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MG MGB Technical - 79 GT Over drive Solenoid

hello all, have been to look at a new potential addition, a very nice original 79 GT however somethign puzzles me.
the owner says that he didnt till recently realise that it had over drive fitted and that a garage told him that a previous owner had taken the switch and wiring out as it had stopped working.
the problem he sumised was due to a faulty solenoid gasket / o ring which leaked oil into the solenoid ?

does this sound likely. im presuming that the whole solenoid would need replacing rather than the wiring, gear lever switch and some gaskets like the owner is assuring me ?

any advise would be very enlightening

regards john

Could be more serious, how much for a recon overdrive?
Neil McGurk

The models with the gear lever switch have problems with the wires to the lever breaking. They are un-fused & sometimes short out & fry the wiring loom. Put an inline fuse in the yellow wire up by the fuse box. The switches are also of poor quality. Solenoids give very little trouble but a dirty retaing plate can spoil the earthing of the coil.
Garth Bagnall

John. You may wish to contact Chris Betson at Octarine Services. Do not know where he is located in the UK, but he has an excellent reputation for the quality of his work, especially on overdrive transmissions. His website has its own technical forum where you can post your questions.

Les Bengtson

Hi John

Just changed the solenoid on my MGC.
very simple to do yourself, you can remove it from the overdrive unit without draining the oil from the over drive/gearbox.change the 'O' rings & gasket at the same time. As a precaution fit a line fuse in the power supply wire behind the dash board switch.
Overdrive Spares Rugby, David Twigger is the man you want 01788540666, cost inc P&P was about 20.

However you should examine what is left of your wiring and do a few continuity checks,
before you spend any money.

If you need a reconditioned O/D Davids price is circa 300. Just bought one for my other MGC.

But obviously the engine will have to come out, then change the clutch at the same time.

Starting to get expensive now I am afraid.

Regards Tim
Tim Hodgkinson

Test it first before digging in to it or spending money.

The gear lever switch and wiring is a bit flaky as it is continually waggled about on the gear lever, and it should be fused as Garth says. On UK spec cars the place to do this is where the two whites in the main harness go to a 4-way bullet by the fusebox. There should be two more whites in the other side, one goes to the fuel pump in the rear harness and the other to the OD in the gearbox harness. The fuel pump should be fused as well, both mine have suffered damaged harnesses in PO hands. If you get two in-line cylinder-type fuse holders with bullets on the ends, and two 2-way bullet connectors, you can fuse them individually. If you get one fuse and one 4-way you can fuse both circuits with the same fuse. Use the standard 17 amp rated 35 amp blow, they are perfectly adequate to protect the wiring, there are two spares in the fuse box, and it avoids a proliferation of fuse types.

You can test electrical continuity of the OD circuit with either a test-lamp, voltmeter or ammeter. Open the circuit at the above conenctors and insert the tester. Turn on the ignition, select an OD gear, and turn the manual switch on. You should see the test-lamp light or the volt-meter register 12v, but these are basically go/no-go tests. If you use an ammeter you should see about 800mA, which check the resistance of the circuit as well as confirming continuity. *Make sure* the test-lamp goes out or voltmeter or ammeter cease registering when you take it out of an overdrive gear, on UK cars you must move the gear lever across the gate away from the 3/4 plane to do this. Move it in and out several times to test the lockout switch. When you are happy with that put it back in overdrive and slide the manual switch back and fore and make sure that breaks and makes the circuit reliably as well.

If all that works take it and test it.
Paul Hunt

This thread was discussed between 18/10/2008 and 21/10/2008

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