Volvo N12 clutch pedal stuck up

My F86 had the same problem. The clutch is air assisted hydraulic. If you do not have air pressure above about 60 psi the clutch has no assist. Don't know about later systems but I believe they are the same. You can change gears by putting your backside against the seat and pushing the pedal with about 50kg force! I have driven some distance like this (Tamworth to Brisbane) and apart from leg cramp everything operated normally.

If the hydraulic side of the actuator goes, the pedal will just go to the floor. You seem to have a good hydraulic circuit.

Mammoth sent me a later model clutch actuator after I got sick of repairing my original.

Bottom line is, you need air. If, as you say air lines are missing there probably is nothing wrong with the actuator and once you get the air system sealed it will probably work just fine.

Just had a look. On this link is the late model actuator Mammoth sent to me (first unit) to replace the original F86 unit. You will see it actually is the unit for your truck so my comments above hold good.

www.uniwest.com.tr/catalog/VOLVO_CLUTCH_SERVO.html

Lang

A Volvo F12 74 model may have the Kongsberg Booster fitted on the SR61 or SR62 box
See
www.kongsbergautomotive.com/globalassets...t-catalogue-2015.pdf
Have a look at this booster for the F12. 626392AM Ø80 mm Clutch Serv
There is an air line going to the port on the booster hydraulic cylinder where the yellow plastic cap is and the hydraulic line mounts between the two bleed nipples at the rear of this picture.

This explains it a bit better


As Lang says until you get air pressure to this you will have an extremely heavy pedal if you can push it down at all. The F86 has a completely different booster but similar principals. Though I have seen the master cylinder play up and do the same thing, by blocking the MCyl outlet port.

yeah our N12 doesn't have a clutch until about 7 on the dial. You get the go pedal at about 6. Ours is a 14sp but I think there were a few options - all with less gears according to the manual and it's a late model 1990 intercooled. Also won't build up air unless the box is in hi, but once it has air leaks less slow in low... And won't go from hi to low if the splitter is in low :huh: I know why it is but can't be bothered to pull the floor out to get to the issue

I was gunna ad my piece to this .... but I'd only be repeating what Chocs and the others have said .... gotta have a good lung full of air before anything happens .... unless you do what I did years ago ..... adjusted the splitter arm a bit too close .... ol mate took it for a drive and flicked the switch ....BANG .. straight through into high split at full loud pedal ....ol mate wasn't impressed as its supposed to only change when the pedal is depressed

Ya know what this thread is missing??
Onetrack..He would sort this in a heartbeat

A massive post from Onetrack with copious amounts of text, cut n pasted from the Volvo Wiki page outlining the history of air actuators from 1820 to present... With some decent diagrams and schematics
Followed by a story about a Volvo he and his brother owned and rebuilt in the 60s, just south of Perth.

Where are ya 'Track?
This one is right up your alley

chocs

d23j is correct about the clutch, with no air the pedal was not movable.

The older models had a 16-speed gearbox, probably ZF as Chocs said. These had a range-change switch on the gear lever and a splitter controlled by quite a large T-handle on top of the dash. Later models had Volvo's own 14-speed box which had both the range-change and splitter switches on the gear lever.

Way back when ZF didn't even have synchros on the S6-80. (6 speed split) Volvo put the synchros on theirs.
In Australia Volvo truck have almost always used their own box, was a 4 speed range and split givin the 16speed in the SR50 and carried through to SR62, which changed to the SR71 14 speed about mid /late 80's The only other box I have seen was a factory fitted 13speed

The only ZF 16 speeds were used in the true "VBuses"

Sometimes the fork on the end of the cylinder lets go on the splines of the throwout lever shaft through the bellhousing and the cylinder is at full travel anyway and there will be no travel left in the cylinder of the booster. There is a distance to be measured from the Cylinder cover to the locknut on the clevis at the end of the spring can't remember what the measurement was 60 or 65 or something not - sure.

Thank you all, that is most helpful. I am amazed by what you can all do with your collective wisdom. Now we will see whether it fixes it.

Thanks, Erik

...one last thing (if it hasn't already been mentioned, and I missed it) the seat and the clutch are the last things to receive air AFAIK...you may need full pressure (around 120 PSI) to have these two charged..

Paul, just a note, the s6-80 is a synchro box. The AK is the crash box