Chevy 3500 not shifting into 4th gear.

[n01tk]

My truck has become terrible on fuel. Noticed vechicle not shifting into overdrive (4th) and the lock-up torque converter is not active? What can I look at to try and troubleshoot this problem? A flowsheet would come in handy. Thanks for any help.

[robertisaar]

do any type of trouble codes get set?

[n01tk]

I acidently cleared all the codes when I took the neg. battery cable off while taking out my ECU. i will have to drive the truck and check for codes. One point I should mention, there was no check engine light on at any time previous to clearing the codes.
I did do some investigating and found the overdrive comes off when the brake pedal is depressed. So if for some reason that input is active all the time, it will not permit the truck to shift into OD. I am looking into this avenue. Thanks for the reply.

[robertisaar]

ah, i have had the same issue in a few cars before, i just added a couple layers of tape near the top of the brake pedal to allow forthe switch to funtion correctly.

that should only prevent TCC lockup though, it should still shift into 4th.