Ronin1014

I recently dropped a 5.7 350 into my 99 K1500, during the process I dropped the CPU that came with the motor into my truck. After getting everything reconnected and attempting to start the truck it simply cranked, and flat out refused to start. Went through and spent three days checking everything from plugs to timing and all checked out. Finally on a whim My brother in law told me to put my original CPU back into the truck and it fired up just fine first turn of the key.

Turns out the truck was in tamper mode the whole time. With everything on the dash flashing I didn't notice the Security light flashing to have given me any indication sooner.

While this may be resolved I am a little bummed that I had to put my original CPU back into the truck, but my truck is alive again and that is what was important at the time.

My questions are, Why why did the CPU that went with the motor send the truck into tamper mode and how do I resolve this issue should I decide I really want that CPU?

sledge.impy

Don't you have to program it to the ignition?

Ronin1014

That idea passed through our heads too considering my brother in law had to do it with my sisters Malibu. That said we called a Chevy dealership and the service rep relayed us to a tech rep who said it should not be the case.

That was literally ten min before trying my original CPU.

tech2

the learned pre release "password" the pcm is expecting will be different from another vehicle.




when ever the ecm/pcm is changed you must do a PASSLOCK Reprogramming Auto Learn and PASSLOCK Reprogramming Seed and Key.


this is right out of the gm service manual.


if the new engine a different displacement? If they both are 350cid, leave in your old one.

Ronin1014

Ok, it's been a few days and I am seeing that my truck is not displaying any of what I assumed were mechanical issues the donor truck demonstrated. I've also done a little more research, all put together I don't want that other CPU. One thing I don't understand is, if that is directly from the service manual, why did the rep say it shouldn't be the case, did I just get patched to a stupid tech rep or something?

When I bought the donor truck, I bought it knowing it was not the original motor. The donor originally had a 305 but the owner located a donor 3500 with a Vortec 350. So he swapped motor and CPU from the Chevy 3500 into the GMC K1500. The donor GMC was difficult to start, if I did not allow the fuel pump to complete its initial cycle it would not fire up, in other words I had to turn the key on, wait three seconds then start the truck. I assumed this was a mechanical issue I could rectify once it was into my truck. there were a few other issues but that was the notable one.

At the time I thought there was a difference in the CPU between 1500 and 3500, so I figured I'd keep that CPU right with the motor. I'm just not finding any conclusive evidence of any difference now that I am actually looking. All that said my original was a 350 and the truck fires up with the first turn of the key (No wait period for fuel pump) so there is no reason to worry about the other CPU now.


However.....this issue will likely confront me again in the future. Right now my concerns have been to get the truck running correctly again so I can put it to work and get back to generating income. Once I have a little more than five and six hundred bucks to put into the truck I intend to put a 454 into it, at that point I will absolutely need to know how to resolve this issue as a CPU change will be needed then.