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I have a 1999 GMC C6500 diesel with a Cat 3126b. 7AS40922 is

Customer Question
the engine serial number. It...
I have a 1999 GMC C6500 diesel with a Cat 3126b. 7AS40922 is the engine serial number. It starts up and runs for a few seconds. Then the voltage gauge drops to zero and then the engine dies. When the voltage gauge drops like that, the engine still cranks and other electronics work. When the voltage gauge drops, is that telling me that the voltage at the ECM dropped to zero? Or is my issue somewhere else?
A few minutes ago, the gauge was showing voltage, so it would start then die, which I did a dozen times. But right now the voltage gauge reads zero and it will crank but it won't start or run. But I'm guessing later today or tomorrow, that will change again and the volt meter will show voltage and it will start then die again... I need to figure out what is causing the voltage drop, and where the voltage drop is happening, so that I can repair it so my truck will start and run good again.
Submitted: 1 year ago.Category: Medium and Heavy Trucks
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Answered in 3 hours by:
7/12/2016
Technician: Daniel Wilson,
 replied 1 year ago
Daniel Wilson
Category: Medium and Heavy Trucks
Satisfied Customers: 5,728
Experience: Owner at DJ Wilson
Verified

Hi Brett, My name is***** will be glad to try to help. Check for the inline ECM fuse between the battery box and engine ECM and for corrosion in the harness for a loss of power to the ECM. also test all your fuses and relays and their connections for any loose connections. If okay test the key switch for power in and out in all positions for a possible faulty key switch. Test your batteries and inspect, ckean, and tighten all battery to starter connections including all grounds as well. Let me know

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Customer reply replied 1 year ago
Okay, I found a few problems. Two ground straps from the engine to the frame were broken, so I replaced them. Still no change. Next, I cleaned battery cables. Then the power wire that goes from the battery up to the the engine bay fuse panel was disconnected at the fuse panel and wire brushed. No change. Next I wire brushed off all of the fuses. And I found an aftermarket alarm system. So I also unplugged power to that. After that, it would start and stay running. And the voltage gauge no longer drops off to nothing.However, when it sat for about 36 hours, it started extremely hard. I cranked it for a minute and nothing. Then I waited a couple hours for the starter to cool off. Then I cranked for about 30 seconds, and it would sort of run, but not on it's own. When I would stop cranking, it would die within a few seconds. After 3 or 4 more tries, it would just BARELY run on it's own, without help from the starter at about 500-550 rpm. After a few minutes, rpm would increase slowly until running at normal idle speed, but it would idle kind of roughly with a lope. After a few more minutes (about 10 total), it would idle smoothly. I did an online search for this problem and the #1 culprit seemed to be the fuel pressure regulator on the back of the cylinder head. So I replaced that with a new one from Cat. I am not optimistic, but I am waiting to see if that made any difference. Any suggestions on what might be causing the hard starting?
Technician: Daniel Wilson,
 replied 1 year ago

Hi Brett, Thanks for the update. fuel pressure should be 58-63 at idle and up to 76 PSI driving under a load. Verify good fuel pressure. If low it can be a faulty regulator or injector o rings, leaks, a restriction between the tank and pump, or faulty pump. Check for bubbles by installing a clear hose between the tank and pump and verify fuel pressure by tapping into the filter housing.

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Technician: Daniel Wilson,
 replied 1 year ago

Speed timing sensors in the front cover can also cause hard starting if there is a loss of signal and also if the cranking RPM is too slow. Let me know

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