My 1996 Chevy k1500 won't go in reverse and doest want to shift when it warms up. Do you know possibly why? It has four wheel drive and an automatic transmission. The size of the motor is a 5.7. I've looked around the internet and someone posted on a web-site the same problem, but also noted that they had problems with the brake lights and someone else suggested something like replacing the brake light switch (I think). This truck has had the same problem with the brake lights but that was fixed before I had problems with it not going into reverse or wanting to shift when it warms up.

SOURCE: 1995 Chevy k1500 Transmission Problems

If check engine light is on ,remove battery cable, you have an electrical problem, maybe fuse?

SOURCE: Transmission Slipping?

Putting it in four wheel drive does not eliminate the transmission. Honestly, it sounds like the transmission is on it's way out. A 4l60e in one of cars was acting the same way. It required a rebuild.

SOURCE: 1994 CHEVY K1500 BRAKE LIGHTS DON'T WORK TOP CAB LIGHT WORKS

I figured out my problem!
From the VERY BEGINING... I changed my brake pedal switch with a BRAND NEW switch, and the lights still didn't work.

Well, I went back down there with a live wire and touched the contacts..and guess what.. the brake lights lit up!

I got a faulty NEW brake pedal switch!! I went a year with this problem, so learn from my issue, test even your new parts if it doesn't fix the problem to completely rule it out!

I hope you guys figure out what's going on with your issue.

SOURCE: truck won't go in reverse

take it to a transmision shop sounds like a problem with the valve body

SOURCE: 1996 Chevy K1500 4L60 Transmission having problems

The vehicle speed sensor is not working, it is located in the transmission tailshaft. The computer uses this speed info to shift the transmission, and tell when the rear wheels are locked up so the ABS will function.

Testimonial: "Right on the mark Put new VSS in and Truck Shifted perfectly Thank You"