Vehicle:
-95 GM Lumina APV Van with 3800 (3.8L VIN L)
General Symptoms:
-Intermittent loss of alternator/generator charging (particularly when hot right after startup or unplugging and then plugging in the generator control connector). -Rare erratic shifting and PCM undervoltage code.
Specific symptoms all the time (even when generator working):
-Voltage at "L" terminal, when disconnected from generator, ~30mV when ignition in 'RUN' position and ~10.5V when engine running. Should be B+ (~12.5V) in both cases.
-Voltage at "L" terminal, when connected to generator, ~30mV when ignition 'ON' and ~11V when engine running. Should be ~1V and B+ (~13-14V) respectively.
Specific symptoms during failure (engine running, generator not working because control voltage to low or erratic):
-Voltage at "L" terminal, when connected to generator, highly erratic on scope (smear from 0 to 10.5V for 50ms and 10V for 10ms or constantly 0 to 8V) averaging 3-6VDC. Sometimes (very rarely) it is stable at 9-10V that still does not allow the regulator to turn on the alternator.
-Curiously; revving the engine usually kicks it out of this failure mode into the ~11V generator control output.
-If driving and generator failing consistently this is sometimes accompanied by erratic shifting or PCM going into limp mode or PCM undervoltage code. This is probably when, due to loads and the alternator not working, the system voltage drops to low.
Diagnostic Details:
-Generator/Alternator good; replaced unnecessarily :-( -C110 OK -Generator connector (PLF/IS) OK -CKT225 OK (Good impedance, no short to ground, no voltage drop from PCM to generator; ie. if it is 10.5V at the generator it is 10.5V at PCM). -PCM voltage good, fusible links OK, etc. -PCM ground good at G105 (checked as per TSB 23-81-06) -Many other PCM outputs that should be at B+ checked and are good. -PCM swapped with loaner from dealer with no change :-(
Discussion:
I have been over the TSBs and large portions of the service manual and am rather at a loss here. Something is pulling or allowing the generator control voltage to go down. The PCM power and ground is good and I have disconnected the blue connector (red and white have to stay in since they have power and ground) and as many sensors/outputs as I can get at to see if one of them is dragging the output down; but to no avail. A few remaining, long shot, possibilities include:
-Second, test, PCM also bad. -PROM screwy -Some sensor/output that I missed is dragging it down
This is similar to this problem that I found on the net: