Expert: Roland Finston - 11/11/2009

Question
i have a 1991 lebaron cnvt w/3.0 v6 at the volts are low below 12v when running i had the alternator tested it was good is there a voltage regulator hiding in there and what else could i be looking for thanks

Answer
Hi Scott,
The voltage regulator is part of the engine control module and is not a separate part. The voltage output is based upon there being 12V on the dark blue wire at the alternator (with the ignition switch in the run position) and that there is a good connection on the dark green wire to pin 20 of the engine control module, and that module should be oscillating that dark green wire on and off of ground in order to create the proper output voltage as measured on the gray/black wire at the alternator. A simple voltmeter won't be able to detect that oscillation, however. The most usual cause for low/no current output or voltage output is defective field coils on the alternator. I would remove the dark green and dark blue wires of the alternator and measure the resistance between the pins on the alternator proper from which those wires were disconnected. You should find that to be about 4 oums. If that is ok (not ok would be 0 ohms/shorted or infinite ohms/open field coils) then either the module is not oscillating the dark green wire or there is something wrong with the interanal alternator contact for the rotor  armature. I have seeen quite often tests done at a parts store that claimed the alternator was good when the coil was in fact open. So do the voltage and resistance measurement. The regulator function of the ecm doesn't often fail so I would not replace the ecm unless I was convinced that the alternator and the wiring was good and/or that the dark green wire was showing a steady 12v (not oscillating).
Roland