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Old 12-04-2012, 04:39 PM
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electrical gremlin at 14v

OK guys, the saga of my electrical gremlin continues. I replaced the positive battery cable yesterday due to corrosion that was leading to persistent low voltage. That solved the low voltage issue. Victory.

Now, here's the weird thing. When I fire the truck up here's what happens. While the glow plugs are operating, if I rev the engine to the point it's at 14v, the electrical system blinks. The glow plugs blink off and come back on, the dome light does the same, etc. PCM does not seem to blink, as my idle controller (where I'm getting my voltage reading) does not blink.

Any suggestions on this? Truck has a nasty habit, as yet unidentified, of frying glow plugs, and I wonder if this is related.

Thanks.
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Old 12-04-2012, 04:49 PM
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Alternator should not be charging with gp's running

I made my Samsung SGH-I997 send this.
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Old 12-04-2012, 04:55 PM
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Thanks, though I'm not sure what that means. Since voltage rises when I hit the pedal, the alternator is charging. What controls that? Does the starter solenoid? The alternator? The PCM?
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Old 12-04-2012, 05:21 PM
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The PCM is supposed to tell the alternator to not charge during GP activity. Unless OBS trucks aren't that way.
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Old 12-04-2012, 05:39 PM
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How do your ground wires look? I would imagine if the truck is starting the engine to battery grounds are o.k. but you might want to check some of your cab to chassis grounds or even add one and see if that seems to help any. Also might want to take some voltage readings from a few different circuits and see if your results vary.
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Old 12-04-2012, 06:13 PM
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The alternator is always charging when the engine is running. There's no interface between the PCM and the alternator. The voltage is lower when the glow plugs are on simply because of how much they draw. If the alternator didn't run while the glow plugs are on, the voltage would be even lower.

Neither the starter relay nor the starter solenoid are in the picture after the engine starts. The connection on the starter relay where the wires go to the glow plug relay and all the truck's accesories is just a junction point; it doesn't use the circuit of the starter relay itself.

If you rev the engine after the glow plugs shut off, how high does the voltage get?
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Old 12-05-2012, 11:29 AM
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Thanks, madpogue. After the alternator shuts off, voltage goes to 14.4 at the batteries and 14.3 at the idle controller in the cab. It will hit 14v when I rev the engine even with the glow plugs on. I do have an HD 200amp alternator. Is it possible that 14v is too much for the GPR?

Thanks for the help.
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Old 12-05-2012, 04:41 PM
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You mean after the GPR shuts off? The alternator should never shut off, while the engine is running.

What brand/flavor of GPR?
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Old 12-05-2012, 04:53 PM
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Yeah, sorry, the GPR. The GPR will blink, shut off momentarily then switch right back on, when the system hits 14v. The timer does not seem to be affected. Even if the system blinks, it will still shut off after 2 min on a cold start or shorter on a warm start. The illumination circuit (dome light/ head lights etc) will also blink. Fuel pump, radio, etc don't seem to be affected.

EDIT: not sure on the brand, but it's a quality relay, only a year old. I replaced it last winter along with new plugs.

Last edited by CharlieC; 12-05-2012 at 05:08 PM.
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Old 12-05-2012, 05:21 PM
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Kinda makes sense; the "timer" is the PCM. If this doesn't happen after the GPR cycles off, then I'd say you're on to something; the relay isn't capable of handling that voltage. 'Course, how high are you revving it to get the voltage that high while the relay is still running?
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