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GM Passlock Electrical Wiring Short
Robo Tech was called to a shop on a 2003 Chevy Van G 25 00, with a 4.8 liter engine, experiencing
a security light on followed by a no start situation. For some reason, the shop replaced
the crank sensor. Because is was a no start, they reason it was probably the crank sensor,
and of course, it wasn't. They eventually saw the security light on, and figured out
that it was a Pass-Lock system issue. They next replaced the Pass-Lock key cylinder,
with no luck either. At this time, the shop decided to call me in for assistance. They
simply didn't have to scan tool to access the B C M or body module for this problem.
When I arrived at the shop, the tech gave me a briefing on the vehicle, and also said
that the vehicle would start once in a blue moon. Ok, that was probably some kind of clue.
I started my diagnostic by performing a general code analysis. I did get code P16 32, or Theft
Deterrent Fuel Disable Signal Received from the B C M. There were no other codes present.
I knew it wasn't the key cylinder, since it was already replaced. There are steps to take
in all Pass Lock diagnostics before condemning a component.
In this particular GM, the signal from the ignition system is controlled by a resistor
embedded in the Pass Lock module, which is part of the ignition switch cylinder. In these
systems, any key that will allow the cylinder to be turned to the on position will work,
the only condition that must be met is that the cylinder must be turned to a position
where a magnet engages a hall effect switch, which connects the embedded resistor to the
system, allowing the Body Control Module to read the signal value, on the yellow Pin A6
wire, and determine if the resistance value is correct.
I analyzed the Pass Lock calibration and determined that it was correct. So, why was this car
not starting? I started to suspect an electrical or connector fault. So I decided to visually
inspect the Pass Lock module and wiring. Everything looked normal, so I then decided to concentrate
on the B C M. As I scanned the B C M data stream for clues, I saw that all of a sudden
I lost communications.
Was the ECM defective? Or, was it an issue with the vehicle network? The fastest way
to determine this was to query the other modules to check for any U codes. Sure enough, all
the other modules had a loss of B C M communication code. This pointed to a B C M communication
issue, either due to the B C M itself or the wiring.
I then decided to grab the oscilloscope to do a quick check on the GM class 2 network.
I connected the scope to the network wire and ground. I saw that the network was alive,
while the B C M was not communication. It is impossible to detect a specific module
on the network using the scope alone. The digital signals are unrecognizable from each
other. But, this was enough for me. I knew then that the issue was probably at the B
C M. So, I searched and uncovered the B C M for testing.
I started to pull the glove box to make a close inspection of the B C M harness, to
make sure it was properly connected. Then, I saw the problem at hand. The B C M had come
in contact with the dash support bar and shorted wires pertaining to the pass-lock circuit.
This was the reason why the B C M was shorting out and dropping from the network. The scope
did not reveal the signal issue because it was simply shorted to ground, but it did reveal
that the rest of the network was alive and well. Often times, you do a combination of
scanner and scope procedures to get the the bottom of the problem. Is not how big the
tool is? It's how you use it.