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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
Technician
Exercise support division
Twentynine Palms, California, USA

23 Replies Received (View Replies)

 

car Vehicle Data

1996 Dodge B2500 5.2L

Engine5.2 L
Trans3-speed Automatic (Electronic)

car Vehicle Data

1996 Dodge B2500 5.2L

Engine5.2 L
Trans3-speed Automatic (Electronic)