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MG MGB Technical - Overdrive problem

About ten years ago I had a good used overdrive box installed by an MG shop into my 74B. It has always worked well and I love it. Recently while driving I discovered if I left it on while shutting down the car that the next time I used the car it would start out in 1st o/d. Later on while checking it out I found if I was in 2nd and hit the o/d lever it would shift into overdrive! Does anybody know If I can do anything to prevent this from happening? Should I take it back to the guy that installed it?
Bob Ekstrand

Sounds like you have a bad overdrive inhibit switch. Until you fix it or get it checked be sure the switch is turned off prior to selecting reverse. If you don't there is a chance you will damage the internal overdrive parts. The big problem with the inhibit switch is it's not easy to get to for repair, the rear of the transmission will need to be lowered. It's shown in the image below, the switch on the left is the OD inhibit switch. No OD in the transmission in the photo, I installed the switch to show where it's located.

Clifton




Clifton Gordon

I think you've been dead lucky you haven't wrecked the overdrive in the last 10 years as it sounds like the fitter didn't include the lockout switch in the circuit! I've never heard of one failing closed, only failing to close and allow OD to engage, and that is usually just a case of adjustment. You do have to be travelling a a particular speed for enough oil pressure to be developed to engage OD, so like I say you have probably been very lucky.

If you disconenct the rear cross-member from the chassis rails and lower the rear of the gearbox on a jack, then remove the arm-rest and tunnel carpet, there should be an panel (big on 3-sync, small on 4-sync but it still helps) which makes access to the switch easier.
Paul Hunt 2

I'm pretty sure the inhibiting switch worked when the O/D was first put in. I'm wondering if perhaps a wire fell off of it and just needs to be reconnected. Otherwise it looks as tho the inhibitor unit screws into the box. perhaps abit tighter or abit looser would be a correct adjustment. What do you think. Also I am very careful with the gearbox, anytime I get in the car I always check the o/d switch to make sure it is off. I'm aware of the risk to the unit.
Bob Ekstrand

One of the switches shown in my image was stuck in the closed position, it was used as a backup switch in a non overdrive transmission. However, it looked like that transmission had gone swimming. Not sure if that could happen to an overdrive switch. The only way to be sure is to check the switch, if it has one. If one of the wires came off the overdrive would never work.

Clifton
Clifton Gordon

The switch should be tight and is shimmed out with 1 or 2 fibre washers to achieve correct operation. However the problem is usually wear which means it doesn't operate, or switches in and out if you pull the gear leaver round in an OD gear, and a washer has to be taken out to get consistent operation again. The circuit is fail-safe so as Clifton says if a wire falls off (anywhere) the OD won't engage at all.
Paul Hunt 2

It sounds like the switch may be stuck in the "on" position. Water can easily get into these switches since they didn't use any o-rings or sealant when installing the plastic cover piece. Once water enters it's only a matter of time before they corrode and start to stick.

I've accessed mine the same as Paul. There's not a ton of room but it is do-able.

Since mine is a daily driver, the switches get a bead of silicone calk around the alloy body and plastic cover piece. So far no failures.

The inhibit switch and backup light switch are the same. If you have a spare trans, the backup light switch can be robbed from that. ALWAYS check switch function before insalling in the car. You'll avoid a lot of hassles if you do. This also applies to new ones!

Tom
Tom Sotomayor

Thanks Clifton, Paul and Tom for your help on this. I've been looking thru my Victoria British mag and I can see its the same as the back up switch. The price is cheap too so I think I'll order one. I really appreciate the help. MERRY CHRISTMAS! Bob
Bob Ekstrand

When checking a switch (new or old) that hasn't been used for a while I'd advise *not* doing it with an ohmeter, but with a 12v battery and bulb. The problem is that the contacts of the switch will develop a film after not being used for a while, and an ohmeter doesn't usually offer enough voltage or pass enough current to burn through it, and so it will show resistance leading you to believe it is faulty. A battery and bulb is more like 'real life' and should burn through such a film. You could then use a voltmeter to see if you have any voltage lost in the switch, which *would* indicate a suspect switch.
Paul Hunt 2

This thread was discussed between 16/12/2007 and 19/12/2007

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