My model is called ASV30, other models are the Terex RC30 or PT30.I have two safety switches, and one or both seem to be dysfunctional as the engine runs very rough unless you disconnect the bar safety switch. However if you disconnect the bar safety switch, the hydraulics do not work. There is a seat switch and a bar switch as far as I know but I do not know where the seat switch is located. I checked the safety relay and it clicks when you turn on the key so it probably is not bad. However the switch on the safety bar works but it appears to work in the wrong direction of the bar. When the bar is up it seems to me that the engine should not start. When you get in and lower the bar (for safety) then the engine should start. However, if the bar is up the engine starts fine and runs fine when you lower the bar the engine either stops or runs very rough like it is not getting enough fuel. Seems like this should be the opposite. With the bar down and operator in the seat the engine should start and stop if you raise the bar. Let me know if you need any additional info.
The engine should start and run fine with the safety bar up. This allows you to start the machine, and get out of the machine to say let the machine warm up or check grade. The safety bar is only so the hydraulic controls won't activate when not needed. If the engine is running well, then your likely on the right track with checking the safety bar and hydraulic controls.
Lap bar, door switch, seat switch, are all wired in series to complete ground for safety relay. If any of those switches are malfunctioning or miss aligned, you will lose all or partial hydraulics. If door switch, is the problem, then you will lose bucket and lift arm controls, because it simulates door is open, so you don't rip the door off by accident. Likewise with seat and lap bar switches malfunctioning, you will lose drive hydraulics, because, it simulates no person in driver seat. There is no connection between safety systems, and the starting and run system. If machine runs good, with lap bar up, but struggles when down, to me indicates eng problem under load, plugged fuel filter, lift pump, debris in suction line in tank. One thing to check is the fuel shutoff solenoid spade connector, they heat up, and get loose, causing symptoms your mentioning.
Does it have Aux hydraulics? I've run into the aux circuit being stuck on and the engine loads down and struggles when you drop the lap bar. Usually a foot switch buried in mud on the floor.
Aux loading is a possible issue, there are some constant flow switches that can sometimes cause issues. I’ve found the wiring harness issues to usually be the main cause of problems on these small but mighty machines. Take the belly pans off and look near the resolver block (safety solenoid block, lock out block). If this machine is used, look for slap-dash repairs and bypasses for the safety circuit. The operator seat safety is a common place to see a scotch lock. Work safe, Todd