Not Starting?

TXZDK

MEMBER
2003 GMC SIERRA
5.3L
V8
2WD
AUTOMATIC
155,000 MILES

I took my truck to a “tuner” because I was having fuel problems and thought it might have been my PCM since I had been told I needed to replace it. Tuner scans my PCM and there’s a lot wrong like timing and other stuff. He then connected another PCM he had laying around to my truck. The truck starts but since it's not tuned to my truck, it wasn’t running properly. He then removes that PCM and when he tries to connect my PCM back in, the truck won't start. Dash lights come on but when you turn the key nothing. Any help?

1 Reply

AL514

MECHANIC
4,120 POSTS

Hello, does the Check Engine light come on when you turn the key to the on/run position? Not starting the vehicle, just key On. It should for about 30 seconds or so.

This is the Starter circuit wiring diagram, first check that the IGN A Fuse 40amp has power on both sides and is not blown. Using a test light hooked to battery negative is the best way when checking starting circuits, it puts a little bit of a load on the circuit. If the fuse is ok, then pull out the Starter Relay and with the Key On there should be power on 2 of the 4 pins where the relay sits. Verify those first. The PCM is responsible for Grounding the control side of the Starter relay, so that will be checked next.
My guess here is that he either was too rough with the PCM connectors when plugging the PCM back in, or he didn't turn the key off when swapping over the computers.
With the key Off, you can unplug the PCM connectors and make sure he didn't bend any pins on the PCM, or that there isn't any corrosion inside the harness side of the connectors, use a flashlight and look deep inside the connector pins for any corrosion or possibly spread female pins. I'm assuming the PCM he plugged in wasn't a stock GM computer, so it might have had wider/fatter pins than your original PCM, so that might have spread the pins in the connectors, causing a pin fitment issue.
I would double check any area he was working or messing around in.

Did he try to do any programming with a scan tool while your old PCM was still plugged in?
Another possibility after looking over service info is that the Pass lock security system needs a relearn procedure done, because the BCM learned the Pass locks password from the previous vehicle his PCM was in. Thankfully there is a 30 minute procedure you can do without a scan tool. If you notice the security light either blinking or staying on. There's no telling what PCM he plugged into your vehicle; I'm surprised it even started with it in. But diagrams 3, 4 below explain the Pass lock security system Relearn procedure.

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