1986 Fuso bus air assist gearbox issues

Hello everyone I have a friend with a 1986 Mitsubishi Fuso Motorhome and he is having issues with his gear changes. It has the air assist 5 speed manual.

The first issue he has was it was grinding going into 4th. He removed the box and sent it to a repairer and they found no problems with it mechanically.
He put the box back in and now is getting weird things happening.
When the bus is stationary he can select every gear without issue, all the correct lights come on when engaging every gear and the bus will move fine.
But as soon as he starts to move and engage second gear it will not select and the light goes out. If he comes to a stop he can easily select any gear again and the lights work. Reverse works as well.
He has tested all the solenoids on the bench and the air tank has pressure. The regulator on the tank is working.
If he leaves the bus over night the air tank is empty so a small leak sounds likely somewhere.

Can anyone shed some light on this weird issue please?
Thanks Grae.

I don’t know if it’s the same style system but we had issues with the air brakes leaking slightly which caused the air assist clutch not to work, was fine to drive as long as you didn’t use the clutch and the brakes at the same time
Gumpy

Out with the soapy water

Is there an inline filter before the regulator?

Not sure if this will help or not.

I have a 6 speed isuzu box with air assist (both lever and clutch)  of that era in the bus.

1. The wrong oil gave me all sorts of problems. GL4 is very different to GL5.    Mine uses GL4. GL5 makes the synros sticky and the brass wears out pretty fast.

2. The cables need to be adjusted incredibly accurately.  The gear lever in 'rest' has align with the 3/4 gate perfectly.  If it doesn't, the gear lever spring (and feel)  AND the gate detents are not aligned when finding 5/6 and 1/2.  Goodness knows how many time I crunched gears until I worked that out.

 

Thanks for the replies. Does anyone know what the regulated pressure is set at from the factory for the transmission?

does anyone know what the regulated pressure is set at from the factory for the transmission?

What do you mean by "the transmission"
Do you mean the air assist on for the cable selector/quadrant box (?)  or the air assist for the clutch(?)  or something else?

Both actually.

A friend has a Fuso import bus (motorhome) of that era. He reckons both tanks are coupled pressure wise (brakes and service).
He measured 110psi on the first tank.

hope this helps.

Thank you very much.