Overdrive sticking and engaging while reversing

[Cumbria Graham]

I've just had my recommissioned my 3A after a very long layoff. I had the work done by a very competent engineer. Complete engine rebuild and replacement of the gaskets and seals on the gearbox to cure the leaks. This involved separating the gearbox from the overdrive but not stripping either. Reversed out ok and fine for the first three miles but the O/D might have been engaged. Couldn't reverse afterwards and O/D proved to be engaged. Checked all the wiring and actuated the solenoid a few time and that seemed to clear it. Just driven it home, about 25 miles and reversed once - all OK. Went to move it again and the O/D again locked. It appears that the action of reversing is causing it to bind up. Any Ideas?

 

Regards, Graham

[RobH]

It's not clear from your description whether you were turning the OD on or whether the car somehow was doing that itself.  If it's doing it itself and you are sure the wiring is OK,  is the solenoid lever adjustment correct and is it free to move?  Has the solenoid become 'sticky' so it is not retracting properly every time when de-energised ?  

As you probably know there is a uni-directional roller-clutch mechanism in the overdrive that is easily (and expensively) destroyed if you reverse with OD engaged.  It is obviously that which is 'locking up' so fingers crossed it is still OK. 

[John Morrison]

Hi Graham, 

if this has happened more than once, suggest you disconnect the solenoid - check the solenoid is 'Out' so you can reverse safely and let us have a look?

John

[Cumbria Graham]

This is getting strange. Drive round the block, park up in the garage, still engaged, then isolate the battery and leave it 36 hours. It's now disengaged so at least I can get it out from under the 4 post lift and put the car onto it to disconnect the solenoid and check the circuits and the solenoid operation again (for the 3rd time) to see if it's live even without either the gears in 2/3 and/or the O/D switch on. It could be a sticky solenoid, a faulty relay, a failure of the oil to depressurise (an MG forum gave a link to O/D spares suggesting that the orifice from the relief valve could be blocked or that the clutch itself is failing to disengage and the oil that came out was fairly murky. The strange thing is that it seems to engage after a 15 yd reverse. Has anyone come across this relief valve orifice blockage?

Voltmeter out  tomorrow and I'll keep you posted. 

[John Morrison]

Hi Graham, 

Overdrive wiring diagram for you;-

H

 

Still think most likely your problem is here.

Re servicing the operating valve, and blowing through the tiny orifice with compressed air, then yes, more than once.

Dismantling is simple enough, best to swop the ball bearing for new, re-seat, and make sure you get the plunger, spring etc., in the correct order and located

or it won't go back as it should. Re set the operating arm as per the book, but these days, with 60/70 years plus casings using the 3/16 drill is not always quite enough, and you need to 'Lean' on the arm to set correctly.

If you want to substitute the relay, to eliminate it, any modern, black box, 30 amp,  4 pin relay does.

John.

[Trev Good]

I had the exact same problem a couple of years ago on my 3A, I did exactly what John is saying and as it was fairly soon after I had redone the Scimitar overdrive, I cleaned out both the gauze filters in the overdrive, the hidden one is under a screwed plate!

Luckily, the overdrive was fine, even after locking up the rear and is now operating perfectly.

[Trumpy3]

First is to separate the electrics from the mechanics. Leave the car overnight to let any residual oil pressure to dissipate. Remove the connection to the OD solenoid and tape it up to prevent any 'shorting'. Take the car for a drive and test the reversing. If all is OK then the issue is electrics. If the problem still persist, the the issue is mechanical.

Electrics, then test as per the wiring diag. If it is mechanical then my first thought is the operating valve. If the valve mechanism is faulty then the oil pressure from the accumulator will flow direct to the operating pistons and engage OD. Remember that oil pressure will be present as soon as the gearbox output/ OD input shaft turns, the oil pump will pump oil and develop pressure in the accumulator. Forward and/ or revers.

The operating valve is a fairly simple ball valve with five parts. Cap (A) spring, guide pin (C) , ball (D), valve seat (F) (machined into the casing), Valve operating rod (J) . It is easy to remove and refit. Undo the cap and remove the guide pin and ball. Then use a thin wire or magnet to remove the operating rod. The bleed orifice they talk about being blocked is in the rod.

The operation of the solenoid turns a shaft that runs across the housing and turns the brass check leaver on the right hand side of the housing. A small cam (K) is attached to the shaft under the operating valve and the shaft turning lifts the operating rod and so ball valve. This allows the high pressure from the accumulator(E) to flow from above the valve down through the seat and on to the operating pistons (L).. Disconnecting the solenoid, relaxes the cam and allows the spring above the ball to push it down on the seat and isolate the accumulator. As the cam allows the operating rod to drop down it uncovers a small drilling (G) down through the rod and lines up with a small horizontal bleed  orifice (H). this allows the pressure in the operating pistons to bleed to the housing thus disconnecting the OD. This drilling is covered when the operating rod is pressed up against the ball and so preventing the oil from bleeding away when OD is engaged.

Check that it was installed correctly and the ball and seat are in good condition. New balls are readily available from bearing suppliers (at least here)..

Edited November 11 by Trumpy3
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