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Although overdrives are reasonably reliable, if problems do occur they are usually expensive to fix. However, one of the most common failures, glazing of the
overrun linings, can easily be avoided by sensible driving techniques. Symptoms are the engine speed drops under overrun in forward gears (when in direct drive) and, worse still, loss of drive in reverse. Both these conditions get worse as the gearbox oil warms up. The sensation in the forward gears is often initially thought to be clutch slip by owners.
The glazing is caused over a long period of time by changes out of overdrive under high overrun conditions - easily identified by a marked jerk as the engine is bumped up to speed as the overdrive drops out. Each time this happens, a momentary slipping takes place on the overrun linings, which after many such changes causes the linings to glaze and lose grip.
To avoid it, change out when the engine is under power - even a small amount will do. Delay the change if necessary - after all you won't need the lower gear until under load. Alternatively, some people prefer to use the clutch as in a normal gear change.
Early MGBs fitted with the D-type overdrive had a vacuum switch fitted which automatically delayed the change till under power again, but this was discontinued in late 67 when the 4 synchromesh gearbox was introduced. Presumably the theory was that the bigger LH type unit would be able to withstand inappropriate changes - unfortunately time has proved that it doesn't.
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MGB unit with LH
type overdrive (Jim Hekker photo)
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Overdrive “Pumping”
Another 10 min miracle cure!
Overdrives tend to be quite reliable, when they do develop problems the Factory manual is very useful in diagnosis.
A key point in this analysis is ALWAYS measure what the hydraulic pressure is doing BEFORE removing the unit from the car (if this is needed). Doing so usually provides a very clear lead on where the problem may be. The manual describes how to go about this, one helpful improvement is to have enough length of hose on the pressure gauge to enable it to be tied to the 1/4 light , allowing it to be observed while the car is being driven.
One symptom not covered by the manual was a “pumping” effect during overrun when the car was driven in direct drive, while in overdrive all appeared OK. The problem (after having removed, stripped, refitted three times the first time I found it and being ready to tear my hair out) proved to be one of those 10 minute miracle cures.
The cause:
With the solenoid deactivated (ie o/drive switches out) the solenoid plunger was not retracting fully, and the springiness in the small o-ring at its tip was enough to push the piston & ball back into its seat once the pressure had bled off. This of course caused the pressure in the actuating system to build again and started the overdrive to engage again. Past a certain level, there was enough pressure to force the plunger back a bit until the pressure bled off and the cycle started again (frequency about every 2 seconds).
Normally, as the overdrive clutch unit starts to move during the engagement process, the car temporarily loses
overrun between when the inner lining leaves its the annulus and the outer lining contacting the brake band. This is so brief it is not noticeable, but in this case the outer lining never got as far as the brake band before being pushed back again, so for about a second neither lining was in contact & the car had no overrun. As soon as the inner lining took up with the annulus, overrun returned with a jerk and stayed fir a second or so until the lining was pumped away and the cycle began again.
The fix:
Very simple - fit a thicker gasket under the square plate covering the solenoid. This allowed the piston to move a bit further back before hitting the cover, the extra movement being enough to make the o-ring slide down its bore rather than simply compress a bit and act like a spring.
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