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MG MGB Technical - 78 B overdrive gone

overdrive just gave up. It went in OD and out again, I left it a while ,put it in again and it stayed in, but did not feel right, nothing I could explain.Next day it would not go in

5 years ago had a similar event (same car) but this happened on and off for days until I put some oil in the gearbox and everything has been fine since.

So, I checked the oil level in the gearbox and its OK.
I took the switch of the gear stick and checked the wiring while driving I touched the 2 wires together, nothing I also tested wires with test light nothing.

Under the car oily and dirty but wiring looks intact.

Any advise very much appreciated.

Thanks Dave FL
DJ white

Dave,

Trying to get a little better understanding of what you checked so far on your OD. With ignition key in "ON" position and shifter if 4th gear, are you getting 12 volts to the OD solenoid or is your test light lighting up?

Wasn't sure what you meant when you said "nothing when you checked with test light.

Fred
Fred Wright

Dave. I, too, am not sure I understand what you are telling us. As Fred notes, the bottom line is are you getting power to the overdrive solenoid? If not, then that is the most probable problem as to why your overdrive is not working.

Put the car up on jack stands or on a lift. Remove the input wire(s) to the coil and wrap some tape around it/them to insulate them from grounding. Turn the ignition switch to the on position, put the transmission in 4th gear, turn the overdrive switch on, and check for power on the wire from the overdrive inhibit switch (left side of the tranny) to the overdrive solenoid. If you do not find power there, look at power into the overdrive switch (factory was on the shifter handle). If you do find power going to the switch, check for power through the switch. If you have power to the switch and not through it, bad switch. If you have power through the switch, you have a problem with either the wire going to the overdrive isolation switch or the isolation switch (they sometimes become loose and do not work properly).

If you do not have power to the overdrive switch, trace back the wiring until you do find power. Your fault lies between the place you find power and the wire going to the switch. Not uncommon for the insulation of the small wires going up the gear shift lever to deteriorate over the years and ground out, causing the wire to burn out. I rewired my OD transmission, on the RB cars, to be activated by a switch on the dash to overcome this known problem

Les
Les Bengtson

Read thread: LH overdrive has a mind of its own.

Insert an ammeter where the whites from the main harness join the gearbox harness. With the ignition on, manual switch on and in 4th gear you should see about 800mA flowing.

If you do then the problem is mechanical or hydraulic inside the OD.

If not then there is an electrical problem. The later manual switch on top of the gear-lever is a useful intermediate point to look for 12v with a meter or test-lamp, with the ignition etc. set up as before. If you see 12v one side of the switch but not the other and the manual switch is on, then the switch is faulty.

If you see 12v both sides of the switch then either the solenoid, or the connection to it are faulty, there should be a bullet connector between the two which makes a useful half-way house.

If there isn't 12v either side of the switch then you need to check the isolator switch high up on the left-hand side of the gearbox. It is a pain to get to, you may well have to remove the centre console, pull back the tunnel carpet and remove the small removable panel in front of (I think) the gear lever, as well as lower the rear of the gearbox on a jack after undoing the crossmember to chassis rail bolts.

Again, with 12v on one side of the switch but not the other when the gear lever is in 4th gear shows the switch is faulty, or perhaps more likely is loose and needs tightening, or worn and needs a washer (originally there were usually two) removed. If pulling the gear lever hard back and to the right engages OD it is likely to be one of these last two.

Voltage on both sides of the switch indicates a break between there and the manual switch, again there should be a bullet connector.

Voltage neither side means a break up towards the 4-way bullet connector in the white wires of the main harness near the fusebox, two bullet connectors in this run.
Paul Hunt 2010

Paul has given a very full diagnostic list but a quick test you can try is to push the gearlever towards the passenger door (USA) - if the OD kicks in then the inhibitor switch has simply come out of adjustment - remove one of the fibre washers under it and you'll be fixed!
Chris at Octarine Services

Paul's "pulling the gear lever hard back and to the right engages OD", and Chris' "push the gearlever towards the passenger door (USA)" might both point to a different cause.

Soft or misaligned engine or gearbox mountings, a too tight/stiff or incorrectly fitted gearlever boot may be causing the gear lever to move away from its normal 'at rest' position in 4th gear.

The movement is not enough to upset the gears ( except in extreme cases, jumping out of gear has been known ) but could affect the OD switch operation.

When drive is increased or released the engine & gearbox move about more than 'normal' if mountings are soft, and so the switch can be intermittently opening/closing.

Jim
J N Gibson

Come on, the gearbox would have to be miles out for pressure from the boot to push the lever far enough to cause the switch to disengage but the gearbox to be still in 4th! If the gearbox moving about is enough to cause the switch to engage and disengage that is precisely pointing to marginal switch adjustment.
Paul Hunt 2010

some really good things for me to check, hopefully this weekend when I get my sister back on a plane to UK.

Sorry about the electrical "kind of test" info, it was somewhat short on explanation.

I'll let you know in due course.

Dave 78 B FL.
DJ white

These are the results following my first look at the problem following the ideas you gave me. I used Pauls notes as my guide.

ignition on gear shift in 4th
I used my ammeter on the whites from the main harness where they join the gearbox, my ammeter jumped of the gauge, (may be fried) Went to a test light and it came on.

I also tested the manual switch on top of the gear lever, using the test light I got a light on the red wire and nothing on the black, took the switch off and tested the wires, again red wire got light and black wire nothing.

So would you say I have a faulty switch ?

Many thanks.
Dave
78B FL
DJ white

What scale was your ammeter set to? If set to 1 amp or higher and it jumped off the gauge then there is a short somewhere. If a full short and unfused as normal it would smoke the wires, unless there is also a high-resistance connection somewhere that is limiting the current to a non-smoking value. In either event that is quite likely to result in insufficient voltage at the solenoid for it to operate reliably.

On the face of it the switch is also faulty, if you had 12v on the red but not on the black *with the switch in both positions*. That will definitely stop the solenoid operating. I'd simply bridge the switch red and black wires while you are in 4th and see what happens to the rev counter.

However on the one hand having 12v at the switch red tends to indicate that your ammeter was set to much too low a scale (800mA needs a 1 amp scale) rather than there being a short. But on the other hand if all that is wrong is the switch open-circuit the ammeter shouldn't have moved at all. So I have a number of question marks in my mind over the tests and results.

Paul Hunt 2010

This thread was discussed between 21/04/2010 and 29/04/2010

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