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I'm having a tough-to-diagnosis problem on a 93 9000 turbo, approx 110k miles. The fuse for the ABS pump blows immediately when the engine turns on, resulting, obviously, in the antilock light coming on.
Following instructions by Saab posted on the excellent Townsend Imports site, I thought it might be a defective diode 303B in the fuse box . This is the diode linking some very hefty black/yellow wiring which eventually heads to the ABS pump.
I checked the diode (actually, two diodes) with the ABS computer disconnected and the battery disconnected, I used a digital VOM with a diode/continuity check function and touched each end of the diode. It showed no flow in one direction, and approx 50 percent voltage drop in the other -- just like it's supposed to.
Next, I tried various combinations of things plugged/unplugged to try to narrow down the possibilities. In all of the following scenarios, the ABS computer is connected:
1. Both electrical connectors to the brake master cylinder/pump, etc. are disconnected. In this setup, the diode circuit continues to be fine and the fuse does NOT blow when the ignition is turned on. (To me, this suggests that a bad pump is NOT the cause of the fuse blowing, since the pump ain't connected.)
2. When the bigger connector (lots of small wires) is disconnected and the main pump circuit (little connector -- two hefty yellow and black wires) is connected, the diode circuit shows continuity in both directions (bad?). When the engine is started, it blows the fuse (definitely bad!). This would suggest a problem in the main pump circuit. However ...
3. When the bigger connector (multiple wires is connected) and the main pump circuit is disconnected, the diode circuit checks out normal. HOWEVER, when I start the engine, it still blows the fuse.
At this point, I'm kind of baffled. I have not gone tracing wheel sensors, etc., yet, because I don't think they'd make the main fuse blow.
BTW, the other fuses are fine. And the regular brakes are fine. And the TCS is fine.
Thanks for any help.
Graham
posted by 68.100.205...
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