MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - overdrive failure

I have a 67 BGT with overdrive.The o/d engages but then soon afterwards cuts out. Most often, at about 50mph, i switch ON the o/d engages and perhaps for a mile or so all ok, then it dis-engages. Switching OFF and then a few minutes later ON does not re-engage. I have checked all wires, the switch, solinoid, oil level, filter, gearbox switch - in fact everything but the o/d unit. Can anyone confirm that this problem confirms that the O/D unit needs replacemnt? A lot of money and work, so I would like to make sure.
Thanks
Davidp Pughe

David,
After the overdrive cuts out try very lightly pushing foward on the shift lever while the overdrive is still on (in fourth gear)and see if it will reengage. If it reengages and stays on while you are pushing on the shift lever you may need to remove one of the fiber washers that is on the overdrive switch. You will have to remove the tunnel carpet and the gearbox cover to get at the switch.
I had the same type of problem when I first installed my overdrive unit last summer. After taking out the washer it work well.
Good Luck, Tony
Tony Shoviak

David,
I forgot to ask if this was a "D" Type overdrive unit which is what I have in my 66 B.
Tony
Tony Shoviak

The BGT is a 3 syncro gearbox, so suspect that it is a D type overdrive
I have already gently pressed the gearlever around when in 4th and there was no change
D Pughe

Before doing anything else confirm that it is indeed a mechanical failure. As well as the solenoid, gearbox switch and manual switches there is also a relay and vacuum switch in the D-type electrical circuit. The simplest way to check for continuity throug the OD is with an ammeter, and is one of the rare occasions qhen I *do* recommend using an ammeter, most faults can be detected with just a voltmeter.

Insert the ammeter at the single connector where the yellow/red from the gearbox harness joins the yellow/red from the relay, which will be on the bulkhead somewhere. When first energising the D-type OD there is a pull-in current of about 17 amps then as the solenoid moves it opens a switch and the hold-in current drops to about 2 amps. If the current is staying at 17 amps the solenoid is not adjusted correctly, and could well have overheated. This could cause a break in the wiring which is closed when cold but open when hot.

If the current initially rises then drops as it should, but drops to zero after you mile or so when OD disengages by itself, then it is an electrical problem. Now you will have to use a voltmeter at various points along the circuit to see where the break is occuring.

First test at the same single connector (now reconnected) as above, and if the voltage drops to zero when OD cuts out by itself, the either the relay is releasing or the white from the ignition to the other relay contact is losing its voltage supply. The easiest thing to do now is connect the white from one relay contact direct to the yellow/red on the other contact. If the OD still doesn't work then the 12v supply from the white is failing. If it does then the relay is releasing for some reason, either because its winding or contact are faulty, or the 12v supply through the manual switch as failed. Further voltage tests will reveal which.

If the voltage at the sngle connector remains at 12v when OD disengages then you will have to check either side of the gearbox switch (should be a convenient access panel under the tunnel carpet) and where the wires goes into the solenoid. If all those still show 12v then the solenoid has probably gone open-circuit anyway, the only way you can check that is by using an ohmeter while it is still 'hot'. If you leave it too long it will cool down again and 'repair' itself.

But if the current remains at 2 amps or thereabouts when OD disengages by itself, then it is hydraulic or mechanical. As a last ditch before getting the unit out you could try running with ATF (auto transmission fluid) as some report that it can act as a cleaning agent and clears blockages. If so, then drain and clean the filters again. I'd then refill with 20W/50 again, although some report that running on ATF all the time 'cures' their problem (it's actually only getting round the still existing problem but it's cheaper and easier than an OD removal and rebuild).

Paul Hunt 2

David

Do you still have the vacuum overdrive switch hooked up? Check for a vacuum leak for the tubing going to the switch and also for the switch itself.

I use a hand operated vacuum pump (kind used for brake bleeding) to check if there are leaks. If the hose is just slighly leaking, it could cause the OD to switch in and out.

I have a set of pictures on the various parts to the vacuum switch that I will send you.
Bruce-C

The vacuum switch closes under conditions of high vacuum i.e. high rpm overrun to keep the relay operated and OD engaged when the manual switch has been turned off. Faults in the vacuum switch can't make the OD *operate* on its own, neither can it make the OD release when the manual switch is still operated. If the vacuum switch is faulty by being short-circuit it keeps the OD engaged in 3rd and 4th regardless of whether the manual switch is on or off, until the ignition is turned off. If it has failed open-circuit, or the vacuum supply has failed, then OD will disengage immediately the manual switch is turned off, regardless of manifold vacuum.
Paul Hunt 2

This thread was discussed between 26/11/2007 and 01/12/2007

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.