blowing rear heater blower fuse

cwus · 10-24-2009, 05:33 PM

I am blowing the fuse for the rear heater blower on my 89 C4. It happens when I turn the climate control anywhere except full cold. I just had the climate control rebuilt (for unrelated problems). Is the motor the likely cause? If so, I can buy the motor, or the full assembly. Why would I need the full assembly?

computamedic · 10-25-2009, 06:11 AM

Make sure you are replacing the fuse with a 30 amp part rather than the 25 amp part referred to on the label. Porsche uprated the fuse to 30 amps early in the life of the 964 but your '89 car will still have the 25 amp part listed.

The blower takes more current than expected and will blow a 25 amp fuse quite readily. Replacing the fuse with a 30 amp part resolves the problem.

If a 30 amp fuse still blows it's likely that the motor bearing has worn and causes the motor to overload when it starts. You can replace just the motor if necessary. There have been several threads on Rennlist on this subject.

Regards

Dave

cwus · 10-25-2009, 06:50 AM

Do I dare trying a 40A fuse, assuming the motor will run, but needs more starting current?

cwus · 10-25-2009, 11:36 AM

and, what is the function of the resistor in the circuit? Can it cause a fuse to blow?

computamedic · 10-25-2009, 01:24 PM

I think it would be a foolish idea to "try" a 40 amp fuse. It's likely to find the next weak point in the system - such as the wiring. Some risk of a fire or charred wiring.

The function of the resistor in the circuit is to provide a means for the blower to run at slow speed. With the resistor short circuited the blower runs at high speed, in circuit it runs at slow speed. It is essential that the resistor is installed in the blower plenum in order to cool it - otherwise there's another fire risk.

The ballast resistor was changed during the life of the 964 to incorporate a safety switch but I see no reason why it would cause a fuse to blow.

Regards

Dave

cwus · 10-25-2009, 02:14 PM

Thanks! I have had 30A fuses in the circuit for some time.

I blew a 30A fuse 2 different times, but tried a 3rd, and so far have not blown it. I guess my motor is "on the edge"?!

I think I blew one of the fuses when I used the defrost button, and did not blow it using the fan control turning the speed up gradually. does that make sense?

computamedic · 10-25-2009, 10:55 PM

It's likely that using "Defrost" will turn on the rear blower on high speed whereas the nore normal use will only use slow speed. There is likely to be some extra load when starting on high speed so that may explain the blown fuse.

Regards

Dave

cwus · 10-26-2009, 10:40 AM

Well, I blew my 3rd fuse today, so will replace the motor. It looks like the housing is relatively easy to remove.

Thanks for your help!

Regards,
Charles