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Old 07-28-2020, 03:22 PM   #1
trmech3
 
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Default Won't run with alternator connected

1989 D21 with 4 cyl. Driving one day, engine shut off, like fuel pump issue. Checked fuel pressure, fine. Checked spark, good. Discovered if I disconnected the alternator belt, engine would run. Assumed it was a electrical issue inside the alternator shorting the electrical system. Replaced the alternator, same issue. Changed relays, fusible links. Also discovered if I left the alternator disconnected and connected a battery charger to it, when I turned the charger up, and the voltage went above 12.8 volts the engine would quit. Changed out the ECM, still the same issue. Let the truck sit for a few months. Decided to take another attempt, replaced the battery, since the other one was 7 years old and had been drained and recharged a lot with the previous testing. Had to move the negative cable to make it fit the new battery, that meant I had to pry up on the ground tab that attaches to the inner fender. With the new battery and movement of the ground cable (also a harness ground attaches at that point) engine started and ran fine. I replaced the ground cable and all has been good for 800 miles. Yesterday, started the truck and it immediately shut off. removed the positive wire from the alternator and it started and ran, was able to drive it home, would skip some at idle, turned the ac on to raise the idle.
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Old 09-19-2020, 10:59 PM   #2
alabama_lowlife
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That’s one weird issue. Makes no sense. When the alternator is connected, exactly how does the truck behave when it won’t start?
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Old 09-21-2020, 02:47 PM   #3
Reserector
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OK, this one is going to bug me...
I'll let you know if I have ANY ideas.
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Old 09-21-2020, 02:55 PM   #4
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Maybe the coil or coil amp is failing above a certain voltage. Do you have one, or two coils?
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