Fans turn off when turning on heat

[jwilcockusu]

My HVAC system turns off when I try to turn on the heat. It works great when running the a/c or cool air, but when I get about midway to the heat, it all shuts off. It doesn't matter if I have it blowing on feet, defrost, main vents or what the fan speed is set at. It still warms the car slightly while it does allow some hot air to flow through the vents, but the car fogs up on the cooler days. Any ideas? My car is a 1990 964.

[peritus]

I had the same problem. The issue ended up being the thermostat on the back of the temp control unit. Pull out the unit and clean it up - hopefully that will solve the problem. It worked for me.

[bcr4d]

I had the same problem, fans run on blue dot, turning up towards the red and front fans would shut down. Defroster wouldn't work either. My problem was a blown fuse, 25 amp, in the engine compartment. Replaced it and the fans are working fine.

[computamedic]

I agree with Billy that the problem is almost certainly the fuse for the rear blower. However, note that Porsche increased the rating of that fuse to 30 amps (see the TSB here) with no other changes to the wiring. The 25 amp fuse will keep blowing due to the high startup current for the rear blower. A 30 amp fuse prevents this problem.

Regards

Dave

[jwilcockusu]

Thanks everyone. It was just the fuse...I just wish they were always that easy.

Best regards,

Jeremy