Overdrive - LH-Type

LH type without ignition relay     LH type with ignition relay - UK     LH type with ignition relay - North America     Location of gearbox switch    
Hover over a wire to confirm the colour
LH type without ignition relay (68-76 and V8):
On cars with the dash-mounted manual switch to protect the most wiring insert the fuse in the white wire at A where it connects to the back of the switch. On cars with the column switch - to avoid cutting wires - insert it at B where the gearbox harness joins the main harness below the pedal box.

LH type with ignition relay - UK (77-on):

On cars with the gear-lever mounted switch to protect the most wiring a fuse can only be inserted at A where the gearbox harness joins the main harness.

LH type with ignition relay - North America (77-on):

A fuse inserted at A will protect both the OD and the TCSA circuits, this is probably where the gearbox harness joins the main harness. However there is also position B: In the wire from the inertia switch a single fuse will protect the OD, TCSA and fuel pump. But a fault in the OD or TCSA wiring will also cut off the fuel pump, so it is better to use two fuses - one in each of the wires coming from the 4-way bullet connector at B.

This picture shows the yellow wire from the manual switch coming out of the main harness, joined to a yellow/red going into the gearbox harness. Insert the fuse here. (Image by David Farrar on the MGOC Forum)

Note: For a few months from late 76 to Feb 77 the original gearbox Overdrive switch operating in 3rd and 4th was used plus an additional TCSA (Transmission Controlled Spark Advance) microswitch operating in Reverse, 2nd and 4th. Wiring these two switches in series allowed the TCSA i.e. vacuum advance to be enabled in 4th gear only whilst Overdrive was still available in both 3rd and 4th. In Feb 77, possibly due to unreliability or cost considerations, the microswitch was deleted and the Overdrive switch arrangements changed to operate in 4th gear only, feeding both the TCSA and Overdrive. With this arrangement the output of the gearbox switch fed both the Overdrive manual switch and the TCSA solenoid directly, so OD was only available in 4th gear.

Is this (A) the TCSA switch? Certainly a microswitch as per the original description I had read, and it closes in Reverse, 2nd and 4th as the gear change shaft (B) moves forwards in those gears:

The location of the gearbox Overdrive and reverse light switches. This is a UK rubber bumper 74.5 to 76, but the switches are the same on all four-synch gearboxes:

Showing the much smaller removable panel on top of the 4-synch tunnel (right) compared to the 3-synch, so you will probably have to lower the rear crossmember and back of the gearbox as well. Image from Moss Europe Incidentally the smaller removable panel is shown the wrong way round by Moss, the hole for the gear lever is towards the rear as shown here, not towards the front:

3-synch: The main reason for this picture was to show how the cover is moved backwards when a 4-synch gearbox is fitted to the earlier car, but you can see the screws in the forward section, and the holes in the rear section which were for the remainder of the screws originally, which has been tack-welded in place, with a filler-strip:

4-synch: The much smaller removable panel, with the hole for the gear lever positioned to the rear, and not how Moss show it in their drawing. By 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. The reverse light switch should be easier to get at from below with channel-lock pliers, as it is lower down and faces sideways. 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 the panel:

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

Duct-tape over the edges of the hole to protect hands:

With the panel removed, crossmember dropped and gearbox levered to the right, you can get at the switch wires more easily. I found I needed to remove these wires from the switch to be able to pull the solenoid bullet connector up far enough, or down far enough, to part or reconnect it. With the wires removed you can get a drift in which will hopefully tap the switch round until you can get a smallish hand in to fully unscrew it. If you can't get a hand in then I think you will struggle to get the switch thread started on refitting:

Switch removed, wires tied back to the gear lever to stop them dropping out of sight:

Original switches (left) have the hex smaller than the terminal end, so you can only get an open-ended spanner or grips on them from the side, which isn't possible with the 4-synch OD switch with the gearbox in-situ. However replacement switches (right) have the hex as the widest part, so I'm hoping with Vee's replacement switch (the old one is intermittent) I can get a socket or box spanner on it to make sure it is tight.

Solenoid assembly ...

... disassembled.

The spacer lifts the coil up, so the plunger is pulled upwards when the coil is energised. Without it the plunger is as likely to be pulled downwards as upwards. However without any of the case fitted, there is almost no magnetic force on the plunger when the coil is energised and it doesn't move at all. With the outer case and bottom cap fitted the plunger moves up slightly as shown. But when the top cap is fitted there is a very strong attraction upwards. You can pull the plunger most of the way out of the bottom, and powering the coil will pull it all the way in and against the top cap with a real smack. It has to resist 400-420 psi of oil pressure in the 4-cylinder OD, and 510-530psi in the V8.

Ball-seat inside top cap

Plunger and ball, small O-ring arrowed, which prevents oil escaping down the inside of the solenoid, and leaking from the cover. The slot in the plunger prevents air-pressure or leaked oil pressure resisting the movement of the plunger. When the plunger presses the ball against its seat oil cannot flow from the inlet to the outlet of the top cap, the pressure rises at the inlet, and OD is engaged.

Top cap. When OD is engaged and oil is not flowing from the inlet to the outlet, the medium O-ring prevents oil escaping past the ball when on its seat, which would result in pressure loss. The large O-ring prevents oil escaping down the side of the solenoid, which will leak past the cover.

In the released position the ball and plunger should be pushed back so releasing oil pressure. With a sound small O-ring the plunger is unlikely to fall back under its own weight, which is why - apart from possibly the first time the coil is powered after the car has been driven - you won't hear any noise from it, unlike the 3-synch solenoid. If the plunger doesn't move back far enough there may be enough residual pressure to prevent the operating pistons and clutch sliding member moving fully back from the annulus to the outer casing so it can't fully engage direct drive. While the clutch sliding member is between the two there is no engine braking (you still have drive as until OD is fully engaged the one-way clutch is bypassing the slipping clutch) and you get a distinctive 'pulsing' sensation. However under normal circumstances the plunger comes back at least 2mm when the solenoid is released, giving a clear path through the valve, as can be seen here.

O-ring set (MS&C)

Solenoid coil, with earthing spring. Flat on Bee's removed coil on the left, whereas opposite sides should be bent up to form a spring, as with the new one on the right.