Amazon: M41 gearbox jumps out of gear

[AdamG87]

Hi all

I am having issues with my M41 gearbox jumping out of 4th gear when not accelerating. The gearbox was rebuilt a few months ago and the bearings were replaced. The gears, synchros etc. were deemed to be fit for re-use. I have noticed the problem getting worse and it happens more often when it is out of overdrive and when the gearbox warms up. When it does jump out of gear, it will grind. To me it seems like the synchromesh, synchro assembly on 4th gear is weak.

Would appreciate it if someone could shed some light on this issue.

[arcturus]

First thing is to check that the lever is securely located in box. Check that the large nut at the base of the lever is securely tightened down onto the box.

[AdamG87]

The lever is tightly secured onto the box although the top cover bush is fairly worn. The lever is still secure and doesn't move around too much. It will jump out of gear even on a smooth surface so it seems like it could be something inside the box.

[Bob Meadows]

No direct experience with this gearbox: ~ I have worked on Riley boxes with a very similar arrangement of springs/ball bearings to locate & hold the selector rods- you may have a weak spring/damaged. The same can be a reason for jumping out of gear.
Good Luck
Bob.

[northNH]

It IS something inside the box...

The 3rd/4th gear brass selector fork, which SHOULD mount perfectly perpendicular onto it’s sliding shaft, is slightly bent toward the REAR (3rd gear end) of the box.

Thus, when you pull the gear lever back into 4th, the bent fork is not allowing the shaft coupling to move fully FORWARD to fully lock the input shaft to the main shaft, allowing it to de-couple and “pop” out of (only happens in) 4th gear at the slightest vibration...

Driving in 4th with hand resting on gearshift knob for years exacerbates the eventual kink; common for this to show up as a problem after a rebuild as the bend is slight and rarely noticed.

Cleverly, and with balls it is possible to fix this popping-out without opening or even removing the gearbox. In fact without even leaving the comfort of the drivers’s seat of the car.

Willing to explain what to do (as revealed to me by my long-dead legendary vintage Volvo guru who swore me to secrecy many years ago), if no one can figure out how to do it...

🙂

[AdamG87]

Thanks for the response.

I would be interested in learning how to fix this issue without removing the gearbox.

[northNH]

You should understand what I wrote above, have a fair conceptual and/or pictorial image of the cutaway drawing of the M40/41 gearbox (e.g. Haynes, FSM), and understand clearly what you’re going to do...

1) Park car with nothing in front of it, parking brake on.

2) Raise engine speed to mid-throttle (~3K RPMs).

3) DO NOT DEPRESS THE CLUTCH PEDAL AT ALL ‘TIL FINISHED!

4) For LHD cars (RHD cars, you’ll figure it out...): With your left hand grabbing the shift lever just below the knob (motor @ 3K RPM) gently and firmly push the shift lever towards 3rd gear AS FAR AS IT WILL GO.
With sufficient engine speed the lever will stop and will not engage 3rd gear.


5) With the gear lever’s forward progress firmly held by your left hand and stopped by further movement forward by the rapidly spinning internal shafts/synchros, with your right hand give one good smack to the top of the shift lever (i.e. shift knob) towards the front of the car.

6) You’re done. Drive it.

7) You may need to smack it harder a second time if you didn’t have the balls to smack it hard enough the first time.

What that smack does while the 3rd/4th gear brass selector fork is internally fixed and unable to move further REARWARD toward 3rd gear is to slightly bend the tweaked selector fork FORWARD, allowing it now to push the shaft coupling far enough forward to firmly lock the input shaft to the mainshaft for 4th gear operation.

Remember, at least 3K, and don’t touch the clutch pedal.
And with all due respect to Volvo Joe from central VT, RIP, the cat’s out of the bag...

[northNH]

NO response? Amazing.
Must be the wrong crowd...

[arcturus]

Originally Posted by northNH

NO response? Amazing.
Must be the wrong crowd...

That's because they forgot to put the brake on and tried it in front of a concrete wall !