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Old 03-25-2003, 07:57 PM
BruceT
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Fan speeds

I have some work going on with my a/c at the moment (a second re-built compressor is being fitted - under warranty).

In the process of finding out why the first one failed, the a/c people said I needed to replace the tx valve (which is inside the car, in the box above the passengers feet). They replaced that, and found it didn't fix the problem, but now I only had 2 fan speeds.
They took the car back to take the faulty compressor out, and spent half a day trying to fix the fan speeds, and said it looked like some control module needed replacing.
I took the car back, and looked through this site (Beav has some great gif's). The fan speed is controlled by a huge 30 amp, 100 volt transistor, and I thought if that had blown they would have known all about it. This transistor is mounted on a finned heatsink, which is mounted through the wall of the evaporator housing in the footwell, and uses the airflow in this housing to cool the transistor. If you look under the glovebox it has a 3 wire connector plugging into it.
As usual with power transistors, if they are bolted directly to the heatsink the heatsink ends up live (it's got power on it). I found directly behind the heatsink was the tx valve, and the grounded metal capillary tube from it was touching the heatsink, shorting out the transistor. Simply pushing the tube away fixed this, and I have a 3 speed fan again!
The transistor is on the ground side of the fan motor, so shorting it out merely turned the fan on, and didn't damage the transistor.

Note, in tracing parts I found the same module is used in Saab cars (look at Saabnet.com, they have some good threads on repairing this module).

Bruce
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Old 03-27-2003, 05:04 AM
svx_commuter's Avatar
svx_commuter svx_commuter is offline
Making tires round, Six now :)
 
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Location: North Jersey
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Hi Bruce,

Thanks for the info. It's nice to have this info after you have found the cause. Makes it easy for the rest of us.
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