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MG MGB Technical - Overdrive wiring

I have a 1970 mgb roadster which I race.
It has overdrive fitted on 2nd 3rd and 4th. The unit was specially prepared to take 2nd.
It recently started to hesitate in engaging or did not engage at all.
A electrical specialist renewed the wiring, checked the selanoid which was ok but found the inhibitor switch to be faulty.
A new switch was fitted and overdrive on 3 and 4 works perfectly, but nothing on 2nd.
Has anyone any ideas why and how it may be fixed.
C Bolton

As it's the inhibitor switch that shuts out 1st, 2nd reverse normally, then renewing it with a standard one wired in the standard way would very likely revert to standard. How to fix? It would be best to talk to the electrical man to establish just how it was done.
Paul Walbran

The switch is operated by a plunger. It can operate in any gear and even in reverse if you want to destroy your overdrive. What operates the plunger is part of the selector linkage in the remote housing.

The linkage part has a shaped edge so that it pushes into the plunger at certain gear combinations. This edge was cut down for the later boxes so that OD only operated on 4th gear. I would assume adding metal to an earlier linkage would give you OD on 2nd also. But simply changing a switch shouldn't affect anything...unless this switch hasn't been shimmed correctly. It's possible the extended metal doesn't engage the plunger as positively as the factory part and the switch may need to be screwed in further (remove it and take out one of the fibre washers).

It could easily be something entirely different of course, without having the parts to look at. Something to try, though, if you enjoy contorting yourself into the tunnel to reach the switch.
Roadwarrior

Only North American spec were modified to deliver OD in 4th gear only, but otherwise I'd agree with Roadwarrior. It's the gearbox parts that determine which gear it is available in, not the switch. So if just changing the switch has caused this then perhaps it is simply switch adjustment/shimming, i.e. the switch needs to be closer to operate in 2nd than it does for 3rd and 4th. Take a washer out.

Access is tricky on a 4-synch, you will probably have to remove any centre armrest and tunnel carpet, and lower the tail of the gearbox on a jack after undoing the four cross-member to chassis rail bolts.

I'd agree the 2nd does need OD, it's a long stretch to 3rd when pulling away on a steep hill.
Paul Hunt

This thread was discussed between 26/05/2014 and 28/05/2014

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