PCM Alternator Field Coil Current
Posted to Technical Discussion Forum on 3/9/2013
23 Replies
My question I was hoping someone here could answer is: What
should the Alternator Field Coil (PCM regulated) total amps
be?
Recently I was called out to change an Alternator on a 96
Dodge B2500 Van, the symptom was "does not charge". Being
that I was unfamiliar with this design (and still waiting
for my 501c Hi Amp Clamp), I simply checked the voltage at
the battery (engine running) which revealed low volts
(11.4V). I removed the Alternator and took it to AutoZone
where it tested good. The next day I pulled a wiring
diagram, stuffed the ALT back in, and this is when i noticed
bad battery + and - connections were dirty and probably just
loose all along. After cleaning terminals and tightening
them up the problem was solved.
I wanted to be certain though, so I checked the the signal
at the PCM, great looking on/off square wave. Duty Cycle was
about 56% with accessories off and 99% with all accessories
on. Charging around 14V. Hooking up a Solus Pro, I checked
the Desired Volts of the PCM, and it matched the 14V I was
seeing.
I then hooked a low amp clamp (on the 20 amp scale) around
the PCM regulating wire. Being that it goes through a 30A
fuse, I expected it to be around 8 to 10 Amps. The wire is
14-16 AWG, so I know it cant be any more than 15A. What i
seen on the scope was current that oscillated a little, but
was also .6A during 56% Duty Cycle and 1.6A when the PCM was
at 99% Duty Cycle.
That seems like a very low amperage total. Am I looking at a
failing field coil?
About the circuit: From the Battery through a 30A Maxi Fuse,
then through the ASD Relay where it splits and goes to the
ALT field coil, then the PCM provides a PWM's the ground to
control the field coil.
Any waveforms you may have captured, or advise would be
greatly appreciated.
Kevin Mowdy Technician Exercise support division Twentynine Palms, California, USA
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