2005 Colorado No Start, Electrical/pcm Issue

fmjhp594 · May 20, 2011

Sorry for the long post, but I've been working on this issue for a while now....

So I was driving my truck and had made many short stops throughout the day. At the last place I shut off the truck, and then came back less than 5min later. Truck won’t start, no nothing.

The only things that work:

All of the exterior lights

The interior lights

When the key is turned to 'on' you hear the TB click once

And the check engine light is on solid

Nothing else works, no fuel pump noise, no lights on the cluster work (i.e. ABS, mileage), no radio, no heater/blower, no starter clicking. I had the truck towed home and then checked the fuses. Every fuse is good. I tried to use my scanner to check for error codes but it won’t link up to the truck. I replaced the main 100amp fuse, no change. I then started swapping relays around in the fuse box thinking one of them went bad, still no change. I put on new battery terminal clamps too.

So I disconnected the battery for about 20min. Hooked it back up, everything powered back on (blower, all the lights in the dash, fuel pump). I turn it over.... it starts but then the instrument cluster shuts off as soon as the motor turns over. No lights on it, no needle movement, but the truck is running now. I ran it for about 5 min then shut it off. Started it up again, everything is back to normal and works fine. Turn it off again, and then I hooked up my scanner for codes, there were none. Try to start the truck again, back full circle to the dead electrical system. Damn.

Disconnecting the battery again, 20min later... It starts up just fine, no CEL, life is good. Shut it down, start it up again. No issues. Third times the charm... shut it off, back to dead key.

So anyone have a guess as to what electrical part is bad here?

Thanks for the help,

Mike

fmjhp594 · May 20, 2011

So I left the battery disconnected all night. Now it starts about 75% of the time. But the cluster is dead with just the red security light on solid. Also the truck now has a slightly rough idle. Once again my scanner wont link up to the truck again.

Anyone with an idea?

asc08 · May 20, 2011

A friend of mine had a suburban that was acting very erratically, sometimes it would start, sometimes not. Occasional stalling out, occasional rough idle, cel on occasionally. He took it to the dealer to have it diagnosed, of all things, bad battery.

rdnkcpa · May 20, 2011

Bad batteries cause weird stuff like this. Make sure the battery terminals are clean. lso, check all the grounds. If they get dirty or corroded they can cause electrical gremlins as well. I'm not saying this is the cause, but maybe an easy first step to eliminate. Good luck.

fmjhp594 · May 21, 2011

I had put on new terminals right after I checked the fuses. So its not them. Battery was sitting at 12.8v when everything is off, but when I turned it on to a dead truck, it dropped to 12.4. I also tried jumping it twice with no change. I did clean the ground right there by the battery and even ran a temp wire from the battery terminal to a good bolt on the motor. Still dead.

This is annoying, I'm starting to think that there is a 'few' bad relays or the PCM is acting up...

markw8 · May 21, 2011

If you changed the terminals because they looked bad, the whole cable maybe bad. This was common on s-10s. Have your battery tested and the starter. Might even be the ign switch. Not likely to be a few bad relays.

It would probably save you money and grief to take it to a dealer or good shop and have it properly diagnosed, if the above doesn't help. You don't want to throw parts at a computer controlled truck.

Mark

fmjhp594 · May 26, 2011

I had it towed to the dealer, right off the bat they said it was either the ign switch or the computer.

After they fought with it for three days... the computer was bad. When they put the new computer in, it started throwing communication error codes left and right. They traced the whole issue down to a short in the wiring harness going to the airbag computer. The airbag computer is under the carpet, in the center of the truck below the armrest/center console. The wires that shorted were under the drivers seat under the carpet. It appears that someone took the seat out in the past and when they put the seat back in, they sorta dropped it on the wire bundle. Theres a good crease on all of the wires right behind one of the seat's feet. Thats our best guess. So who knows how long it took to short out, but at least they found it. Just cost me $875 though....