external temp sensor. This is housed in a metal tube in the alternator cooling hose in the left front wheel well, behind the front splash shield. Normally the tube is screwed to the splash shield, which makes it messy to separate without damage. The lead from the sensor has a 2 pin connector on it, which being out in the weather can lose connectivity - check this for clean, rectify, and test your heat control again. Mine was badly corroded, and I had to clean and resolder the pins. See here for the mounting mod I did:-
https://rennlist.com/forums/928-foru...placement.html
jp 83 Euro S AT 57k