K3500 4L80E won't upshift out of 1st in 4WD LO

[john8662]

Here is an odd one.

I was testing the 4x4 on the 1996 K3500 Dually after getting the right sized tires all the way around.

4WD works, light doesn't come on, but it's working.

Anyways, thought to try 4WD LO, the transmission then won't shift out of 1st gear, Took the shifter down to 1st and tried to manually shift, no go, engine goes to red line.

Kinda odd, I'll never use 4LO, so don't really really care.

What's busted though? Or, is this by design, stupid as it may be.

This truck has had the later engagement solenoid in the front differential, there is a long wire that runs down to the harness on the transfter case up to the solenoid itself. Who knows if they did it right, Wiring WAS a mess on this truck, now better, I didn't mess with the 4x4 stuff though.

[JohnC]

Codes?

Are you using the right sensor for output shaft speed? Does the PCM know the transfer case is in Low?

[john8662]

Thanks for tackling this one.

Could be wiring, or a sensor. I haven't messed with the xfer case. I had the transmission out during the engine swap, but didn't mess with the case. I'll go over the wiring.

Codes, no check light on, but haven't had it scanned.

At least it's gonna work in 4hi for these next two days of snow, ice, and nastyness approaching again.

[JohnC]

The scanner should show if the PCM thinks the transfer case is in Low or not. If it thinks it's still in high, the speed calculations will be all wrong and it'll probably default the trans to second, which would be hard to differentiate from first in Low range. Pulling the lever to 1 would not kick it out of 2, so you'd think it was 1.

Or, I could be completely up the wrong tree...