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· Registered
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Discussion Starter · #1 ·
Hello all,

I've been perusing forums trying to find a solution to my problems for a weeks, but haven't quite found the exact solution to my problem, so here goes the whole story. (I'll try and condense as much as possible)

This all started about 2 years ago when the Colorado spun a rod bearing. I hadn't been around , and this truck was sort of the family spare vehicle, so my brother pulled the engine out, put a new crank in and rebuilt the bottom end. Some time went by and I finally put the motor back in and hooked everything back up, but when I went to start it, I had the main power and the switch line shorted on the starter, so as soon as you would push the clutch in (M/T) the starter would engage, regardless of if the key was on or not.

So, I fixed my shorted wire situation, but the starter would still engage with the clutch pushed in when the key was in the on position. We thought we must've fried the lock switch mechanism, so we replaced that, problem continued. Our cheap little code reader wouldn't read anything, so we let it sit for a while because we didn't have time to mess with it.

A few weeks ago, My brother and I decided to tear back into it and do some more troubleshooting. I have access to a Snap-On Modis scanner so I borrowed that, and I couldn't get it to communicate with the vehicle at all, I keep getting a Class 2 Serial Communication error. Thought maybe the body control module was junk because when we pulled the kick panel out, the control module was wet, so I got a used one from ebay, but I'm having the same problems.

Now, I'm thinking it's the engine control module (ECM/PCM) but I don't want to order parts and have the same issues that I'm having now.

-All fuses are good
-No OBDII communication
-Fuel Pump will only run when bypassing the relay
-Starter will only engage when bypassing the relay (Starter no longer engages when clutch is pushed in)
-Problem persists with both body control modules
-Passlock light is on

Is there any way to test the PCM at a component level? If the PCM functions properly, then I know that I have a wiring problem somewhere.

TL;DR - Can't communicate with OBDII, think the PCM is junk but don't know how to test it.

I should also note that I took a multimeter and probed some of the wires going to the PCM, and I measured 5V from the PCM ground to vehicle ground, which I didn't think I should see. Not sure if that means anything to anyone.


· Premium Member
4,464 Posts
:welcome: to the Nation.

Check ground point G105. It is the ground source for the activation side of the Run/Crank relay and the Fuel Pump relay. If the Run/Crank relay isn't operating there will be other systems affected besides the starter.

G105 is a small black box mounted to the left fender wall in the engine compartment. It is located aft of the battery and outboard of the antiskid module.. Remove the single bolt and clean the mating surfaces of the box tab and the fender wall. There have been a few instances of corrosion/broken wires inside the box. The box can be pried open, if necessary. The only thing holding it together is a couple of molded in tabs and the friction of the terminal pins. It might take a bit of force to overcome the friction of the pins.

· Premium Member
3,536 Posts
^^^^ Good advice !!!
Good luck finding the Gremlins !!!
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