I have a 1965 TL-14 wheel loader. It has a TwinDisc power shift 4 gear (+ 4 reverse) transmission that seems to work fine in 1st gear but barely moves the loader in other gears. Clutches engagement is fine, I can definitely feel them engage and tractor creeps ahead/backwards, however in order to move at any appreciable rate it needs to be revved to the sky.

Today I checked the pressure gauge (as I understand connected to the forward clutch pack) while cycling through the gears in idle, both forward and reverse. It behaves the same way - around 30-40psi in neutral, then move shifter to any gear and the pressure goes quickly up to 250psi and you can feel the clutch engaging.

There is enough oil as can be seen through the sight gauge, it also looks good, ie light colour, no foam. I don't know when it was changed last time since I got the loader only recently.

The fact that it behaves exactly the same in all 8 gears gives me hope that it is not clutch packs for gears 2,3 and 4 burnt/slipping. Or does it actually mean exactly that, say if the oil pressure creating part (pump/converter) is functioning fine but the clutches are gone?

The pressure seems to be tad higher than the book states (150-220psi) but could be just the accuracy of the gauge. Also I took these readings when the oil was still cold.

I know from different forums that when the strainer is plugged there is not enough oil for the pump to raise the pressure but in my case it seems that the pressure is not a problem?

I did read somewhere to watch the driveshaft between converter and gearbox while in gear and not moving, the shaft should not move if the clutch is good. Did not try that today, is this a correct procedure?

Can someone help me to troubleshoot this issue, I really want to bring the old girl back to life.

Many thanks everyone.

Thanks Eric, yes I do have an operator's manual that has pressure values and measuring ports shown. I however do not understand how this is supposed to work. The pump must be OK as it definitely creates and holds pressure (not sure about low pressure at idle though). I would guess that the torque converter is also OK because when in first gear it seems powerful and spins wheels if on softer ground.

I wonder if it needs more flow of oil in higher gears  that it doesn't have or simply all the clutches bar first gear are shot and slipping. Also read somewhere that it is not clutches but the seals on the clutch shaft that start leaking...