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4L80 Not shifiting out of 1st - intermittent and dash lights flicker before it does

T

TruckCanuck

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I have a 2000 year 2500 Silverado 6.0l with 187,000km. Tranny fluid and filter changed last year. No strange or sloppy shifting before this.
Intermittently, the dash lights will flicker (red brake light and a few other lights). Then, the tranny will stay in 1st. (I have to take the highway regularly so this is a pain.) 98% of the time it shifts regularly and smoothly. It is only after the dash light thing that I notice the problem. Then, after shutting the truck off overnight, it is fine. Dealer took a quick look and is stumped.
 
Grounds, remove any fasteners holding down a ground cable, wire or strap and sand them to a bright and shiny clean. Put them back together and see how it does then.
I know knotttt where all the grounds are located on a gas engine so You’ll most likely have to do some searching for them.
Usually there are at least three, four or maybe five on the engine and possibly one on the transmission too.
Welcome to the forum TC.
 
Welcome.
I agree with Marty- you definitely are fighting an electrical issue. Start at the battery cables, remove and clean. Load test the battery while it’s disconnected.
then chase the harness removing abd cleaning the grounds as you go. The grounding issues gm trucks have is universal regardless of engine or transmission used.
 
Check the ABS ground while you are at it. I don't recall if it applies to your rig but the ground for the under cab units is on the frame and that is known to be a problem with corrosion.
 
Welcome to the forum, I agree there is defiantly a ground or an electrical gremlin hiding under the hood. might also check your tranny connections for any fluid seepage inside the main harness connector. sometimes tranny fluid can cause a short condition. if you have a code scanner, try reading the codes and see if it pulls up anything. when is gets stuck in what feels like 1st, it's actually going into limp mode . if all grounds and other connections look good, you might also try disconnecting the smaller connection to the alternator. it's rare but possible that the alternator can start sending spikes of voltage that can mess with things.
 
Scanning for codes is the first place to start. Post what you have and we can point you in the right direction.

For what its worth I have put a TON of fuse boxes in those trucks for all sorts of issues. As well as ground issues at the back of the heads.
 
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