Heater & Recirculation fault finding

Eiger · Oct 13, 2009

I have two issues (fan & blower works):

1) When I push the recirculation button on the dash nothing happens - although it appears as if recirculation is ON - as the fresh air inlet flaps are up/closed when the car is running.
2) Turning the heat dial doesn't introduce heat into the cabin (ie not broken clip)

I checked both solenoids by connecting directly to the battery - both work (ie allow air to flow through them when powered) - and when connected up they seem to be allowing air to flow in the vacuum system. So this solenoid must be powered by default correct?

There is vacuum on the supply feed from the engine bay, and through to the valves on the inlet flaps - if I disconnect the hose at the inlet valve the flaps open. Presumably pushing the recirculation switch to OFF (ie out by default) should disengage the solenoid and break the vacuum allowing the flaps to open for fresh air.

I've kept the supply vacuum feed on the Y joint that feeds to both solenoids but swapped the other two hoses around - but no change in the state of the system.

I noticed the heater water valve on the rear of the engine doesn't move and stays in the "AUF" position ie link pulled out. Turning the heat dial doesn't change that position and I assume doesn't allow hot water to flow in the Matrix/Heat Exchanger and heat the cabin.

Under the dash the solenoids share a wiring plug with four connectors. There appears to be a pair of brown thin wires and another pair. Which solenoid should the brown thin wires control (ie the recirculation or heat function)? If I know this then I can connect the hoses to the correct solenoid.

Pushing the recirculation button on the dash doesn't make either solenoid click, nor does turning the heat dial. Perhaps someone could check theirs and let me know if you hear a click in a solenoid when you push the button or turn the dial to heat.

Many thanks if you can assist with this fault finding - it was 4 degrees this morning!

Riverside · Oct 13, 2009

What happens when you press the windscreen demist button? The demist button should set the fan to max, heat to max, close the footwell vent, open the winscreen vent and on cars without aircon open the recirculate flap to allow fresh air in (on cars with aircon it closes the recirculate flap & switches on the aircon). All of these settings are hardwired as a failsafe in case the HVAC computer fails so barring a physical fault it should always work & is therefore a good test to see whether the fault is electronic or physical. If the windscreen demist button works as it should your hardware is fine & you have a faulty sensor or heater control unit. If the fault persists it's a hardware fault. On a 20+ year old car it could be anything and it could be more than one thing. There is an excellent fault diagnostic procedure to follow in the Porsche workshop manual downloadable from [link=http://www.promax.com]www.promax.com[/link] (other sources are available) in section 87 (Vol3-chassis).

Eiger · Oct 20, 2009

ok I got my 944 back yesterday and was able to try the DEMIST button - and nothing happens. What kind of hardware fault do you suggest looking into? I downloaded the manual but I don't find a heater diagnostic procedure only an air-conditioning fault finding - I don't have AC. I also checked the "clips" in the passenger footwell - both the temperature and mixing valve are intact - but neither move when the heat dial is turned or anything else on the control unit. Not the sliders, the demist or recirculate buttons. No clicks whirs around the HVAC unit under the dash at all. I even tried pushing on the little arms to get some movement but they are solid.

In fact it seems like the only function working on the control unit is the fan blower speed dial. Is it possible that the control unit has failed for all functions except the heater blower speed? Any ideas folks.

Riverside · Oct 20, 2009

ORIGINAL: Eiger ok I got my 944 back yesterday and was able to try the DEMIST button - and nothing happens. What kind of hardware fault do you suggest looking into? I downloaded the manual but I don't find a heater diagnostic procedure only an air-conditioning fault finding - I don't have AC.

The diagnostic procedure in the manual is for all varaints - if you don't have a particular component you just skip that bit. I suggest you read through all of section 87, if you don't feel able to carry it out yourself take it to a specialist who will be happy to go through exactly the same diagnostic procedure & charge you by the hour.

Eiger · Dec 18, 2009

Turns out the dash control unit was faulty - I fitted a replacement bought of eBay and it's all working. Toasty.

moriarty · Feb 6, 2017

I have a 911, (996) 2004, to prevent misting i have to have the air con on manual and on low (so its freezing) fans work fine but on some setting i get a mist from the air vents and an unpleasant smell, any ideas?