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MG MGB Technical - Testing the inhibitor switch on OD

Hello,

I have a tested my OD inhibitor switch and it completes a circuit in every gear. Does this mean it will allow the car to go into overdrive in any gear?
What would be the cause (and fix) of this?

Many thanks
Alex Davies

Alex,

You're right, if the circuit is closed then rge OD will engage.

SO DO NOT DRIVE THE CAR

Because if you try to use reverse with overdrive engaged you will destroy the overdrive.

Have you checked the wiring going to and from the switch? THey often get snagged and worn where the gear lever moves, and this could be causing a short that looks like it's the switch. Have a look under the gear lever gaitor.

If it is the switch, they are a pig to get at. Some people say you can get at them if you lower the gearbox crossmember to get more room, and I've also heard of people cutting the tunnel to make an inspection hatch.

The picture is what you can see from the front with the engine out.

If and when you can get access they can be adjusted by washers to get the right clearance/operation.

Can you get the reading to chenge if you push the gear lever sideways?

David

D Balkwill

Closer view

D Balkwill

Alex. The overdrive switch, on the transmission, is a normally off/push to operate switch. It has one or more fiber washers associated with it which are used to set the spacing between the switch and the top of the transmission. If these washers are not present, the switch may be turned downwards too far and will be always on. If there are too many washers, the switch cannot be depressed sufficiently to turn it on.

The only failures I have seen of the basic overdrive/reverse light switch has been failures to turn on. I have not seen a switch fail to always on as you describe.

Thus, the first thing I would look at is the shim pack for the switch and if it is in place. If there are no shims, that is the most probable cause of your problem. If there are shims, you should unscrew the switch slightly and see if it will turn off when moved upwards slightly. If it does, you need to remove it and and an extra shim. If turning it upwards does not turn the switch off, it will need to be removed and bench tested for proper functioning.

As to being able to drive the car, I do not see any problems, as long as the main overdrive activation switch is turned off. Power flows to the main switch, through the main switch, then to the transmission mounted overdrive inhibit switch, then to the overdrive solenoid.

Be aware that I have seen one car with the inhibit switch bypassed, which would give the indications you describe.

Bottom line is you will have to get into the area, examine the switch and its connections, then decide what might be needed in the way of repairs.

Les
Les Bengtson

I would say that the plunger is stuck due to a weak return spring inside the switch.
Steven 67GT

I have a 64B and had a Blue label LH O/D installed about 3000 miles ago prior to a long road trip last summer.It performed flawlessly but after approx 2000 miles it would kickout from cold after 10 miles once warmed up. I replaced o-rings etc in the OD and it engages/functions OK running it on Jack Stands. Before I put the Tranny remote access plate back on and reglue my carpets I am considering replacing the 4th gear overdrive inhibit switch in case that is the issue. Winter prevents me from road testing the OD . The access to this switch is better on the earlier B's but still wondering how I am going to remove it since there is minimal wrench room . Any suggestions ? Thanks !

RK Derrick

Lower the rear crossmember on a jack.

I've never heard of a switch actually failing but things can wear. The symptom of that is OD engages when you push/pull (depending on which side you sit) the lever towards the right rear corner of the car and that engages OD, but easing it towards the front left disengages it. There is no adjustment but originally they were provided with one or two fibre spacers, try removing one of those. Failing that you need an ammeter inside the cabin in series with the manual switch and see what happens when OD drops out. If it still showing about 1 amp, then it's hydraulic/mechanical and not electrical.
PaulH Solihull

Thanks Paul , I have ordered a new switch and will lower the crossmember. Do I need to unbolt the front drive shaft u-joint ? The complete engine/transmission must tilt and flex on the engine mounts to allow removal of the switch with a spanner.
RK Derrick

No, the UJ and sliding joint will accommodate all the movement for this exercise. I've used that method to get at the later OD switch, which has very limited access indeed by comparison.
PaulH Solihull

This thread was discussed between 19/04/2011 and 11/03/2013

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