Rebuilt engine - now electronics don’t work

[WestCoastie]

1st post - probably should have been in introductions but I need help fast.

I have an 2003 Yukon XL 1500 with the 5.3.

Bought it with just under 100k miles the beginning of last year.

After about 10k miles started to get a battery light and random small issues (like fuel gauge bouncing). Cleaned up all the grounds I could find (chassis/engine). Battery light faded a bit but persisted. Battery eventually wouldn’t take a charge. Replaced the battery and alternator. Battery light faded almost all the way but wasn’t quite gone.

immediately after this engine developed rod knock. Drove less than a mile after.

Truck sat for 6 months until I was able to rebuild the engine.

Rebuilt the engine, almost stock. Did a few mods for oiling and oil pressure and installed LQ9 cam. Engine fired right up and runs smooth.

Everything was good until all the electronics in the cab stopped working. Engine light came on and so did blinker lights in cluster, they both stayed on solid whether the key was in or out. Also, all gauges were frozen where they were when problem occurred.

I disconnected battery and weirdness went away. Until, I turned the truck back on. After about 30 seconds electronics stopped working again and blinker lights came on again. This time when I shut the engine off it over-ran/dieseled. That junction under parking brake lever has a relay that is making a clicking noise and the IGN 1 relay under the hood is clicking as well.

Even with electronics not working engine still turns over and runs. Thought occasionally it stumbles and sputters and now always over-runs.

I’m thinking it’s the BCM but I have no idea. This is definitely the strangest issue I’ve ever had in a car.

And before anyone says anything, I have used the “search” function a lot but nothing seems to really come close to what I’m dealing with now. Any help would be appreciated.

 

[iamdub]

BCM? Funky ignition switch? Possible flood victim? Rodents chewed wiring harness, causing intermittently shorting?

 

[WestCoastie]

BCM? Funky ignition switch? Possible flood victim? Rodents chewed wiring harness, causing intermittently shorting?

No to flood, it spent its life pretty much in the desert before I got it.

BCM is my guess.

By ignition switch do you mean the actual keyed switch? Might be something to check.

Also, I looked through as much wiring as I could, everything looked fine.

 

[nonickatall]

Did you check for error codes?

Did you measure how many volts your have in your system? It should be on engine running 14.5 - 14.7 Volt.

I would losen all connectors, spray contact spray and connect again. As well i would look for more grounds. Maybe someone can make a list, where all the relevant control units sits.

But ignition switch sounds as well plausible...

 

[WestCoastie]

Did you check for error codes?

Did you measure how many volts your have in your system? It should be on engine running 14.5 - 14.7 Volt.

I would losen all connectors, spray contact spray and connect again. As well i would look for more grounds. Maybe someone can make a list, where all the relevant control units sits.

But ignition switch sounds as well plausible...

I definitely tried to check for codes, but scanner just kept saying error, it wouldn’t read anything from my truck, and yes it does work correctly in other cars. I assume the same communication issue I’m having between inside and outside car is effecting that too.

Did not check voltage with a handheld meter but voltmeter in truck (before it froze) was showing good voltage.

We did spray and clean up grounds that are supposed to be there and didn’t find any that weren’t supposed to be.

Ignition switch may be the problem, but I would think that with the engine running ok’ish and only interior electronics reallly being knocked out that that wouldn’t be the problem. But I have definitely been wrong before.

Spent almost 2 weeks straight working on it. Now I just need it back. Had it towed somewhere that can hopefully diagnose and repair it. I only have a basic scan tool and no way to look more deeply into the problem or flash a new bcm if it comes to it. I’ll let you all know if we find anything

 

[nonickatall]

But that is fundamental, you absolutely have to measure how many volts the on-board network has. Of course, if the voltage regulator of the alternator is defective and you have far too much or too little voltage, something else can happen. So i would recommend to get a multimeter and measure the voltage first.

 

[WestCoastie]

Looks like rodents were the issue. We’ll one issue anyway. After some intense digging found some chewed on wires. Fixed those and most electrical problems are gone.

But, now there is a code for 02 sensor P0141, which wasn’t an issue before.

Now, oil pressure seems good on Start up, but at idle it drops to 10, and then continues to drop. At least according to dash gauge. No unusual noise from engine. Don’t have a mechanical gauge to check.

And… it wont shift in 4wd and has service 4wd light. This was a problem immediately preceding the Rod knock. While engine was out I replaced encoder motor hoping that was it, but doesnt seem to have done the trick.