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MG MGB Technical - Overdrive not engaging correctly

While returning from a day out yesterday and traveling at approx 65mph I tried to engage overdrive, the result was what I can only describe as the classic learner problem of kangaroo petrol. It felt like the OD was cutting in and out repeatedly. As soon as I switched the OD off it stopped but when I tried it again the same thing happened. I let the speed drop to below 60 and without accelerating tried again, this time it engaged smoothly and stayed engaged even when accelerating to 70. It stayed working until I had to slow and disengage it, as I sped up and tried it again exactly the same symptoms occurred.

Until now it's always worked without issue, engaging nicely with about a 1 to 2 second delay and dropping out immediately in both 3rd and 4th.

The car is a 72 roadster albeit with a 73/74 engine and as far as I can remember it's a blue label type OD. The gearbox is filled with Castrol 20/50 and the last time I looked was full but I'll check later today. The oil has been in for approx 2 years.

I couldn't check last night as it was too late but I plan to check as much as I can this evening. Any suggestions would be appreciated.

R.A Davis

Simple things first-assuming you have checked the level and found it OK.- Drain and refill but clean the filter and fit a new gasket. Check for any debris-mine had bits of friction material in it.... Fit an ammeter in series by pulling a wire off the dash(?) switch: connect to the meter and a wire from the meter back to the switch terminal. Takes about 1 amp when engaged. Does this drop to zero as the o/d cuts out? If so it's electrical. Most common culprit is the inhibitor switch in the gearbox. This can be tested by holding the gear stick in 4th and pulling towards your thigh. If that makes no difference but the meter dropped to zero you will need to check every connection from the fuse box to the main loom by the bulkhead and the sub loom from the main that goes to the o/d,for bad connections.

If the meter does not drop to zero ie there is still current engaging the solenoid, then there is likely to be a fault with the o/d itself. If there are bits of nastiness in the oil as you drop the o/d sump, you will need to pull the whole lot to replace the O/D. Exchange from usual suspects. I had mine rebuilt by Overdrive Repair Services as I am rather suspicious of "exchange" items....

If no obvious bits it could be the solenoid asscoiated O rings. See Haynes /WSM for how to remove/replace. Solenoids themselves are usually pretty reliable

Re fit with a fuse in the circuit-there isn't one at present...unless a DPO fitted one and it's blown.... see checking every connection etc!
Michael Beswick

Mine was a frayed wire at transmission, only a couple strands of wire holding things together.

Oh, and the new gasket on the case has never fixed the leak around the cover plate to the filter/screen.
R.W Anderson

2 seconds sounds slow

in addition to what Michael has put, as I was reminded here earlier if you've got engine oil in the g/box then it should be changed and o/d filter cleaned every 24k-miles/2 years(?)

there are three John Twist videos on o/d servicing and others on info about the o/d in his 200+ vids -

it doesn't sound like you're a clutch dipper but if you are then I suggest you don't
Nigel Atkins

Drop the OD sump and check for bits of friction lining material - my guess is that the outer friction lining has come off the cone clutch or possibly that the sun wheel cir-clip has broken - you'll find that on the filter gauze too!

You'll need a new filter gasket, you can't successfully reuse the old one as it tends to tear apart on removal.
Chris at Octarine Services

Pulsing in and out is a symptom of low oil level, but unless you have suddenly lost a significant amount of oil it is unlikely to be that.

You can check the electrical continuity by putting an ammeter in series with the manual switch, it should show about an amp with the engine running and OD engaged.

With two gearboxes to drain and refill I bought one OD sump gasket, and one came off OK and the other didn't, such is life.

The speed and fierceness (or otherwise) of engagement can be controlled by the number of shims in the valve assembly.
Paul Hunt


Checked oil level last night and it's halfway between the high and low marks. I can't check the current drawn until the weekend, but I've ordered a complete set of O rings, gaskets and a filter and intend stripping it this weekend as per John Twist's videos.

R.A Davis


Got a complete set of O rings and gaskets including ball bearings (the latter probably not necessary) Watched John Twist's videos and set to.

The oil was a bit darker than I would have expected but it's been in for a couple of years. The OD sump came off no problem but the plug for the pump put up a fight. In general everything looked in good nick, the filter did have a bit of swarf on it, see image, but no more than I would have expected.

Put it all back together, refilled the gearbox with 20/50 and last weekend took it for a 60 mile round trip.

It didn't work badly before (prior to playing up) but now it's better, it engages almost immediately and seems a lot smoother when it disengages. It used to disengage with a jolt if you didn't have just the right amount of throttle or were going fairly slow.

A worthwhile morning's work.


