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 - my overdrive has packed up!!!!!

I have a 1978 RB BGT. The overdrive recently started to disengage its self once in a while then rapidly degenerated into going on and off whilst the switch was engaged and now (less than 100 miles later) it won't engage at all. I have recently (five hundred miles ago) had a new engine fitted and I changed the prop shaft straight afterwards. knocking banging the switch does not have any affect so I would be suprised if the switch on the stick is faulty. I recently discovered a large patch of oil in the street where I park the car. Innitially I didn't think it had come from the B (its a busy public road where people park all the time) but now I'm not so sure.
Would an oil leak give these symptoms without other gear box noises etc? Does anybody have any suggestions I am really concerned that the unit will need replacing which will require removal of the new engine and being a huge cost to me since I don't have a garage to do any major work in Is there anything I should look at to start with? How do I look at the oil level in the gear box?
nik

Nik,
There is a plug on the right side of the gearbox, about half way up. Take this plug out and fill until it runs out. If I were you, I would first drain all the oil from the bottom plug and then refill with fresh oil. I use 30W non-detergent, others use 20-50.
The wiring up to the switch on the gearshift knob gets a lot of movement and is susceptible to failure. The switch might be fine but check the wiring. Also, the inhibitor switch, located forward and to the left of the shifter, can be at fault. This switch limits the use of overdrive to only fourth, and on some cars third, gear. Hopefully it's not that switch because it's a real pain to get to.
From your description my bet would be on a wiring fault somewhere.
Rick
Rick Jaskowiak

Nik, By all means check your oil level. The OD, while electrically actuated, is indeed hydraulic. Insufficient oil will cause exactly what you are experiencing.
Ken Lessig

Nik. You should be able to use a voltmeter or test light to check for voltage at the solenoid. If you have voltage, oil is probably the problem. If not voltage, check backwards until you find the break. If good oil and voltage, you need professional help. Les
Les Bengtson

The oil is the first to check - yes it produces these symptoms without the level falling far enough to damage the gearbox - so no noise from there.

Chreck the wires leading up to the gear knob. Undo the four crosshead screws holding the gaitor retaining ring and lift up the gaitor. Under, you will see two bullet connectors and the wire running up the gearlever. The wire is prone to fracture near the bullets.

Use a meter or battery & bulb across the two wires whilst operating the switch.

The interlock switch is truly a pain to get at but can be tested by checking for continuity between the gearlever switch connection and the solenoid connection under the car. If it proves to be faulty, it is just possible to change it by undoing the gearbox crossmember and drooping the box at the back - it is then possible but difficult to reach the switch by taking off the tunnel top plate.

Otherwise it is an engine out job to change the OD unit. Check my website for more OD detail.
Chris Betson

Nik;

Danged if mine didn't display the same symptoms a month ago. There are a number of things that can cause this that have already been accurately described;
in my case it was the plug under the steering column where the switch plugged into the harness. The internal connector had come apart inside the plug. (!)

Dave
Dave Munroe

A 2 1/4", or 2 1/2" metal cutting hole saw in your drill will get you into that inhibitor switch in about 30 seconds, and then it is plane sailing to adjust, remove, check. Some self tapping metal screws and some mild steel or Gal sheet metal will seal it up just as painlessly.
Ken R
Ken Rich

Nik

There are two wires that come up the gear lever to connect to the switch. Each of these connect to the o/d wiring with a Lucas (God of non-working overdrives)sleeve / bullet connector. Check that these are making good contact, You can get at them by removing the gear lever boot. A loose connection in either of the wires here will cause exactly the problem you described.

It will also be well worth your while checking that the insulation on the wires is not frayed. It is not uncommon for the insulation to wear through by rubbing on the metal parts or the foil on the insulation blanket. If this happens to the wire from the fuse, the wire will cook before the fuse blows and melt the rest of the insulation in the loom. A good precaution is to put in a 5 Amp line fuse on the wire to the o/d unit.

Cheers

Ian f
Ian Fraser

The lockout-switch can get marginal which means the o/d cuts in and out if the lever is pulled to one side or other or under accelleration/decelleration.

The reason the loom burns out if the wires up the gear lever short out is because there is *no* fuse in the circuit. Adding an in-line fuse, particularly to cars with the gear-lever switch, is always a good idea. There is little point in using other than the standard 17amp rated, 35amp blow fuse, the wiring will cope with that and it avoids a plethora of different ratings in the car.
Paul Hunt

This thread was discussed between 14/12/2001 and 15/12/2001

MG MGB Technical index

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