rsathre

Hi everyone. I'm new to this forum. Actually I'm new to porsche ownership.

I have a 1990 C4 Cab

Intermittently the 30 amp fuse for the fan blower keeps blowing. I replace the fuse, and it works fine. One thing that seems to make the fuse blow is trying to turn the system off when the key is in accessory position rather than when the engine is running. Any ideas?

Raghu

Rocket Rob

How comfortable are you at working on your car? I would want you to remove the rear blower to inspect the motor's bearings. I suspect yours is getting iffy and occasionally blowing the fuse. If this is the case, you can replace it. To remove the blower is pretty easy (less than 15 minutes). You will need a screw driver to remove a few hose clamps and a 10mm socket & ratchet to remove the blower itself.

Bertrand

First, what exactly do you mean by 'trying to turn the system off'?
So the key is in the accessory position. Then what do you do, step by step?

rsathre

Lets say, the fan motor switch is in position 1, 2, or 3 (not 0) . I turn the key to accessory position 1 or 2. Before I crank, I try to turn the fan speed switch to 0. —> blown fuse.

The other time it blew was when fan motor switch was in position 1, I cranked the engine, motor did not immediately rank, and I recranked the ignition…blown fuse

max3.2

the 30A fuses the ballast and the fan, right? so check those. pull the ballast and see if it got any cracks. if its still in position...
then proceed to the fan as Rob said...if the bearing is gone - and they do fade - the electric motor can short circuit itself. especially when loosing rpm (which actually stabilizes the rotor, like a bicyle wheel.. ), it can tumble and hit the wall...so turning it off and cranking the engine (power loss...) might be a hint

Bertrand

Puzzling. I too would see the electric motor as the prime suspect. As you can see below, there is not much between the 30A and earth, so I too would check the ballast resistor and perhaps the relay (see if there is no arcing trace - a shot in the dark really), and if all is fine, check the electric motor.

After 22 years the bearings of the motor are likely to be worn, and as said above it can lead to blocking.

See https://rennlist.com/forums/964-foru...wer-motor.html for the reference of the motor.

ToreB

Maybe this could explain a bit of the strange fuse blowing behaviour:
The rear fan is supposed to run at full speed when the engine is stopped (and ignition is off) to help cooling the engine. This depends on the engine and ambient temperature. Sometimes my rear fan also starts blowing at full speed if I turn on the ignition again after I've stopped at a gas station or something. I guess this is the same cooling function. Trying to run the rear fan at full speed with a damaged(shorted) motor will blow the fuse immediately.
The rear fan also helps pushing heated air to the cabin in normal operation. However, the behaviour depend on the temperature and fan speed settings on the CCU. Trying to run a shorted fan motor at low speed could cause excessive heating of the ballast (series) resistor, and it will eventually cut the power due to the internal bimetallic fuse, and turn it on again when it cools. The 30A fuse might stand up to the current through the ballast resistor.
Cheers,
Tore

rsathre

How do I check the ballast. I found several links on changing the blower motor.

ToreB

It's fairly easy to check the ballast. Pull it out from the air hose, disconnect the two wires and check the resistor assembly with an ohmmeter. A failed bimetal switch in the ballast will disconnect the resistor and you will read an open circuit.
Cheers,
Tore

rsathre

How do you check if the ballast for rear blower motor is functional, or needs to be replaced?

ToreB

Why do you ask the same question again rsathre? Use an ohmmeter, or do the test described on my DIY page.
Cheers,
Tore

rsathre

I used the ohmeter and found the old resistor to be defective. I changed the resistor, and everything worked great for about 2 weeks and then fuse blew again. I am under the assumption that the blower motor was changed because, I see some duct tame on the discharge shroud, so it looks like someone used the longer mounting bolts which is the replacement blower. Should I change the relays, or just go straight and replace the blower motor?