Montana 911

My searches have not yielded a direct hit to this question, so my apologies if it is redundant:

I just bought a 1991 Carrera 4 with low(er) mileage. The heater fan works for about two or three seconds when the car is first turned on (or when turned to the "on" position before cranking the engine). Then it shuts off. I can feel heat coming from the vents so the heater must work. Once the fan stayed on, but I have not been so fortunate since. Could this be an issue with dirty contacts in the climate control unit? The prior owner (for two years) claimed he never used the heater or a/c because he only drove the car in moderate weather.

Thanks for your help. Please note I live in Montana and heat would be nice this time of the year!

HoBoJoe

I had a similar problem with my 89 C4, no blower once the temperature dial was turned to warm. My issue was the fuse for the rear blower motor. Does the car blow air when set to cool? Check your relays and fuses first, then search "heat servo" next.

Computamedic

The rear blower creates the pressure differential to open the flaps allowing the hot air to reach the mixing chambers. If the rear blower doesn't run - for whatever reason - the front fans will stop after a few seconds.

You can check the rear blower by setting the temperature towards the red dot - the rear blower should start up immediately on slow speed. If it doesn't, check the fuse (MUST be 30 amp), check the relay (can swap oil cooler relay to check) and check the ballast resistor. The later type ballast resistor has a thermal cutout that requires a manual reset. It's clipped into the blower plenum on the left opposite the fuse panel in the engine compartment.

Although the CCU is known to have it's problems it's not the most common area of fault.

Regards

Dave

Montana 911

Thanks for your replies and the information. Upon further inspection last night, it does blow air when the dial is set to "cold." I think there were a few threads that detail the problem and possible solutions. But first I will start with the areas you mentioned, Dave.

--Andrew

Gelignite

I have this problem and it is the rear blower fan. It kept blowing fuses and when I replaced the fuse everything was fine for a while until the fuse blew again. Unfortunately this time it has melted the whole fuse holder, probably due to a duff fuse. I suspect the motor is shot, either bearings or bushes, which seems to be a fairly common problem now.

andy73

I have the same issue so this is a helpful thread. For me, this problem began occurring shortly after replacing the blower motor with a blower motor bypass pipe. For the first few weeks / drives the fan still worked fine but no only blows when I initially turn on the ignition. shall try a few of these checks too.

titleistaddict87

Checkout Tore’s site at BergvilleFX. Great troubleshooting

ToreB

Not a problem with the CCU, but with the rear fan. If this does not work, the CCU will shut down thw two front blower fans.
Details here: https://www.bergvillfx.com/porschehvac1
DIY test here: https://www.bergvillfx.com/index.php...on-system-test

Cheers,
Tore

andy73

great info, thanks Tore. May sound like a silly question but is the rear fan fuse the same as the LH & RH front blower fan fuse? Just can’t see a specific reference to it in the fuse diagram.

ToreB

Rear fan fuse is in the engine bay fuse box.
T

andy73

Got it. Thank you.

Pickleshenry

Hello! I have a 1990 964 - my problem is that all fans shut off unless on coldest/blue setting via the temperature selector on the dashboard. Can't get the fans to run when I call for heat.
I checked the blower fuse in the rear fuse panel and it is good. But there is a connector hanging out there, the white plug in the middle/right of the first image, and another connector outside the fuse panel. Is this my problem, and where should these be plugged in?
Many thanks!

systemsc

The 964 CCU is very reliable and rarely fails. Most likely you have a wiring problem in the engine compartment, e.g. the fan resistor.