Overdrives

Introduction

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:

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 swtich (this is the switch in the gear remote which prevents overdrive engaging in reverse, 1st and 2nd)  These can also fail completely, but its most common fault is its sensitivity to adjustement.  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 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 everdrive 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 huydraulic faults well and describes how to go about the pressure test. 
• 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. 

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.

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 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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