Porsche Enthusiasts Club Forum

I thought my recently repaired aircon had failed already, but actually I've found that the heater is stuck on hot. The distribution system is working OK (the air can be directed up, down, or to the centre with the lower slider) but the temp slider does nothing.
This has happened since I last used the car. What could be causing this fault? Could the cold weather have anything to do with it? The temperature in my garage is only 5 degrees centigrade.

Hi

That fault is usually caused by 1 of the temp sensors (it was on mine)

2 sensors...

outside sensor in alternator cooling hose that takes air to rear of alternator...

inside sensor , behind little grill thing near middle of dash ...

Outer sensor is easy to change....

Inner one has a little fan that SUCKS air into it and its an ABSOLUTE pig to change and its usually the little fan that fails...

The way you test is to get a cigarette and blow smoke towards it and see if its sucked in....lol

The workshop manual actual say precisely that

All the best Brett

There are multiple faults which could give rise to heater stuck on hot but it's easy to narrow down the culprit by following procedure in workshop manual.

First step is to disconnect the connector on the setting motor (lives above your left knee - need to remove driver shelf to get access). Measuring the resistance across two of the pins as you adjust temp slider will tell you if the setting motor is getting a correct signal for demanded temperature.

If the resistance reading is within spec then problem lies with setting motor/link to flap which mixes hot/cold air.

If reading is out of spec then need more simple tests to see if internal sensor, external sensor or the control unit itself is faulty.

All fully explained in the Heating volume of the workshop manual. Do you have a copy? John Speake can get you a legit copy of the manual CD which was scanned by Jim Morehouse in the states. Also contains wiring diagram and loads of technical service bulletins.

I think Brett posted while I was writing last post!

He is very likely correct in suspecting internal or external sensor.

I recently replaced internal sensor and fan (both were fubar) and now appear to have fully working climate control.

Only thing I'd add is that if the cabin sensor fan fails that will just cause very sluggish behaviour of the heating system, though I suppose that could be confused with a heater stuck on hot. The internal system consists of a thermistor and a separate fan to draw cabin air over it - the thermistor can fail/go out of spec which was my experience, but fan could still work fine.

Changing the internal sensor requires removal of centre console or almost complete removal (I found it easier with whole thing off and sitting on my desk!). Not necessarily pig of job but fiddly.

Hi Adrian

Mine had the same symptoms a few years ago, i.e the damn thing would ONLY give me hot air no matter where I put the slider....

So i changed the outside sensor which was the easiest one....of course it wasn't that lol

Then changed the inside sensor and mini fan....and all worked again <<<< I made that sound a LOT easier than it actually was, I seem to remember a 10 or maybe 8 mm bolt that I could only turn around a 16th of a turn at a time, and with each turn I lost some skin from my knuckles

All the best Brett

Hi Brett,

Sounds like we had same problem.

In my case fan motor hardly turned, but real problem was the internal sensor was about 1500 ohm high out of spec so the signal to setting motor was always at full heat level at top of resistance range (even with temp control at minimum).

Wonder if the knuckle skinner was the pesky bolt behind grommet in glovebox, which secures top left corner of console? That's the most awkward bolt (actually a self-tapper i think) as very deep so easy to lose when it's free!

I just fitted a new internal sensor, mainly because the outer part was broken, possibly where somebody had tried to lever it out of the dash. The motor behind was fine.
I'll have a look at the external one as that's an easy job. However I thought there was supposed to be a microswitch that gave you cold, or ambient air by default when the temp slider was on full cold? So even with faulty sensors you should get cold/ambient air, but no graduation to full hot?

That rules out internal sensor anyway. I always wondered why so many had chipped outer parts and thought it was people trying to remove them the wrong way. Some may be but a lot get cracked because of expansion of the dash in sunlight apparently.

Yes there is a microswitch but lever must be pushed firmly left to operate it. Check the resistance of total sensor string at setting motor first. Once you have readings for this then look at individual items including the controller where you can test both slider and the microswitch at the long side connector.

Did you remove console entirely to replace internal sensor? Checking the controller doesn't require you to remove console at all, just remove trim piece around it and undo the 6 screws holding in controller and the aircon switch strip below.

Or as you know internal sensor ok (almost certainly - unless connector was left loose?) then you could test external sensor at connector under centre console by removing driver carpet side of console, and controller by removing it as mentioned.

Do look at setting motor first though to see if it moves at all. The hvac is complex but logical so if you're methodical you'll track fault down.

I've looked at this, and the heater is not actually "stuck on hot" at all....as the temperature in my garage is only about 4 degrees C the internal sensor is telling the heater to warm the car up. I know this as I put my hand over the sensor to channel some warm air into it from the vent next to it, there was the sound like flaps moving from behind the dash, and the heater started to blow cold.
I assume that when the interior is warmed up this will happen automatically.
This means I guess that the only fault is with the ambient air override microswitch, (heat control fully over to the left) which is clearly not working.

Sounds like int sensor ok then Lightning. Is it's fan also ok (blow smoke or just listen for whirr).

Hvac controller v easy to remove without messing with console removal. Just h-shaped trim piece and a handful of bolts. Once you pull it out a few inches you can reach round and undo the two edge connectors. With controller freed you can then check the microswitch with any continuity tester but if you have meter you can confirm temp controller moves through spec resistance range.

If you don't have workshop manual (was it you or Sidbounce who found microfiche version?) then I can quickly look up the pin numbers you should check for microswitch test.

Oh a d merry Xmas!

An update on this, I found that the little motor behind the sensor on the dash was not running, which means that the climate control is slow to respond.
It was working when I had the console out to replace the sensor, so there was nothing for it but to remove the console again. I found that the motor was jammed against a horzontal metal support behind the dash, and this was stopping it from rotating....but only when the console was pressed fully into position, half an inch out and the motor turned.
Somebody had clearly had this trouble before, as 2 alternative motor mounting holes were present on the back of the console, about a half inch from the originals. Moving the motor solved the problem.
I guess I must not have noticed this when I dismantled it last time, and refitted the motor in the original factory position.

Good to hear all resolved now Lightning.

Lucky wasn't the motor - mine cost about £110.

At least you are now pretty quick at removing/installing centre console...

And only one part left over this time! Last time I had a washer/screw, a spring clip, and a plastic cable clamp. This time it was just a plastic spacer.

Racking my memory about the spacer. You could be in luck to replace it easily, as I think it just fits on leg of the centre vent which pulls out from top of console. Makes it a snug fit in console (these are well-engineered cars aren't they?

I think it's almost an MOT requirement that 928 ashtrays and centre armrest hold a few odd screws etc from previous work around the console!

My armrest and ash tray is full of screws too!