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About Roland Finston
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I specialize in Chrysler Corp. front wheel drive cars and minivans for the years 1982-2005 based upon my 50-year DIY experience and factory shop manuals. I don't have expertise/manuals for Jeeps, Trucks, SUV's, Imports. Today I will take questions one at a time from 8am-2pm, and 6-8 pm CST, answering you within 1/2 hour. If I am shown as "maxed out" try again, in an hour, as I may be answering another question or otherwise busy. There are answers to ten thousand Chrysler questions at your finger tips: put a few key words in the "find" box (above) or click on "view past answers".

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Chrysler Repair - 98 Ply GR Voyager: Alternator Field Coil

Expert: Roland Finston - 11/4/2009

Question
Roland -

I hope you had a good vacation. Based on some AutoZone schematics and your other answers to alternator troubleshooting I think I almost have it figured out. Ititially, my alternator light was randomly coming on, but for the past two weeks the alternator is not producing a charge. The voltage is staying at 11.5 - 12.5V depending on the current battery strength. I have had the alternator tested twice by O'Reilly's multiple times each visit and they claim it is producing 14.5V as it should. I reinstall the alternator and no voltage. I have measured the contacts and the coil of the Auto shutdown relay and they will change as the key is progressed through a start, so the relay is OK. I ohmed out the wired from the alternator coil to pins 6 (DG/O) and 8 (DG) and they are good. But when I check resistance from pins 6 to 8, I measure an open. What should the alternator coil measure? What is the sequence of operation that the PCM expects to see/output?

Sorry I cannot send the .pdf schematics

Thanks, Steve

Answer
Hi Steve,
I checked the pin assigments at the pcm and found that the ends of the alternator coil go to pin 4 (dark green) and pin 6 (dark green/orange). So the Autozone schematic is in error. It should read a modest number of ohms because it is simply the resistance per foot of the copper field coil winding times the length of the winding. I don't happen to know an expected value but anything other than 0 or infinite is probably OK. One end of the field coil is oscillated rapidly to ground (with varying pulse lengths) by the pcm at pin 4, while of course the battery voltage is solid on the other end via the dark green/orange at pin 6. That varying oscillation is what produces a voltage output at the desired level. You may not be able to see the oscillation without an oscilloscope or similar device.
Roland

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