Hello All,

I hope this is in the right forum and I thank you for reading this. I'm really stumped and trying to figure this out. I've always referenced LS1Tech, but haven't had much of a need to post.

Here's my long winded story:
My buddy has a 2007 GMC Sierra 1500 Vortec MAX. It has the 6.0 L76 motor. That's also that weird transition year, and this is the newer second half the of the year model. The engine started running rough, and LONG story short, it's freshly rebuilt after a thrown rod. He got it back from the machine shop as a complete long block.

We slapped it into the truck and it fired right up, ran absolutely perfect. Great oil pressure, everything. He went down the street to do a little test drive(100 yards away) and it stalled on him. He went to start it back up and the start just pulsed. It would engage for about a second, then disengage, then re-engage, just over and over and over. It wasn't random either, like a loose wire.

I blamed it on a weak battery. So he put in a brand new battery, same issue. After trying to research it on multiple forums, I thought it might have been the crank position sensor. He replaced that a few hours ago. Truck started right up and idled perfect for about 10 minutes. He turned off the truck and started it up about 5 times. Then it started doing the pulsing thing again.

I'm really at a loss here. The ONLY thing I can think of is to try starting it and make sure that its doing the weird pulsing thing. Then with the key still on, jumping the starter to see if it still does it or if it continues to crank.

No CEL either. Could this be some sort of weird security issue?

I'm really hoping the experts on here can point a finger in the right direction or have some more suggestions on things to check/try. Before the engine rebuilt it didn't do this either.

Thanks a ton for the help!

 

I don't have a handle on this "pulsing", can you be more specific? When your bud pulled the crank sensor what did the end look like, was it mangled? Sometimes the CPS need to be shimmed to keep the reluctor from hitting it...

 

Basically the starter engages, then disengages, back and forth like that about 4 times then stops. Its not sporadic, its very much a computer controlled thing. You can actually turn the key to the start position, and then let off and the starter continues to pulse.

 

Last night I ran some more tests, like what John and Shane suggested. All the grounds are hooked up, exactly where they were from the factory.

I swapped the starter relay with a headlight relay, nothing changed.

No security lights are on.

I disconnected the ignition wire that goes to the starter and when you turn the key to the 'start' position, it gets constant power, around 12 volts.

I hooked my own wire to the starter and touched it to the battery. The starter would just spin and spin, but the truck wouldn't start.

When I hook the original ignition starter wire back to the starter, it starts with the on/off of the starter.

Right now its very consistent: When you turn the key to the start position, the starter engages, then dis-engages a few times then stops. You can hit the key to the start position, let off, and the starter still pulses on and off.

I'm really at a loss here. Sounds like it might be time to take it to the dealer?

 

Sorry just realized I forgot to answer another question, no the CPS wasn't mangled or scratched or anything. It was in perfect shape.

This is a video of it: https://www.youtube.com/watch?v=ydYr...ature=youtu.be

 

That is bizarre! Aftermarket alarm system?

 

Negative. My buddy got fed up and took it to the dealer. Apparently there was one little ground wire on the backside of the motor that would touch, then not touch. He said it would be really easy to miss, which we did.

The truck fired right up, but said there was something else that was wrong with it. I guess the dealer is fixing it, not sure what it is.

But thanks again everybody for the help. I was really trying to avoid my buddy having to fork over a wad of cash to the dealer, but that's what happened.