Gearbox Switches

Location of the reverse light and OD switches on a 3-synch gearbox ...

... and a 4-synch (in this case a UK rubber-bumper with the side filler/level plug, chrome bumper had the same dipstick as 3-synch) gearbox:

Showing the large access panel on top of the 3-sync tunnel (left), as opposed to the small one on the 4-sync. This should be enough to get at both the OD and reverse light switches on the 3-synch, whereas on the 4-synch the rear crossmember and back of the gearbox has to be dropped as well. Image from Moss Europe Incidentally this shows the 4-synch removable panel correctly with the hole for the gearlever towards the rear, it is shown the wrong way round by Moss.

Chrome-bumper cars (both 3-synch and 4-synch) have the dipstick for gearbox oil level, rubber-bumper cars have the side level/filler plug. 77 and later models have the same switch arrangement as earlier 4-synch, but different wiring to cater for the gearlever-mounted manual OD switch.

If you have a 72 and later car with the centre arm-rest and cubby, cut the carpet around this access panel as well as round the gear lever hole, which means you don't have to pull the tunnel carpet back to remove it.


Keep the section from over the panel, and drop it back in place for noise reduction before re-fitting the arm-rest.

Accessing the 4-synch OD switch is really tricky. But by removing the small panel on top of the tunnel, removing the bolts from the removable cross-member and lowering the tail of the gearbox as far as it will go, and levering the gearbox towards the right, I can get my hand in.

Switch out. Incidentally this shows that the tunnel carpet was cut with a large hole round the gear-lever hole - slightly too large really. But at the front it was really helpful as it meant I could remove the small panel without having to pull the tunnel carpet back, as I've had to do in the past on the roadster. So I'm going to cut the roadster carpet round that panel for the same reason, but stick the cut piece back on top of the removable panel for it's sound deadening. For the V8 I have an off-cut of interior carpet I can use.

Reverse light switch - different orientation, so can be reached from below with grips.

Switch actuation: 4-cylinder - round shaft, notch under the switch in the reverse and 1/2 plane, with a smooth transition at it moves to the left to activate the switch anywhere in the 3/4 plane. This gearbox uses selector lever 22B 386 and isolation switch plunger 22B406

V8 with OD on 4th gear only - similar notch under the switch. This gearbox uses selector lever 22B 726 and isolation switch plunger 22B 727.

Normally the plunger only moves to the left as 4th gear is selected, and on Vee only goes about half-way, and has a different shape with a sharper transition than the 4-cylinder plunger.

Once in 4th if the lever is pulled further across towards the driver the plunger moves even further, which is not a problem. But when in neutral and 3rd gear it also starts to move if pulled across like that, so adjustment of the switch spacing is pretty critical. With a new switch and a thinner 40 thou spacer (in an attempt to get a more reliable engagement) the switch was operated all the time. With the original 50 thou spacer the switch operated reliably with the gear lever used normally, but it also operated if pulled towards the driver in neutral and 3rd. It took an additional 15 thou of spacing (front wheel bearing shims!) to stop that happening, but still leave it operating in 4th.

Typical switch and spacer. It's always been said they are fibre and there are two per switch. However Vee's OD switch spacer (here) is copper and there is only one. Most suppliers show it as fibre, and only one (as does the Parts catalogue) but Brown & Gammons shows the reverse light switch spacer as fibre and the OD spacer as copper, the same part as the master cylinder banjo washer.

Vee's reverse light switch and spacer. One fibre, thinner than the copper OD spacer above, smooth all over so not squashed down by the switch.

4-cylinder OD switch and spacers. Two, different sizes and thicknesses, one the same as Vees copper spacer the other slightly thinner.

Vee's original OD switch on the left and a modern replacement on the right. The original design means that you cannot get a socket or ring or box-spanner over the end of the installed switch, only an open-ended or grips from the side, which isn't possible with the 4-synch OD switch with the gearbox installed. On the modern switch the hex is the widest part, meaning a socket or box-spanner can be used, making removal and replacement on an in-situ gearbox much easier.

Switch Internals

The plunger end, with the moving part consisting of two contacts. Very oily - and sulphurous, so gearbox oil has worked it's way up and in, rather surprising seeing as the switch is at the top.

The terminal end, with the fixed contacts. Two pairs - each bridged by one of the moving contacts, so two chances of making a good connection, particularly should the one that makes and breaks first becomes burned.

Once cleaned of oil very little signs of burning, so I don't know why operation had become erratic. This switch operated with quite a small movement of the plunger, but then became erratic as the plunger was moved further, and needed very firm pressure at full travel to make contact again.

Plunger with a O-ring (hasn't kept oil out!) and an 'overthrow' spring ... not for 'come the revolution', but to allow the plunger to carry on moving once the moving contacts have reached the fixed contacts.

Each moving contact bridges one pair of fixed contacts, and either will allow current to flow. No less than 18 individual components.