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2005 sierra 1500 4.3l automatic 2wd
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This is a strange one for me and a bit lengthy so please bear with me. Been troubleshooting it for a few weeks and done alot of searching.

2005 gmc sierra 1500 4.3l v6 automatic 2wd , farm truck

check engine light off, no codes, passlock light off

Got the truck and was told they had already had to replace the ICM before and that fixed the no start issue it had.

When I got it it was not cranking. They had another ICM so I jut went ahead and replaced it. Checked all fuses, good.
Pulled out the starter and found it bad so put a new one on. Fired up good. Ran good for a week.

Back to crank, no start. Tested ignition coil and ICM electrical connections. Got good readings. It fired up and ran, for a week.

Then cranked, but no start. Pulled the plugs, they were ugly so replaced then, checked the plug wires, replaced them. Started and ran good. For a week.

It died while sitting at idle one day. Back to crank, no start.

Fuel pressure 56-58psi at "ON", slowly drops to 45-40 over maybe a minute then holds for awhile. Starter fluid not working.

Cleaned distributor cap and rotor. Would fire up sometimes and run for a few seconds then die. Fuel pressure would bounce in the mid 50's range.

Pulled a couple plug wires and was getting no spark now.... wtf? So i checked the ICM again. +12v to battery (-) good. Ground to battery (+) good. Used multimeter on AC volts with alligator clip to pierce signal wire from ICM to coil with other lead on battery (+), all components connected. It fired right up. .... ? ..... reading 2VAC on ICM to coil signal.

So... twice now testing the ICM to coil signal wire caused the truck to start and run fine. Drove it, shut it off, started up, repeated several times just fine. I'm sure it'll stop again shortly. SO... Why? My thinking is of course that there is a component that is on the way out and by bridging that circuit to the battery gives it a little extra kick. But which one?

also, would getting a free diagnostic software (obdwiz for instance) for my laptop and plugging it into obd help diagnose this kind of thing and verify my ECM and BCM are good etc?

thanks all
 

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2010 GMC Sierra WT RCSB
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406 Posts
follow this guide: https://easyautodiagnostics.com/gm/4.3L ... -the-icm-1

It sounds like you are shorting the switching signal from the ICM to the coil when connecting it to the battery by AC voltmeter. This is like connecting a large resistor between the battery and coil. You want an on and off 12V signal from the ICM to the coil under normal conditions. An AC voltmeter is just going to give you RMS (average) value.
 

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This is a strange one for me and a bit lengthy so please bear with me. Been troubleshooting it for a few weeks and done alot of searching.

2005 gmc sierra 1500 4.3l v6 automatic 2wd , farm truck

check engine light off, no codes, passlock light off

Got the truck and was told they had already had to replace the ICM before and that fixed the no start issue it had.

When I got it it was not cranking. They had another ICM so I jut went ahead and replaced it. Checked all fuses, good.
Pulled out the starter and found it bad so put a new one on. Fired up good. Ran good for a week.

Back to crank, no start. Tested ignition coil and ICM electrical connections. Got good readings. It fired up and ran, for a week.

Then cranked, but no start. Pulled the plugs, they were ugly so replaced then, checked the plug wires, replaced them. Started and ran good. For a week.

It died while sitting at idle one day. Back to crank, no start.

Fuel pressure 56-58psi at "ON", slowly drops to 45-40 over maybe a minute then holds for awhile. Starter fluid not working.

Cleaned distributor cap and rotor. Would fire up sometimes and run for a few seconds then die. Fuel pressure would bounce in the mid 50's range.

Pulled a couple plug wires and was getting no spark now.... wtf? So i checked the ICM again. +12v to battery (-) good. Ground to battery (+) good. Used multimeter on AC volts with alligator clip to pierce signal wire from ICM to coil with other lead on battery (+), all components connected. It fired right up. .... ? ..... reading 2VAC on ICM to coil signal.

So... twice now testing the ICM to coil signal wire caused the truck to start and run fine. Drove it, shut it off, started up, repeated several times just fine. I'm sure it'll stop again shortly. SO... Why? My thinking is of course that there is a component that is on the way out and by bridging that circuit to the battery gives it a little extra kick. But which one?

also, would getting a free diagnostic software (obdwiz for instance) for my laptop and plugging it into obd help diagnose this kind of thing and verify my ECM and BCM are good etc?

thanks all
Did you Find out what the issue was ?
 
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