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MG MGB Technical - Lack of power to overdrive

Have just purchased a 79 MGB LE, one owner , low miles, am having trouble finding why I am not getting power to the O/d switch on the gear lever. Conections seem fine, also does the lockout switch. Would like to find a solution before dropping the trans to get a better look at things. Anybody got any ideas?

Kim R.
K Rutherford

Kim,

I just had a look at the wiring diagram for a '77 (the newest I've got) and you can have a look for a white/brown wire coming out of your fuse box at the #5 position (third fuse down on the forward side of box. The fuse-ends are numbered but hard to see). This same terminal also connects to the positive side of coil, rad cooling fan and the ignition switch relay.

This wire runs from the fuse box to the switch on your shifter. Check for continuity as the wire itself may be causing your problem.

Cheers
Simon
Simon Austin

Hi.

There is a common fault with the gearknob mounted overdrive switch where the insulation of the wire chafes through on the lever, and shorts the (unfused) 12v feed, resulting in fried wiring.

I don't know if this is related to your problem, but I suggest you fit a fuse in the feed while you are working in the area.

Don
Don

A late model North American spec car should have a feed from the ignition switch on a white, through the inertia cut-out switch, to the fuel pump and the overdrive manual switch. There should be a 4-way bullet connector after the switch, with one output going into the rear loom for the pump and another into the gearbox loom for the OD. This is the point you should be checking. The diagram shows two additional 2-way bullet connectors between the 4-way and the switch. One will be at the base of the lever and should be accessible by lifting the cent console and pulling the tunnel carpet back to expose a removable plate on the tunnel.

The 4-way connector is a good place (the closer to the supply the better) to put an in-line fuse, which will protect the wiring to both the fuel pump and the OD. As well as the gear-lever OD switches having been known to short and burn looms, both my cars have had shorts at the fuel pumps. Both cars now have fused ODs and fuel pumps.
Paul Hunt

This thread was discussed between 08/09/2004 and 10/09/2004

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