t 02:47 PM 3/7/02, Lee, Howard S wrote:
>I have a blower problem on the climate control system which I would appreciate your advice to help fix. It was working fine last week. Noticed yesterday that the blower only works when the fan switch is at the highest setting, 4. Does not blow at any other setting. A/C, Heat, Temp and Vent controls seem to be working normally. Rear A/C, blower system seems to be working normally.
>Is this problem due to the documented resistor pack issues? I'm not sure since the maximum blowing isn't intermittent, and still coming on only at setting 4.

Two different resistor pack problems.

The blower motor is powered in two different ways. The speed knob sends power thru different combinations of wire resistors in the resistor pack mounted in the blower plenum under the hood. Top speed bypasses all of the resistors. If the blower does not run on any of the lower speeds, but will run on 4, the problem is usually a broken resistor wire or bad connection in the resistor pack.

The blower motor has an independent power source thru the DEFROST setting. If the blower will not run on any knob speed, including 4, but will run on DEFROST, the problem is probably either the blower relay or fuse.

The "magic blower syndrome" is different. Inside the resistor pack, there is a thermal switch that bypasses the resistors, running the blower at full speed. Apparently, Porsche engineers were concerned about overheating the resistor pack or the plastic plenum where it is mounted. If the blower is on a low speed, with low airflow thru the plenum, and perhaps the blower motor is pulling more than normal amperage due to bad bearings, the resistor pack can overheat. The thermal switch will close, bypassing the resistors, and the blower motor will go to full speed. The increased airflow will cool the resistor pack and the thermal switch, the switch will open, and the blower will go back to a low speed.

if the thermal switch gets too sensitive, the blower will continually be switching to high speed and back by itself, thus the "magic blower syndrome". The cure is to take the resistor pack out and adjust the thermal switch, or to replace the resistor pack.

Wally Plumley
928 Specialists


At 04:38 PM 7/16/2003, John Eifert wrote:
>If your car obviously has the Magic Blower Syndrome, even if the fan is already at the full setting already, does that mean that there is a specific problem with the resistor pack or blower motor? The fan would already be moving at Magic Blower Syndrome speed anyway, if it's working properly. Correct?

"Magic Blower Syndrome" refers to the operation of the safety system built into the HVAC blower system. The blower speeds are controlled by a multi-position switch that routes blower power thru an array of Nichrome wire resistors that is mounted in the blower plenum under the hood.

At low speeds, the resistors generate heat, and if the heat build-up gets excessive, it could overheat the plastic plenum, causing a fire. Porsche put a thermostatic switch in the resistor pack to prevent overheating.

Up thru '88, if the heat build-up in the resistor block gets excessive, the normally-open switch closes, applying full voltage to the fan, bypassing the resistors. The combination of more airflow and no heat generation for the bypassed resistors will quickly reduce the temperature of the resistor pack. The switch opens, and the blower goes back to the set speed.

After '88, the normally-closed switch opens, cutting off blower power completely. After the resistor pack cools, the switch closes, turning the
blower on again.

If your blower works on Defrost, but not on heat or A/C, the problem could be in the resistor pack or in the control head - assuming that the fuse and the blower relay are really OK.

Wally Plumley
928 Specialists


Here's a few tips I learnt you might find helpful to put on your website?

1) Completely remove the rubber piece between the blower and HAVC box; it makes access much easier. Some people says to juts pull it back, but removal is best.

2) Once blower bolts are undone, you have to turn the blower to the right and push it back into the corner on the bulkhead; again it allows access to get your arm into the resistor.

3) Undoing the resistor; pull the plug first and push it down out of the way for access to the lower retaining screw. Then undo the top retaining screw almost all the way first but leave it in, then undo the lower screw undoing the last few threads as not to loose the screw. Then put your arm into where the resistor is and hold it whilst undoing the top screw and then pull the resistor out. Do the reversal to install the new resistor.

4) Be careful putting the fan retaining screws back in; if you drop them into the bulkhead they're near impossible to retrieve. Good idea is to find some new screws as spares just in-case!

5) the blower back in and secure first before putting the rubber sleeve back as you will need the space to fix the upper fan retaining screw.

6) Hoover out what leaves/debri you can whilst things are undone/moved around.

7) COVER both wing tops with a soft towel as you will be leaning over them a lot so scratches are a possibility otherwise!

I'll let you know on the fan controller.

best, Dave.