Okey Dokey, We found the problem. It was the Main #1 SCR. It would break down under load. Truck is running good. Thanks for the replies

I am not familiar with that model truck, but if it has regen or auto braking it might have a problem with that circuit it is obviously not your logic as you have replaced that but it could be to do with a sensing wires for that circuit.

Regards shane

When fault occurs by-pass and high speed do not kick in. Runs on half power with throttle pedal released. It happens when truck is under load.Works o.k when wheels off the ground. Resets when put into neutral and reoccurs when put into fwd/rev and throttle reapplied. Throttle assembly is not sticking ( no cigarettes butts) It does have micro switch and test good. Throttle output to the card is within range and does not hold when fault occurs.

does it do it every time, or just 10 minutes before 2nd shift is over?
is it high speed, or just creeping forward? Can you be sure it is not a mechanical binding of the accelerator pedal on the floorboard, or a cig butt underneath the foot pedal preventing the pedal to return? (and we can be sure this unit has a good seat switch?)

Hi there. Its not an electric truck then?

Had similar problem before but problem was in throttle box. Is this the one with a switch and pot in assembly?