it is possible but not trivial to check each SCR and diode. first you should be positive your battery is in good shape, as battery problems can show the same symptoms.
The tech will need a tool called a "flightsystems handyman" (flight systems is a company that first made these testers available to the forklift industry).
You may save a lot of time and money getting the entire chopper panel removed (be sure to mark the 6 heavy cables as you remove them) and send the entire chopper panel to a re-builder like flight systems.
I have been using the truck as it does what I need it for albeit slowly. I have had new forklift engineer out to it and he has replaced pedal switch but says that it is one of the three SCRs breaking down. Is there any way of find out which one is at fault as he has checked with his meter and they are OK but one is breaking down under load. Can this be simulated in any way or do they have to be replaced in turn until faulty one is found?
Thanks for your help
of course since this problem was posted in May 08, and it's now October 08, I hope you haven't left this unit in the corner and burried it under other trash.
The first thing _I_ would check is the repair company's phone number is in the waste bin, and then check the yellow pages to find someone that has a clue to repair your (not new) unit.
This is not easy to guess about this problem, it could be related to the chopper controller or the accel pedal and it's switches. it does sound as if it is getting what we call a "pulse monitor trip", where the rectifiers are not correctly pulsing, one of the 3 major SCRs (silicon controlled rectifiers) or the related large diodes is shorted closed. but on this old of a unit, this could even be caused by a low battery charge.
Yea, First thing I would check is #1 SCR or called REC, Should read at least 50000 omhs of resistance between cathode and anode, Upon initial acceleration the card will check for shorts ,if the #1 scr fails to shutoff the truck will jerk excessivly and the card will detect and shut off traction circuit.could also be an associated #2 rec