GM Passkey 2 operation
I have this 1998 Venture which was towed in as a no start. It starts and runs for 1 second, even when holding the key in the crank position. Scan the pcm, and retrieve code P1629 for theft deterrent fuel enable signal not received. Check the fuel enable circuit at the pcm, and I do have a square wave signal, but it is only about 1 volt, not 5. Unplug the bcm per service manual, and I should have 5 volts there koeo. I have 1 volt koeo, and it goes to 4.8 volts as soon as the key is cycled off, then drops to zero at some point. The diagnostic table condemns the pcm based on this test, but I am always hesitant about condemning them without all the necessary info.
I assume that the pcm is supposed to generate the 5 volts, and the bcm is supposed to toggle it to ground, but I cannot find that info published anywhere. Is that true? Is there any other circuits I should be checking?
A fuel enable signal is bcm Reporting that it has seen the correct passlock voltage signal and the pcm is allowed to start the engine. What data are you getting from the bcm? Does the bcm indicate that the passlock voltage is a valid signal? The ignition lock cylinder produces a specific voltage when rotated through the use of a Hall effect sensor. Have you attempted A relearn yet by turning the ignition to crank then leaving the ignition on for 10 minutes until the security light stops flashing. It’s commmon for the lock cylinders to become worn and produce an incorrect passlock voltage.
None of my scanners have the capability of scanning the bcm. I just checked with my snapon, autel, and otc genisys. I now have no communication with any module, and pcm powers and grounds are good, but both 5 volt reference wires are at 2 volts. Unplug the pcm, and communication returns to abs, and airbag computers. Looks like I have enough info to condemn the pcm.
I’d agree with that. Sounds like pcm pulling bus down. Do you also regain communication with the bcm with the pcm disconnected? I’d be curious to watch he class 2 data on a scope with the pcm plugged in and unplugged. It should toggle 0-7 volts.
Unfortunately none of my scanners even list the bcm. I have an old Ease pc based scan tool which used to support way more gm computers that all others, like alternative fuel modules, but I haven't used it in years, and I don't know where the usb dongle is for it right now.
Sean, I was talking with Martin, the GM instructor in Canada, a few weeks back, when there was a similar case to this on (another site) and there are some early model BCM that do not talk to scan tools. They self-diag through lights and chimes. IIRC he called them "type IV BCM". If you need me to find the emails or the TDF case email me.