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MG MGB Technical - No overdrive in 3rd

Overdrive in 3rd will engage if the gear lever is held toward the driver, ( right hand drive), otherwise it’s jumping in and out all the time. Someone hinted it’s a solinoid or something that’s in an unaccesable place on the overdrive but would welcome idea’s. Thanks
Trevor Harvey

Trevor, here is some info on the gearbox switch from Paul Hunt's website. Maybe adjustment but awkward to get to.

http://mgb-stuff.org.uk/overelec.htm#switch
Craig M Harvey

Inhibitor switch failing, come loose, or not adjusted right.

3 synchro bodies have a removable top panel on the tunnel for access.

Full synchro bodies had a wider tunnel, switch can be reached and undone if the gearbox crossmember is released and the box lowered into the chassis crossmember and moved to the right as much as possible. Awkward but do-able.
Important note is to disconnect the speedo cable from the overdrive first to avoid the risk of kinking it when the box is lowered.

Adjust via removing one of the fibre washers under the switch.
Paul Walbran

As the overdrive works OK in forth does that mean the inhibitor switch is OK and that the problem would be solved by just removing the one washer or should I just fit a replacement inhibitor switch..
Trevor Harvey

Yes just needs tightening or a fibre washer removed. The 4-synch has a small removable panel on top of the tunnel which does make access a little easier, still needs slim hands and tape round edge of the hole in the tunnel for hand protection.

If you have the original switch as attached a socket won't fit, only an open-ended or grips, but I've been able to tap it round with a hammer and drift. Replacement switches take a socket and with a 16-point socket on a 1/2" extension bar the socket will go with a very limited 'throw'. But by turning the socket on the bar 90 degrees at a time and with patience you can effectively get 32 points.





paulh4

Made a start on the job yesterday, dropped the gearbox removed the transmission bits as advised. No way could I get to the inhibitor switch so I cut away an access panel in the side of the tunnel, the switch came away using a drift, removed one of the two washers and it seems to have done the job will need a longer run to be sure. Thanks all for your advice. Trev.

Trevor Harvey

Crikey Trev. not exactly keyhole surgery.
William Revit

Yes William, not the way I would liked to have done it but I could see no other way of getting access without taking the whole lump out. Once I am convinced it's done the job I'll weld the plate back up to restore strength. I made three cuts then folded it down. More like butchery than surgery.
Trevor Harvey

I'd put a slightly bigger plate back on with sealant and self-tappers. Welding may not be a good idea if it's oily behind there, and flying sparks. Besides which with work hardening the bend won't flatten out.

With auto boxes I believe you have to do much the same thing to get at that end of the kick-down cable.

That also shows the gearbox breather (same as on the rear axle) nicely, just ahead of the switch.
paulh4

Yes Paul that's probably the best way as if further access is required at a later date it will be easier to remove again.
Trevor Harvey

Had a chance to give the car more of a test today to see if removing a washer had done the job. It is working fine now. Thanks to all for the as usual excellent advice. Trev.
Trevor Harvey

This thread was discussed between 09/03/2024 and 25/03/2024

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