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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
Exercise support division
Twentynine Palms, California, USA

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car Vehicle Data

1996 Dodge B2500 5.2L

Engine5.2 L
Trans3-speed Automatic (Electronic)