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clarification
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Posted by DaveW (more from DaveW) on Tue, 19 Nov 2002 12:17:59 Share Post by Email
In Reply to: Sorry dane... (long), Aero9k, Mon, 18 Nov 2002 23:21:53
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Just so I don't confuse anybody by some of my misuse of the terms here...pin 2 of the BPC is connected to 12 VDC (which is the +54 bus in the schematic). I have the Trionic manual for '93, and that line is not fused on the '93.
Most likely you have either a shorted BPC valve and/or a bad output transistor in the ECU. The bad BPC you USUALLY would diagnose by swapping it with a known good one. But - in this case, it sounds like your ECU might have been damaged, in which case you don't want to just try somebody else's BPC, because you'll then have to buy them a new one too! Try these steps:

1. Double check the resistance of the BPC with the cable disconnected, being careful not to short the test probes to each other. If you get about 3 ohms for each winding (pins 1-2 & 2-3), the BPC is ELECTRICALLY ok. If you get either a much higher or lower resistance, it is bad. If you get close to 0 ohms, you definitely do not want to use that BPC with any other ECU, as it will damage the ECU. Even if it is electrically good, the valve itself might be seized.

2. Depending on whether you have an autoranging meter (I'm assuming you're using a good digital VOM), select a high-resistance range. Disconnect the ground strap to the battery, wait a few minutes, and measure the resistance between pin 1 & ground and pin 3 & ground on the connector cable (cable disconnected from the BPC valve, meter connected to a good clean ground on the engine or body). You should get greater than 1 MegOhm (it may read as infinite, or open). If the ECU output driver (transistor) is shorted, you'll get close to 0 ohms. If this is the case, you don't want to just try another BPC, as you'll damage it - you'll also need to get an ECU. I think a used ECU should be about $250. Note that some people will tell you NOT to try this test, as it's possible to damage a good ECU if you're not careful. Just be sure to discharge your body to the engine block before handling anything, and try not to touch the connector pins with your fingers.

3. The test I mentioned in a previous post which uses a scope or dwell meter & a resistor should only be done if everything else checks ok. In your case, pin 1 on the connector would be connected to pin 1 of the cable with a jumper wire (insulated, with a small insulated alligator clip on one end, so you don't short anything out), pin 2 to pin 2 again with an insulated jumper, and pin 3 to pin 3 using a 3 ohm resistor instead of a wire. With the engine running, varying the rpm a bit, depending on your choice of instruments you should see: across the resistor a slightly varying DC voltage that is less that 6 VDC; a varying PWM waveform of about 12V peak-to-peak; a varying dwell reading. If all of the above check ok, then everything is ok electrically - but you can still have a stuck BPC valve, or some other reason (TCS, cruise?) that the ECU is holding it to base boost. At this point, you could then check the BPC off-car with a specially built test rig which would provide a differential waveform to the valve. In my case I just tested it by providing current-limited 12VDC for a VERY limited time (much less than 1s), as you can easily overheat the solenoid coil and damage it permanently.

posted by 169.207.1...
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