Charging system info non mopar

[70Swinger510]

What's up guys, I have a question, well a problem and it's been bugging me a lot lately. One of my work trucks, Isuzu NPR, has a GM 350 vortec motor. I've had a few services calls, guy's truck wont start, cranks but not starting.

I go check it out, it cranks fine, but no spark from the coil, check the voltage on battery it's at 10.56 volts I hook up jumpers and bam it starts. I check the charging system, strait out the alternator 14.5 volts, positive terminal on battery 13.9-14.3 volts. (running) i check the volts with engine off and it sits at 12.5volts, So i figured he left his flashers on, battery voltage dropped below 12volts and would not allow the coil to fire.

My question,
What should the battery voltage be with engine off? higher than 12.5 correct? This is a used battery, I pulled from another truck and with that truck, even if the battery was low if it would crank it would fire. Why would be coil not fire if under 12 volts?

My next step is a new battery, and i'm gonna redo the ground wire from the battery with 2/0, and re run the Charge wire from the Alternator to the battery with a thicker guage. See if that solvs my problem

I have this same problem on my 83 toyota 4x4 It will crank but no spark soon as i jump it it starts.. any ideas?

Thanks

-Chris

 

[Guitar Jones]

Sounds like a bad battery. Just because a battery has 12 volts it doesn't mean it's good. It may have a weak or partially shorted cell. Still measures 12 volts until you put a load on it then it goes away. While this may allow the starter to work, it's sucking all the juice out of the battery and there's not enough left to supply the ignition system properly. I assume this is a computer controlled fuel injected engine right? If computers don't get the required voltage they can do funny things.

 

[70Swinger510]

It started doing it, i changed the battery to a new one, but had low CCA was 630, then still did and i changed it to a known good interstate battery with 800CCA, and it still does it.

 

[70Swinger510]

Oh yeah, i also hooked up a chargin system/ battery tester, it says my alt output is a tad low, but still in the green margin, but at the alt i get good voltage, I will put a new battery and replace the wires and see from there.

 

[ramenth]

Check your wires going to the starter. Just an idea I'm throwing out there, but I've worked on more than one GM in which the positive lug is a tad loose, causing a voltage drop. (A lot of GMs route the alternator through the starter.) Since your tester is hooked to the battery and not the alternator this could explain the variance between your tester hooked to the battery and your direct alternator test.