Servicing the gearbox and overdrive is pretty much limited to changing the oil and cleaning the filters in the overdrive. It is impossible to do this job cleanly- oil will drip and splash and so although its not difficult I thought I'd present my experience in case you've not done this before.
Draining
The gearbox drain and level plugs are accessed from under the car from the right hand side. The pic shows both-and is taken from the rhs. The drain plug is at 7 o'clock on the bottom of the box and the level plug is higher up on the side of the box at 1 o'clock.
spills. ALWAYS CHECK THE FILLER PLUG CAN BE UNSCREWED BEFORE YOU DRAIN THE BOX (ask me how I know....).
Draining is straightforward- you should drive the car for a mile or so to warm the oil, park it level and raise it on 4 ramps at back and front so that its raised on the level. Both plugs are 17mm and seem identical. I use a growbag support tray from a garden centre in combination with a drain pan to collect the oil and guard against spillage so once that's in position remove the filler plug and then the drain plug and leave the oil to drain.
The overdrive
The rubber bumper cars are fitted with a Laycock overdrive situated at the rear of the gearbox. The picture shows the service plates. The round plug covers the oil pressure relief valve and its filter, the square plate with the blue printing covers the solenoid and the large steel moulded plate is a small oil sump and contains a gauze filter and covers the oil pump.
My overdrive was working well, this is simply a service so I'm not too worried about cleaning everything out and I'm not going to disturb the oil pump or solenoid. However blockage in the filters or oilways can stop the system from working in which case a more fundamental strip in situ and clean can restore function. There is a series of videos in the John Twist series although frankly they don't really show this very well.
I removed the sump first- this is retained by 6 BA2 or 8mm bolts. Clean the underside of the unit before removing them. The overdrive will contain some oil even if the box has been drained so remember to position the drain pan beneath it as you loosen these bolts. The sump then comes off- it can be held with a vacuum seal (but never with silicone gasket goo) and if its a little stiff then there is a tab on the left hand side that can be used to break the seal by levering gently with a screwdriver. The gauze filter was partially blocked so I separated this.
The filter comes off the sump- note the two square magnets in the base of the sump- in my case these were covered in a fine sludge of particles which I cleaned off thoroughly. In this pic the magnets have already been cleaned.
I thought that the gauze filter- although now clean had been roughened by the removal process and I was worried it might not reseal, so I got a new replacement, shown here. Clean up the mating surfaces on the sump plate and on the base of the overdrive under the car.
When both surfaces were clean I refitted the gauze filter. Looking at the exposed sump below you can see the oil pump cover as the round unit with two holes for unscrewing it, and the central pickup pipe as the black screwdriver slotted spigot in the centre. This has to protrude through the gauze/gasket and I positioned the gauze hole over this before pressing around it using a small socket to push the gasket material down around the spigot without bending the gasket as a whole. Once the gauze gasket is pushed on the spigot its held in position and the metal sump plate can be bolted on.
I then intended to clean the oil pressure relief valve filter. This is contained under the 19mm plug so unscrew this (it can be really tight)
Here I would point out something that isn't mentioned anywhere
The diagram above (taken from the owners handbook) shows an O ring immediately beneath the plug, the plug itself has a copper sealing washer. The O ring is NOT held in, and as there will be residual oil beneath the plug it will almost certainly come out with the oil. I found it in the drain pan and had to sort out where it had come from! The drain plug actually has a circular groove...
... which fits the O ring perfectly so make sure its there when rebuilding.
The oil pressure valve should just slip out, but its actually a complicated piece of equipment fully exploded in this diagram from the factory service manual. The filter that requires cleaning is number 48 in this diagram. The valve should pull out easily with a hooked probe... not really! I found that I couldn't shift it and reached the stage where I thought further attempts to hook it out would damage it or in particular the second O ring number 49 in the diagram. If I ever do get this part out I will investigate whether it could have a short thread cut inside so that I could use a bolt as a pull handle, but for the time being and since there is no problem with this valve, I simply sprayed it out with carburettor cleaner and then refitted the plug and O ring.
As the solenoid was working well I didn't want to disturb it or its associated O rings so this completed my service. Ill add a description of the other tasks to this section if/when I am obliged to do them. I could now refill the gearbox.
Refilling the box
Clean and replace the drain plug, tighten it home.
Refilling is frankly a nightmare. you can try using squeezy oil bottles or a syringe but we are talking about quite a lot of oil- around 3-4 litres and its a lot to fill that way. I fill from under the bonnet- is you look down to the side of the motor on the right hand side you can find a clear line of sight to the filler plug. I use a long hose threaded down this gap and into the filler plug hole. I can then insert a funnel at the top end and fill from a standing position.
The pic shows the blueish oil hose threaded down and into the box.
Oil will flow down this hose but its slow- it helps if like the statue of liberty you can raise the funnel as high as possible but patience is a must. There is however quite a lot of tube volume so you need to ensure that the pipes aren't too full as the gearbox approaches capacity. Eventually the box will fill and the excess will start to trickle out and into the drainer pan- you can hear this happen and in any event its visible down the side of the motor. Hopefully you don't have too much oil in the hose so you can get under the car, pull the hose out of the filler hole and let any oil remaining in the hose run into the drainer pan. This will be wasted so hence the need to put less into the system as you approach "full".
Insert the filler/level plug and tighten home. Clean up any spills.