R.A Davis

Good news Bob

many owners don't give enough credit or thought to low tech solutions like regular servicing most of which boils down to cleaning and lubing

those that do regular engine oil and filter changes forget or don't consider the same for the g/box despite it having the same oil

>>almost immediately<< is exactly how I remember mine being and would expect others to be and how I describe it

you may already know this - on o/d changes don't lift from the accelerator (well not fully anyway) and don't dip the clutch
Nigel Atkins


I don't use the clutch on either engaging or disengaging, I do general lift off completely engaging OD until I feel it drop in, on disengaging I kept my foot on the throttle except when slowing for a junction. Switching it out while slowing used to cause quite a jolt if I didn't have enough throttle, it's much better now but what I tend to do is leave it engaged until I change down and then switch it out while changing down. I've got a later gear knob switch fitted in addition to the dash mounted switch, this gives me the option of using either, I find the gear knob switch far more convenient.

R.A Davis

certainly with the (later) o/d you want to keep it loaded (might not be the right word but hopefully you know what I mean) - I've had this confirmed by a reliable source as my memory sometimes isn't good - try not lifting off on either engage and disengage and see how it is

the slowing down whilst in o/d 4th is where it loses out to a 5th gear - the o/d works best as flicking in and out in 3rd at higher speeds on country roads or on a motorway slip road or as a 5th gear on a long (more mundane) journey where it will remain in 4th o/d for many miles

trying to use it at lower speeds and as a 5th gear at slower speed and less open roads doesn't work as well

I tried it as I can sometimes be a bit tight about fuel consumption and found it totally counter intuitive to keep the accelerator pressed as I wanted to slow down and drop out of o/d, unless I had lots of road before the need to slow

yes dash mounted switch could have you with both hands off the wheel (not that anyone would do that of course), I think it's just the additional wear on the wiring on the gear mounted switch but it's a much more sensible position, I had this on my Triumph (before my present Midget) - that's when I checked I was operating the o/d correctly as that one played up, the advice I was given then wasn't how I remembered I operated the o/d when I had a '74 BGT
Nigel Atkins


Thanks for the input. Reference the wear on the wiring to the gear mounted switch, I've got a real of trafficator cable that I inherited from my father who was an auto electrician. This is a very flexible cable designed to feed the light in the end of trafficators, I use this at the point where it needs to flex during gear change.

R.A Davis

great stuff about the wiring to the lever switch

personally I'd disconnect the dash overdrive switch just in case it caused an electrical problem there or elsewhere but you probably know more about electrics than me (which wouldn't be difficult)
Nigel Atkins

The factory specifically said not to use the clutch when engaging or disengaging OD. I've never done that, or lifted off when engaging, or deliberately lifted off when disengaging. I've had the roadster for 24 years and 60k and the V8 for 20 years and 100k and been doing near full throttle engages and disengages all that time with - so far! no problems. It's just not that delicate, especially in the 4-cylinder. The V8 is exactly the same design although beefed up for the greater torque. A pal with a supercharger had to have his rebuilt to V8 spec as he was damaging the friction linings.

I've added fuses to the OD circuit in both mine, even though the roadster has the dash switch and the V8 the column switch, it's essential for the gear-lever switch. I can flick the roadster out of OD with a finger-tip with that hand still on the wheel. And if I want to go from OD 3rd to straight 4th my OD lock-out circuit does that for me if I allow the gear lever to move out of the 3/4 plane between the 3 and 4.
Paul Hunt

One vexing symptom of an Overdrive problem that is not mentioned in the factory manual is a “pumping” effect during engagement when the car is driven in direct drive, even though when driven in Overdrive all appears normal. This is caused by a problem that is very simple to fix. When the system is operating correctly, a buildup of hydraulic pressure from the pump is directed to the two operating pistons and moves the sliding annular clutch (conical clutch) unit. Its outer brake surface then comes into contact with the stationary brake ring, and then the complete sliding member and the sun gear will consequently cease to rotate. With the solenoid deactivated (i.e., Overdrive switches off), the solenoid plunger does not retract fully, and the springiness in the small O-ring at its tip is enough to push the piston and ball back into its seat once the pressure has bled off. This in turn causes the pressure in the actuating system to increase, which in turn causes the Overdrive to engage again. Past a certain level, there is enough pressure to force the plunger back a bit until the pressure bleeds off and the cycle is repeated again (at a frequency of about every 2 seconds). Normally, as the sliding annular clutch (conical clutch) unit of the Overdrive unit starts to move during the engagement process, the Overdrive unit temporarily loses engagement during the moment between when the inner lining of the sliding annular clutch (conical clutch) leaves its seat on the annulus and the outer lining contacts the stationary brake ring. This is event so brief that it is not noticeable, but in this case, the sliding of the annular clutch (conical clutch) never travels far enough for its outer lining to contact the stationary brake ring before it is pushed back again, so for about a second neither lining is in contact. As soon as the inner lining of the of the sliding annular clutch (conical clutch) engages the annulus, engagement returns with a jerk and stays for a second or so until the of the sliding annular clutch (conical clutch) is again pumped away, and the cycle then repeats itself. The solution to this problem is very simple: fit a thicker gasket under the solenoid cover plate. This will allow the piston to move a bit further back before hitting the solenoid cover plate, the extra movement being enough to make the O-ring slide down into its bore instead of simply compressing a bit and functioning like a spring.
Stephen Strange

This thread was discussed between 19/08/2013 and 06/09/2013

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