Yeah, that's an old General Electric M210 panel. For some reason Clark thought it would work best if they installed it under the battery. I guess dripping acid probably cooled the electrics.
You can find a pretty good guide here: fsip DOT biz/en/content/technical-documents
My Mac said their certificate was bad but it's a safe website. You can also google Flight Systems. From the main FSIP page navigate - resources, technical documents, GE Controllers, M210- M310
The GE guides are pretty good. Also, if this trouble started after a long period of sitting idle, make sure your battery voltage is up and stays up when you try to travel.
Good luck with it.
Left side has lift pump and lift pump motor. Right side has electronics. Not pretty though.
Everyone is talking about this being an EV-1 system but that's not what I see in your photo. That card in the middle almost looks like an M210 but I don't see a transformer or capacitor nearby. Have you opened the side panel under the battery? There should be electronics in there. If your fuses are all good you want to be looking at the control panel. A picture of that could probably identify it. (though by the age of those contactors I wouldn't be surprised if it had resistors) If you can identify the control panel there will be repair manuals for anything GE made for this truck.
Find 29 wire on card put neg probe to neg on unit put positive on wire 29 at card should have 3.5 -4 volts put unit in neutral and push accelerator the voltage should drop to around 1/2 volt if it does your pot is fine
To check fuse besides continuity lgrab your meter put negative to negative on unit then put positive on fuse should have battery volts
Then try to drive as the meter hooked to fuse if voltage drops then you have a bad fuse
I checked all the fuses and they read ok. I know this is against what Roy said to do but I don't have any replacement fuses handy. My thought it to just jump them for a quick test to see if that is the reason of the failure. Next up would be the trim pot.
Frequently the cause of No Travel with directional contactor closing is an open/blown power fuse. Typically 140amp to 175amp fuse. Do not check the power fuse with a meter, it can fool you. Replace the fuse with a known good fuse and that will verify if that's the problem. If still no travel, proceed to the pot circuit.
EV1 system uses control card input terminal R4 as negative reference point (or you can use battery negative). R5 is wire 29 as pointed out by John Bradley and is the accelerator pot positive input. Be advised, there is also an AGX 0.25amp glass fuse in the pot circuit. This fuse typically doesn't blow but the glue holding the fuse element to the glass case will come loose from driving over rough terrain.
To do a quick check of your scr system, see if you have half battery volts at T2 using negative meter lead on battery negative and positive meter lead on one side of the 1Rec as your test points.
Also note, battery volts means the correct battery voltage, not what you see on the static battery reading. A 36 volt battery that is only at 30 volts after sitting for a long period of time may operate the lift circuit (although at a much lower RPM) but will not operate the EV1 traction circuit.
EV1 control card version 1H3 is a very reliable control card. If you see 5H9, the card itself may be the cause of your problem.
Good Luck Amigo !!
Since it has been sitting you might check
The drive motor cables also look at wiring from front to back mice have a habit of chewing wires I'm assuming it is EV1 system ? If so you can get diagnostics on internet also each
Position on card and the values
Check wire 29 on card for voltage it should be around 3-3 1/2 volts and drop as accelerated
To about 1/2 volt you don't put it in gear as you test it leave in neutral