Aliison tranny problems

[tcat10001]

I have a 02 duramx and it wont shift into reverse most of the time the reverse light just flashes when it is cold it seems to work fine but as soon as it gets up to tem it just flashes. I was told it was a sensor under pan but need to no which one. Help please?

[Kennedy]

The first place to start is the NSBU selector switch. This is on the side of the transmission. It slides over the shifter shaft and then the linkage goes on the end. Early ones were black and then they were replaced by a tan colored switch for easy identification.

[tcat10001]

replaced ncbu switch and still same problem most of the time if u turn motor on and off a few times it usualy goes into reverse ,and if its a long back up it usualy just kicks itself out of gear and starts flashing again