Mk2 conversion to All-Synchro Transmission ... Overdrive won't engage

The transmission migration to the all-synchro is up and running.
Quite nice to have first gear synchro and a little less gear noise when accelrating in first.
The new box with O-drive is about 5 inches shorter than the old one so I had to make a new mount and have the drive shaft lengthened.
All seems fine except the overdirve won’t engage. I can hear the solenoid click, but nothing happens. I took the little cover off the engagement control lever and verified that it moves freely.
It is full with new 90wt gear oil. I drained the old oil and checked the o-drive filter screen which all looked fine. I am going to drive it a while and see if it breaks loose.
Does anyone have any advise on how to diagnose this? I don’t know the history of this prior to my purchase. Hopefully the o-drive was never run in reverse. If it was, is there some easy way to tell?
Anyone need my old Moss box? It works fine, but leaks like a sieve.

Are you sure the overdrive pump is putting out pressure. If the slide on cam for the gearbox mainshaft was left off, no pressure would be developed. There is a port at the bottom of the unit that the factory gauge can be attached to to determine the pressure being developed which is stated in the workshop manuals. IIRC it varies somewhat depending on what engine the overdrive is fitted to.

There is a adjustment on the operating valve , have you checked that :thinking:

I’ve looked in the Haynes manual but can’t see what the weight of the oil should be but 90W sounds too heavy to my mind?

Overdrive transmission should have 30wt non detergent oil or I’ve used Redline MT90. It’s a thinner oil like 30 WT.

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In my experience most of the OD problems are because:

  • not enough oil
  • wrong type of oil, OD is a bit picky for oil type.
    I agree with others, 90wt may not be ok, try what was recommended for
    Jaguar gearbox 60 years ago (was it SAE30?) or Redline MT90.

According to my documentation (2 separate sources), 90 weight is correct for the all synchro gearbox \od. 30 for the Moss box unit.

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My '63 Mk2 was an automatic and I had a conversion done several years ago. The transmission came from a '66 model, with the later short overdrive setup. I sourced a genuine “shovel” type rear mounting and fitted that up. It all looks original.

Gearbox oil 90EP !

I believe the next step should be to verify the positioning of the solenoid operated engagement lever.
If that looks ok, then measuring the pressure is next. I need to figure out where to connect the gauge.
Ken Wallace

If pressure and the position of the operating valve both check out another issue can be that the sliding member is simply stuck to the annulus inside the overdrive unit. I encountered the same issue with 2 overdrive boxes that sat uninstalled for unknown periods of time before I got them.

I went through all the trouble of installing one in my S1 XJ6 during an engine rebuild and manual transmission conversion without bench testing the overdrive. All I did was confirm condition of the box’s gears and synchros, replace the OD filter and confirm solenoid function and travel, and change fluid. When I installed everything the OD would not engage.

After removing the box and disassembling the overdrive I found that the sliding member’s friction material had become extremely stuck to the annulus from sitting unused. I had to resort to some careful violence to get them unstuck but once I did I sent the sliding member off to be re-lined. Everything else inside checked out and when re-assembled with the newly resurfaced sliding member it worked perfectly.

I hope you find an easier-to-fix issue but unfortunately it is a possibility that you have the same problem. The good news is that Overdrive Specialties in the UK can rebuild the sliding member for a reasonable price.

Thank you RustFreeMike. Your scenario is what I have been fearing. This transmission has been sitting for years. Does the sliding member get torque applied to it if I trigger the solenoid, but it remains un-shifted?
I am wondering if I drive it with the overdrive switched on long enough, that it will break loose?

This is a picture of how I built the mount to use the existing bolt pattern to support the transmission mount five inches nearer the front of the car.

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When you engage the solenoid the 2 actuating pistons try to push the sliding member forwards towards the case/brake ring… So really if it’s stuck the only thing trying to unstick it is the hydraulic pressure acting on the pistons which in my case couldn’t unstick it. Maybe violent acceleration might cause it to release but I didn’t try it. I wasn’t babying it but didn’t work it over either because the engine was still in break-in mode. I kind of doubt that there’s much if any slippage between the 2 except during the short periods of application and release and not during normal driving.

I literally had to break the sliding member and annulus apart by chucking the back of the case containing the annulus in a vice and putting a 2x3 on the edge of the sliding member and hitting it with a hammer. It was seriously stuck but a careful beating unstuck it with no damage.

I see a guy on the TV years back un sticking a engine with boiling oil !
Not saying boiling but maybe very hot thinner oil would be worth trying , no need to go for a ride , just jack the back up so wheels off the ground !

Just for info, there are warnings in the workshop manuals to ensure one does not try to back the vehicle up IF the overdrive is engaged or stuck in the engaged mode. Severe damage will occur to the one way clutch internal to the unit.

If the overdrive is stuck due to sitting so long, it would have to be in the un-activated state. I have reversed several times since installing it without any terrible results, so I think, after hearing others experiencing a stuck sliding element, that I will remove and disassemble my unit to un-stick it. Hopefully the friction lining will not be damaged as I don’t know where one would go to get that reworked.


The good news is that with these compact type overdrives, at least with my 3.8S, they can be removed with the engine/gearbox insitu. Do remember to remove the pump plunger when refitting so it does not foul the cam when sliding back in place. Have you been able to check the operating pressure first?