Expert: Roland Finston - 11/6/2009

Question
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

---------- FOLLOW-UP ----------

QUESTION: On my 98 Ply Gr Vovager, the DK GRN wire lands on pin 8, my pin 4 slot is empty. I have ohmed out the wiring per the attached dwg: PCM pin 6, Dark Grn/Org, connects to Auto Shutdown relay & alt coil; PCM pin 67, Dbl YEL, connects to Auto Shutdown coil; PCM pin 8, DK Grn, connects to Alt coil.

Steve

ANSWER: Hi Steve.
I don't understand the discrepency between what you say is how yours is wired/Autozone and my '98 FSM shop manual about the pin assigment for the dark green wire. But in any case, if you are getting a reading 6 and 8 so be it. Why not check it at the alternator plug proper? This is the first I have heard of an alternative schmatic. I do notice that the '02 manual does show the pins to be as you suggest but not my '98. Maybe it was changed in '99 and you got a modification of a late '98 model.
Roland

PS Is it possible that the PCM that you have is not the original one (or the wiring harness was changed?) either of which could result in the field coils of the alternator not being wired up as required to make the alternator produce any voltage??? If by chance there are indeed two different pin setups for '98 I could see that happening and that would explain why you have no output.

---------- FOLLOW-UP ----------

QUESTION: Nothing on the van has been upgraded or changed in the 10 years that I owned it. The VIN # is 1P4G44R3WB580914 if you want to check if there was or when the wiring change occured. By the way, the Autozone schematic does not have pin numbers listed, just wire color.Would you like a picture of the wiring plug on the PCM unit?

Anyway, I have solved the alternator problem. The fiels coil in the alternator was effectively open, greater than 20K ohms, the coil on a new alternator measures about 4 ohms. The new alternator is producing voltage as expected.

Thanks for the help.  Steve


Answer
Hi Steve,
I'm glad you found the problem, it makes you wonder about the accuracy of O'Reilly's salesperson. I think I know the pin/wire colors, so thanks for the offer. What is strange is the year discrepency that I notice as it hasn't come up heretofor. Thanks for letting me know the solution to the riddle.
Roland