98 Pontiac Montana Wont idle with Alternator harness connected, will run fine off battery till it dies.

1998 PONTIAC TRANSPORT
100,000 MILES
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MONTANAGUY
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Have a 98 Montana with 100K, oiginally it was throwing a P0300 code and had 10 psi fuel pressure, replaced: fuel pump, fuel pump harness, tps, iac, plugs, wires, coils, rebuilt injectors, and CPS. Car starts and runs smooth now but wont idle, it just dies. I disconnected the wire harness from alternator and it runs fines till battery dies down, so I replaced alternator and no change, with wire harness connected, car wont idle. It starts and is smooth but after a couple seconds dies. I will mention the CPS appeared burnt when I pulled it. There are no codes or pending codes, longterm and short term fuel trims are staying at 0 and not adjusting. I also replaced ecm (had it calibrated by the vin number) and its the same with either ecm installed. Any thoughts on what would cause it to die with harness to alternator connected? or ideas on how to dx this farther?

any input would help
-thanks
Jan 9, 2012 at 4:15 AM
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CARADIODOC
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Replace the battery. Since the '87 model year, GM went from the second best generator design to the world's worst pile ever. They develop huge voltage spikes that take out the internal diodes and voltage regulator, and those spikes interfere with computer sensor signals, and the computers, which are very sensitive to system voltage. As the battery ages, it can still crank the engine but they lose their ability to dampen and absorb those spikes. It's real common to go through four to six generators in the life of the vehicle, but many professionals are finding out that to reduce the number of repeat failures, just replace the perfectly good battery at the same time.

I don't know how you figured out that unplugging the generator would provide a clue, but that IS a common test. On the off chance the battery doesn't correct the problem, look for any wiring harness that are not routed in their original orientation. Also be sure there's no undesired resistance in the fat wire between the back of the generator and the battery or fuse box. That added resistance makes it harder for the battery to dampen those spikes.

In technical terms, as the lead flakes off the plates in the battery over time, it's "internal resistance" goes up. That's a term usually only heard by electronic designers and technicians. In practical terms, the old battery can still provide the current needed by the starter, but for a much shorter period of time compared to a new battery.
Jan 9, 2012 at 4:38 AM
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FIXITMR
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check for AC voltage at battery with alternator connected. i've heard of up to 30v AC which means bad alternator.
Jan 9, 2012 at 6:06 AM
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MONTANAGUY
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Battery tests good and has 12.6 volts, car starts runs for about a second and dies. With alternator to ecu harness disconnected it appears to run fine. Seems when alternator output hits 14.8 the car shuts off, does the ECU shutdown the vehicle when at or above 14.8 volts? Is there any other scenario besides a bad regulator in the rebut alternator where you would see over voltage? (i have two calibrated ECU's for the vehicle, its same with both)
Jan 14, 2012 at 8:26 PM
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CARADIODOC
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14.8 is just above the industry-accepted limit of 14.75 volts but it is not excessive. Did you try a new battery?
Jan 15, 2012 at 12:22 AM