Thanks for advice but that won't help.
Clutch plates was so worn that there was no coatings on them.
And plates was twisted and burn as well.

Case closed.

You check the strainer first. The strainer is situating in the inlet port of oil pump in transmission box. Remove the strainer and clean it well by petrol. then fix back again. Start the forklift and check for the present performance. If the performance is better than before, You replace the strainer, Transmission filter and oil for Transmission. You can use the oil SAE 10w. as your transmission oil.

good deal
i was fixing to mention that generally the clutch packs give the most trouble but you found it :o)

Thanks for the update report.

Hello again!

I've had the gearbox overhauled and it runs fine again.
It had the following types of faults:
-worn clutch plates
-worn and twisted metal plates
-broken piston rings of the clutchpacks pressure control

All those parts, including oils & filter, cost me about 1000 euros (~1300$) so it was worth making.

Pressure of the packs rised 32psi to 152psi (10,5bar).

Luckily the oil pump and torque converter was ok.

Thank you all for your interest! This topic ends now.

Maintenance does't work.

I changed new oil filter and new TO-4 quality oil.
I also clean control valve unit and tranny cover from inside. Furthermore I empty and clean the oil cooler.

Clutch pressure rised to 72 psi (earlier 32 psi) but drops when engine runs about 2 minutes.

It's time to lift the tranny out. I think oil pump is my problem...

- Gary -

oh that's not too bad, if it is only that much
good luck

Thank's guys, you saved my day!

I try to find a new filter in somewhere, change new oil and try what happens.
If that works it saves me to a big renovation!

Btw Jplayer, there are that silver goop only nearby that suction filter and the quantity of it is about 2-3 tablespoons. Therefore L1ftmech has a good point...

- Gary -

The part about the strainer being dirty, but not so dirty that it won't let oil go through.
That is not exactly saying the strainer is OK.
That strainer has a very fine mesh and if you can see anything at all on the strainer, its capacity to flow volume has been reduced.
I'm sure that the strainer was designed with an overabundance flow capacity and I don't know at what point (% of blockage) problems would surface related to reduced flow volume (which will directly affect the ability to build pressure).
To my recollection the suction strainer on those vintage units was the only filter present. At least the ones we had did not have an additional external filter.
You may want to try jdplayer's advice and try a clean out and new oil.
My take on the situation is: You have little risk in trying and if you find out that you need to go inside later, the new strainer will still be good.

silver goop is a sign of friction disc and metal wear, its pretty common on a tranny that hasnt been serviced. I'd clean it out, change the filter and fluid, if theres a sump screen on the return hose at the bottom of the tranny i'd take that out and clean it too.

BTW... how much silver goop are you talking about? Just a film in the bottom or is it alot?

Then put her back together and fill it up and hope for the best, it may just come back to life. Lets just hope so anyways.

Done some research today.

- oil out from tranny, it looks like coffee (!)
- oil filter out, it's real dirty but not that dirty it wont let the oil go thru
- control valve unit off, no broken springs or other failures
- side cover out, no major damages (what you can see on that small gap between tranny and chassis). There are lots of silver paste in the bottom of the transmission case. It might be on disc plates?

I think that next I have to remove the engine and check the oil pump... :(

Before you jump in to replacing the pump, make certain that the suction strainer is not clogged.
The pump can be replaced without removing the transmission from the truck............but you will have to remove the engine in order to access the pump that way.

Make sure the chrome spool valve for the inching control is retracting all the way in also.

Yes, I did rev the engine.
The pressure is lower on idle. This 32psi is max pressure even if I rev the engine up.
Is it possible to check/replace the oil pump without taking the tranny out of lifter?

yes it does sound like the pump is worn letting oil bypass causing low pressure. You got this pressure at idle? or did you rev up the engine also? it should increase when engine rpm is increased. If it doesn't then this is indicator that pump is indeed worn/damaged.