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MG MGB Technical - Electrical Help

Have just bought a 1977 MGB that has been standing for 19 years under cover.
The overdrive does not engage when the switch on the gearlever is moved to the in position in either third or fourth.
I have a multimeter and have put it across the two wires on the gear lever and found a 12v supply.
When I set the multimeter to bleep when the two probes are touched together and then check the back of the switch i can’t get a bleep with the switch in either position. This makes me think the switch is faulty but i also have another switch but this is the same, are they both faulty.
a goldup

"Are they both faulty"? Perhaps. Since you have measured a 12V input, why not hook up your volt meter and see if current will flow with the switch connected and turned on? If current flows through the switch, it is good and the problem lies further down line. Check for power to the isolation switch, then, through the switch to the overdrive solenoid. Do not know about the home market cars, but the US models only had over drive on the fourth gear, not third and fourth as the earlier models did.

Les
Les Bengtson

Have you checked the gearbox oil level is correct? Low oil will stop the OD working.

UK cars always had OD on 3rd and 4th. Switches can go a bit high-resistance over time which means that resistance and continuity checks can indicate a fault that isn't really there, you are much better doing a real-world test,

Simply test for 12v on both sides of the switch with the switch in each position, you should get it on both sides in one position (on), just one side in the other of course (off). If only on one side regardless of position, then both switches are indeed faulty. If you find you lose 12v from *both* sides of the switch in one position, then there is a bad connection back towards the main harness near the fusebox.

If you get 12v on both sides in one position, and your meter has a 1 amp (or next higher) scale switch it to that and connect the probes to the two wires that go to the switch with the ignition on and the gearbox in 3rd or 4th. If current flows then (should be about 800mA) but not when in series with the switch in either position then again the switches are faulty.

But if no current flows, then the problem is elsewhere. The lockout switch on the gearbox could be worn and not closing when it should, if pulling the gear lever in 4th back and towards the right then works it is switch adjustment. If not it could still be the switch, but could also be any other disconnection between manual switch, gearbox switch, and solenoid.

The gearbox switch is a pain to get at, for the purposes of putting a probe on the spades you may get away with removing the centre cubby, carpet and small removable panel, but to replace the switch you may have to lower the rear of the gearbox on a jack after undoing the rear cross-member to chassis rail bolts. When you can get at the spades again check for 12v on both with the ignition and manual switches on and the gearbox in 3rd or 4th. If only on the one, then it looks like the gearbox switch is faulty or needs tightening or adjusting. If on both, and a test drive still shows no OD, then the problem is inside the OD. But to be sure that is the problem you would need to do the current test while you are driving. If operating the manual switch, and going in and out of 3rd/4th, shows the current going up and down as before then the problem is definitely inside the OD. Could be a faulty solenoid valve, or something more serious.
Paul Hunt 2010

New switch fitted today. Houston we now have overdrive.!!!!
a goldup

I have a 1978 MGB, and just the other day I lost turn signals, and fuel gauge. I can't find the fuse's for the car?
Scott Doll

Scott. The fuse panel is located on the right inner fender about mid way in the engine compartment. The fuse that controls that circuit is the one on the bottom. Very common that corrosion builds up on the two clamps holding the ends of the fuse. If the entire green wire circuit was bad, you would loose the tach, oil pressure gauge along with the turn signals and fuel gauge. Check out what is not working, get a copy of the car's wiring diagram (available on line in color coded format) and see where the loss of power might be coming from.

Les
Les Bengtson

If it was the fuse or its holder then you would lose *all* fused ignition circuits with the exception of the cooling fan, i.e. tach and temp gauge (which would probably be the most noticeable), brake lights, heater fan, wipers and so on. It should also be the *second* fuse up in the panel, white one side and green the other, not the bottom (brown and purple) which feed horns and courtesy lights. As well as a bad connection inside the fuse, corroded end-caps or fuse holder, and corroded spades and connectors, there is also a rivetted connection on the back of the fuse panel which can corrode.

If some of these things were still working then the problem is elsewhere, and in fact likely to be more than one fault, as although there are many branches and tees in the green circuit the tach and the indicators are fed from the same branch. If there are two separate green wires at the fusebox, rather than two wires in one connector, then it could also be one spade connection faulty affecting some circuits but not all.
PaulH Solihull

This thread was discussed between 26/06/2010 and 31/07/2010

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