Fiat Allis FR12B won’t drive after sitting for the winter

Helping out at the family farm, I’m a mechanic but don’t have any heavy equipment experience. This is a high hours late 80s/early 90s loader and I believe it has the ZF 4WG-120 trans. It has been working good for seasonal chores the last few years without skipping a beat. This spring it won’t move. We at least all the time, it managed to drive a short distance a month ago now nothing. I looked it over, found some broken wiring for the transmission solenoids, thought for sure that was it. Nope. Took apart the controller, no corrosion inside. It’s a DW-1 controller if that helps. The power fuse for it is good and I’m getting 24V at the solenoid connector rotating through gear selections. I found a couple wiring diagrams on this forum that are very close to this machine but not an exact match. I was able to jumper the park brake and clutch cutoff switches and that made no change. I tried every gear thinking it’s the CCO and still nothing. The trans pressure gauge does not reach the green section, the needle gets right on the edge with a lot of throttle but the wheels barely move. Most I can wiggle it about a foot, and it’s on a slight incline and rolls back, so there’s no brakes dragging. I’d think with low pressure it would still drive but just not full speed, I could be wrong. I checked the hydraulic filter and suction screen in the tank, nothing wrong there. The gauge does flicker with each gear change so that’s telling me the solenoids are doing something. I also get a red light after pressing either brake pedal and it stays on until shutting the machine off. I can’t figure out what it is, it’s some kind of a circle inside a triangle, the red is faded compared to the other lights so this probably has been on for years. I haven‘t operated this machine before and my in laws don’t remember if that light was on last summer, they certainly don’t remember what the pressure indicated. Just start and go. Any ideas where to look next? Why would this quit over one season? Any regulator or adjustment anywhere to turn up the trans pressure? Is there some kind of safety lock out I’m missing? Thanks!

This transmission has external suction pipe on trans from the sump, check it for damage, cracks or leaks. I am also in MN

I didn't see you mention checking the fluid level in the transmission itself. I am not at all familiar with this loader, but I have a hard time believing that the only place to check fluid is the hydraulic tank. There must be a sight glass or dipstick for the transmission somewhere.

Does this transmission have a de-clutch of some sort connected to a brake pedal that disengages the transmission so you can rev the engine to lift a heavy load?

Trans level checked with machine running

Hyd level is good, right at the full cold mark. Haven’t been able to get any temp in the fluid just idling stationary for a hot check. The left brake switch is the CCO/de-clutch, I checked for 24v to it and through it, and bypassed a jumper wire just in case. It sure acts like it’s de-clutched, there is just no torque to the wheels at all but the bucket operates with full power. I also found out a friend stopped over a few weeks ago and it drove just fine in reverse a couple hundred feet, then barely crawled going forward to its parking spot, then no reverse again. They cut open some filter and there was no metal, I’m not sure which filter they pulled. I didn’t know there’s another suction screen. Hard to see anything, the frame really covers up the trans. A neighbor mentioned that too, said to look for two main hyd lines and the lower probably has the screen. I will look for this the next time I’m out at the farm. Thanks for the comments!

A person call me years ago with same problems .the fix was removed filter started machine to see flow, he go oil installed filter and work good . ps big mess

Ive not seen a ZF trany that used the hydraulic oil tank for its reservoir. The way you described your symptoms it sounds like low oil level.

zf trans has it,s own reservoir and dip stick

Update, still doesn’t work. I didn’t realize at first the trans is separate from the hydraulic system. My father in law was topping it off with JD hyd fluid. After a member kindly sent me a ZF manual, we found it takes engine oil. Drained it, checked the suction screen, replaced the filter, and filled it with diesel engine oil. No change, still won’t drive. There is another clue, whenever you press either brake pedal after starting it, a red light with a triangle and a circle inside turns on and stays on until shutting off the loader. I don’t really know what it is but obviously something with the brake system. I’m thinking this could be acting like the clutch cutoff switch. Searching on this forum for ideas it could be the brake accumulator. Again I’m not familiar with these loaders. I see there’s tanks in the back of the loader. How do I test and how much pressure is it supposed to have? Thanks in advance!

2nd update, success!! I’m an aircraft mechanic and a saying we have in our industry working with other mechanics is ‘trust, but verify’. After countless times asking that the oil level was good, I finally pulled the dipstick myself, bone dry. Finally drives again after filling to the correct level. The trans pressure gauge still doesn’t go into the green but who cares, it’s old and it works. The brakes definitely don’t work at all. It does build air pressure to the green but beyond that something else is bad. That’s for another day…

You need to install transmission pressure gauge ,if do not have one in dash!!!! there is pulgs on trans valve 1/8 pipe will fit, look it up in the manual!!