Yep its a single speed unit ! The transmission was overhauled by another tech approx 100 hours prior to this solenoid issue arising. It had all new oil,filter etc done at that time.

Single speed forward / reverse transmission unit?

Oil & filter changes are important on these units.

Hi, Im new to this forum. have been working on an 8fgcu25 transmission problem. it had an intermittent loss of forward gear when hot.i found the shift solenoid had high resistance compared to the reverse solenoid and to a new one. i replaced the forward solenoid and the problem was solved....for about 2 months/600 hours and its back.now found the forward solenoid goes open circuit when hot!! supply voltage is 14 volts consistent (when tested) i have installed another solenoid (oe this time) and it works. just wondering if other than a crappy aftermarket solenoid if there is something im missing??

Have you actually separated the sections of the control valve yet? Or are you only extracting the spools and springs for cleaning?
There is a fine mesh strainer (Toyota part # 32804-23330-71) inside the control valve that may have a broken section of mesh which is allowing metal grit from the pump to enter the shuttles and bores of the valve. The control valve sections will have to be separated to get at the filter. This will also permit you to blow out all the very small holes in the separator plate inside the valve.
There may be some metal flakes stuck in some of the tiny holes.
If the shuttles/spools are sticking, grit has entered the valve body.
That is what it is beginning to look like from here.
What was the reason for the new oil pump?

the donor control valve also stopped working opened it up cleaned it and made sure the springs and parts were not getting stuck(forward spring and parts were stuck) and it worked again and is still working on the original forklift. Now if i put it back on the other forklift it will work 1-2 then stop working again,(forward spring and parts will get stuck again and stay stuck) I have checked both control valve for scratces marks or anything that might make the springs and parts get stuck but there is nothing the original control valve will work on the donor forklift but once installed back it will work 1-2 then stop working will only work in reverse and yes all parts on the transmission where inspected i have also inspected all wireing no shorts or cut wires have been found

Are you saying that with the borrowed control valve, the transmission regained its forward direction temporarily?
But then stopped working forward again with the borrowed valve still in place?
Have you returned the borrowed control valve to the donor truck and if so, does that truck now have both direction capability of has it too lost its forward?
If the problem stays with just one truck, I would believe the trouble to be stripped splines of the Forward Clutch drum to be the problem.
Have seen those strip out and then resume operation (but usually for just a few feet) later.
You cite having done some fairly major stuff already but have the clutch drums been taken out and inspected?