An overdrive is a two speed, electrically activated, supplementary gearbox which is attached to the rear of the main gearbox.  It was a popular period option which easily converted normal production 4 speed gearboxes into 5 or 6 speed units, and were a very common fitment to MGB’s.

They fell out of favour once 5 speed gearboxes became more routine coupled with the lack of room to fit them in a transverse front wheel drive arrangement.  Fortunately they continue to be well serviced (by and large) for parts availability.

Driving technique

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.

Fault diagnosis

Overdrives tend to be quite reliable, when they do develop problems the Factory manual is very useful in diagnosis.

In general, faults fall into 3 categories:

  • Electrical – usually external of the overdrive, those that aren’t can always be fixed without removing the overdrive.
  • Hydraulic – nearly always can be fixed with the overdrive in place as most items are easily accessed from under the car.
  • Mechanical – usually require removal and stripping of the overdrive unit.

Tips for tracking faults:

Test gear

If the first two simple electrical tests below don’t provide an answer, then it is best to connect up a test light with a long enough length of wire for the bulb to be observed while driving the car.  It should be wired in parallel with the solenoid (fit  a double female bullet at the solenoid plug) and will provide you with complete information about if and when power is getting to the solenoid.

The hydraulic pressure test shown in the workshop manual is best done with sufficient hose length to attach it to the 1/4 light so the car can be driven.  That allows a really useful comparison with any observed symptoms and system pressure.  Mine also has attached to it the test light, so together the two provide a very useful picture of what’s happening or not happening as the car is driven and the faults experienced.

Electrical faults can occur in:

  • the dashboard switch – wiggle the switch side to side and up & down to see if it makes any difference.  A marginal switch will respond to this.  However a completely dead switch won’t and will need further checking.
  • the inhibitor switch:  In the overdrive circuit there is a switch, known as an inhibitor switch, in the gearbox which is triggered when 3rd or 4th gear is selected, but makes no contact in other gears.   It’s purpose is to prevent damage from the greater torque at the output of the gearbox (where it enters the overdrive) when in 1st and second but particularly in reverse where working in the opposite direction could get a bit ugly.

It’s a plunger type of switch, triggered by the sideways movement of the gear lever (ie across the gate) and doesn’t need much movement to operate it  – so the set-up of it is quite critical.  When it is out of adjustment, the results vary between allowing overdrive in first and second and no overdrive in 3rd and/or 4th.  Most commonly, out of adjustment results in the symptoms you have, as the switch is almost making contact in 3rd/top and pulling the gear lever over to the right is enough to complete contact. These can also fail completely, but its most common fault is its sensitivity to adjustment.  If adjustment is an issue, it will often show up by the overdrive dropping in and out as the gear lever is moved.

To check for this:  engage overdrive in 3rd gear and while in gear pull the gear lever as far to the right as you can.   This engages the inhibitor switch just a little more so if its setting is marginal then it may be enough to make contact and bring the overdrive in.   Then move it back to the left – opposite process, will make the overdrive drop out if marginal.     Repeat the process for top gear.

Any sensitivity to this action indicates the need to adjust the switch, done via washers between the switch and gearbox remote … accessed on 3 synchro B’s by removing the top cover of the transmission tunnel, but from under the car on 4 synchro models unfortunately.

  • the wiring – the only common wiring fault occurs in models with the switch on top of the gear lever.  It is wired in by a couple of wires which run down to the base of the gear lever, thence to the rest of the loom.  These wires fatigue and will (after many many miles) break as they flex back and forward with the action of the gear lever.

Important tip!

These wires have also been known to break in a way that creates a short and burns out the loom!  A good precautionary measure I’d strongly recommend to anyone with a gear lever swtich is to have the feed wire fused – unlike the original set-up!

  • the solenoid – if the test light consistently shows you are getting power to the input of the solenoid, your next step is to remove and test the solenoid.  Make sure you remove it complete with its steel casing, inclusing top and bottom.  Make sure you don’t lose the little steel ball in the snout!  Losing this and not realising it should be there is a common cause of non-working overdrives!

I find the best way to check it is to connect the coil wire and a suitable casing earth to a battery so it is activated, then sit the unit on some scales (bathroom scales will do) and push down into the little hole in the end of the snout until the solenoid plunger moves, noting the load on the scales when it does so.  Anything less than 7 lb means the solenoid is marginal.

Hydraulic faults (see also “overdrive pumping” below):

  • If there is power to the solenoid, and it has enough grunt in the above test, then do a hydraulic pressure test.  The manual covers hydraulic faults well and describes how to go about the pressure test.  If you haven’t access to a manual, then the take-off point for the test is the hexagonal plug on the underneath of the unit, next to the square cover for the solenoid.  This covers the pressure relief valve.  Remove it (carefully noting the position and order of the bits that come out with it), drill a small hole (3mm) right through it, and counterbore and tap the outside end to take the adaptor fitting for the gauge.
  • It’s surprising how often an overdrive is removed and stripped before doing this test, which makes diagnosis very much harder!  Generally speaking, most pressure-related causes can be addressed externally, but if the pressure is still holding while problems occur then it’s an gearbox-out job.

Mechanical faults:

  • If nothing shows up in either the electrical or hydrauilc tests, there’s nothing else to do than remove the whole unit and strip it.
  • Unfortunately this means the whole engine and gearbox, as while it is just possible to remove the overdrive of the gearbox while it’s still in the car, there is no way you can get the alignment needed to engage the mainshaft into the two sets of splines inside the unit when putting it back from under the car.
  • 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.  (Yes, I know I’m repeating myself but as I said it’s surprising how often this doesn’t happen so I thought I’d emphasise it.)

Overdrive “Pumping” – A 10 min miracle cure!

One symptom not covered by the manual was a “pumping” effect during overrun when the car was driven in direct drive, while in overdrive or going fowrads in direct drive 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 second (or single thicker) gasket under the square plate covering the solenoid. The hole in the centre of the gasket should be just large enough to allwo the plunger through, rather than the full diameter of the original gasket.  This allows 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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