heater only heats on demist button

captainsmelly · Sep 9, 2013

Now that it's no longer summertime, I really do need to sort my heater issue out.

Has only blown hot air with the demist button since last winter, before that the temp knob did work although I found that the temperature would rise and fall with hard cornering.

The fan knob does work on all settings, but heat is only cold regardless of knob position. The demist button however is seriously toasty. Love that button!

Where should I start?

Riverside · Sep 9, 2013

There are three thermistors: one in the centre face vents, one at the end of a tube behind the small round vent next to the cigar lighter, and one on the drivers side fan inlet (under the bonnet).

The inlet one measures ambient temp, the one in the face vents measures output temp & the one in the dash measures cabin temp. I would guess one of the in-car ones has broken & the computer thinks the interior is warmer than it actually is.

Parts from Porsche are expensive, I fixed mine years ago with a soldering iron & a few pence worth of thermistors & resistors from the local hardware store.

hth.

Paul 290T · Sep 9, 2013

I believe the Factory manual lists the resistance vs temperature for each thermistor. You could find the bad one with a multimeter, a hair-dryer and the use of your fridge. When I had the fan inlet thermistor fail i think the resistance was ok sometimes, but it went off the scale when I took it up to summer temperatures with the hair-dryer.

captainsmelly · Sep 17, 2013

Thanks, I shall have a tinker when I get some time. Doesn't sound so tricky at all. Are the thermistors easy to access? Or is it a dash strip out jobby?

Riverside · Sep 17, 2013

You need to remove the glovebox for the interior one, the face vents/instrument surround for the outlet one & just remove the rubber bulkhead strip & bend the heater motor/windscreen wiper cover out of the way for the inlet one under the bonnet.

To follow the diagnostic procedure in the manual you'll need to remove the heater controls too. Again not that hard but be very, very careful not to snap any of the trim surrounding it as it's quite hard to find good replacements.