Hi folks,
I wonder if anyone has had the same problem as me. I drive a 2006 Volvo FM400 bulk tipper/blower with ishift, and this morning it developed a problem engaging the PTO. You normally select ‘N’ and flick the switch, ‘PTO’ appears on the dash and bobs your uncle, you can hear the PTO engage - then use the cruise control to set the revs to where you want.
This morning after flicking the switch, the ‘PTO’ symbol appears on the dash but will not engage UNTIL you press the accelerator pedal, then you can hear the PTO engage and she will tip as normal. However if you lift your foot, the PTO jumps back out of gear again. Ive tried setting the revs with the cruise control in the hope of keeping the PTO in gear but it doesnt work, the accelerator pedal must be pressed. I managed to get the load off by using a piece of wood of the right length to press the accelerator.
I also noticed that while reversing slowly, when I lift my foot from the accelerator she disengages reverse and she would actually free-wheel off down the yard! Its very jerky when trying to reverse slowly due to it appearing to jump in and out of gear. Also setting off from stationary in ‘A’ it takes more accelerator to actually make the clutch bite, and she is jumpy when setting off.
My first thought was the accelerator pedal itself but weve tried another one tonight and its just the same 
Diagnostics says electrical or mechanical clutch fault, but gives no more detail, clutch was replaced by Volvo about 250K Kms ago so it should be good for another while… wondered if any of you guys had encountered anything similar??
While I can’t comment on the PTO side cos I didn’t have that, the other symptoms you describe were similar to what I had on a Renault premium with ishift. Turned out to be a module on the side of the gearbox that was knackered. That’s about as much as I can tell you though.
Cheers 