Posted to Technical Tips Forum on 2/14/2016
2004 Trailblazer 4.2l Blown PCM Sensor Ground
Another shop called me a while back. They had a PCM that was
generating high voltage codes for both of the APP's, one of
the TPS's, the iAT (plus a few more). I had them test and
the PCM wasn't supplying a ground for all of these inputs.
By temporarily bypassing the PCM's ground the vehicle
functioned normally. Once they proved that the PCM was bad
they had me program the replacement and set up the security.
I stressed concern that something didn't feel right and this
may well have been caused and didn't just happen. They
assured me that they looked and didn't find anything.
A week later they called back, same problem.
Upon arriving I retested and confirmed that the ground
circuit had failed inside the PCM. Opening it up, it was
easy to see the trace that was burnt off of the board. It
was also easy to repair with a jumper soldered into place.
That left the question as to "why"? Worse yet how can we
find it without tearing the whole car apart. It turns out
this ground path affected five ground circuits. Somehow
power was being applied to one of them. To find it
efficiently was going to take some creativity.
All five grounds involved were labeled and removed from the
PCM connector. They were then jumped to ground through a
fused jumper. Now this would work with one current probe but
it would be more tedious and would require first testing a
group of three and then the other two in hopes of capturing
a current spike. Having three current probes allowed for the
The grounds were numbered 1-5.
Grounds 1,2,3 were captured with one current probe connected
to channel A.
Grounds 3,4,5 were captured with one current probe connected
to channel B.
Grounds 2,3,4 were captured with one current probe connected
to channel C.
Making a logic table a spike on only channel "A" was ground
#1, on channel "B" was #5.
A spike on two channels "A & C" was ground #2, and "B & C"
A spike on all three channels was ground #3.
The spike occurred on B and C indicating ground circuit #4
which was the AC pressure transducer. The idea of course at
this point was to identify the affected circuit so that only
it had to be closely examined. It was notable that the spike
only occurred when hitting bumps, under load with the
headlights turned on. What was found was a parking light
feed wire that was sticking out of its harness and was
touching the pressure transducer harness where it exited the
engine harness. These two wires, the parking light and the
AC Pressure sensor ground both had a tiny spot abraded where
one strand of the wire was visible and when they touched was
putting 12v into the sensor ground circuit for the PCM.
I hope you never need this routine, but if you do I hope it
John from Pennsylvania
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16 Replies Received