I pulled the engine, on this old Yale, to replace the ring gear. I also changed the oil and transmission fluid. After some research I used "Triax HTF Forklift " fluid. It's been slipping off and on ever since. Someone suggested using a "thicker" trans fluid in it. The old manual I have says use various kinds of "dextron". What would you suggest?
You did not use the fluid recommended in your manual? Why?
I don't know, just a dummy I guess.
I would try the Dex first, before worrying about a possible problem.
Ok, it says "Dexron C-2" is that the same as "Dexron II" ?
yes, and you can sub Dex III - DEX V, at least in transmissions.
What is the difference in those?
Updated, better for newer transmissions, but backward compatible.
Which would you try the 2, 3 or 5?
Fluid viscosity isn't going to do much of anything for slippage due to glazed or worn clutches. Thicker fluid might help for a minor clutch piston leak but that's just a bandage. Hopefully there isn't damage to the clutches by using a non-recommended fluid. Additives in different oils can react negatively with the friction material. I'd try the correct fluid first and pressure test the clutches. If its still slipping with the correct fluid and clutch pressures are good you're most likely looking at pulling the transmission.
I would suggest draining what is in there, refill with Dexron II, install a new filter, use it a few hours. Drain that while hot and install a new trans filter, refill with Dexron II and back to work. This is as close as you will get to "flushing" it to avoid pulling. You might be surprised, rinsing fluid out the (old?)possibly clogged filter sometimes work wonders..