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MG MGB Technical - Overdrive in 2nd!!!!

When I first got my Jubilee running (after a lay up of two years by the previous owner) when trying to get reverse it suffered from an occasional type of clutch slip, this I found out from this BBS could have been a suspect overdrive unit.

I had intended tolerating it for a few weeks on the road whilst I found out what else needed doing engine wise. It passed it's MOT last week and was taxed for the road at the weekend and I have discovered to my surprise that the overdrive does actually kick in on third and fourth (although not all the time), sometimes however it seems that it is also active in second.

I should also say that the car has only covered about 20 miles since going back on the road, so it may not have fully settled down yet.

Any ideas or suggestions?

Thanks

Andrew.
A I McGee

I would be concerned about the O/D coming on in second gear. It may mean the lockout switch is defective. You certainly wouldn't want it engaging in reverse!
CDD Dewey

The lockout switch is definitely intermittently defective or is is misadjusted. Eithe way, it whould be rectivied without delay as forgetting and shifting into reverse will ruin the O/D unit (this is the cause of the "occasional type of clutch slip" you have experienced). Good luck - Dave
David DuBois

Just flip the manual switch in and out while driving at around 3k rpm in 2nd. If the OD is engaging in that gear it will be obvious. As to why it doesn't always engage in 3rd and 4th this can be due to wear on the lockout switch - assuming the lockout switch is still doing its stuff.

However there may be other problems inside the OD causing intermittent operation, which makes detecting false operation a bit hit and miss. I'd find the connector between the white in the main harness and the yellow/red in the gearbox harness, pull it apart, and connect and analogue voltmeter on its 12v scale or a test lamp between the two halves. Then with the ignition on you should see the test lamp glow/meter indicate 12v *only* when the manual switch is on *and* the gearbox is in 3rd or 4th. With the manual switch off, or the gearbox in any other gear, there should be no indication. If you see an indication in any other gear the lockout switch has been bypassed. If occasionally no indication in 3rd and 4th then possibly a worn/loose lockout switch or bad connections somewhere. The switch could be loose in the gearbox casing, or you could try removing one of the (usually 2) fibre adjustment washers from under the switch. You will probably need to lower the rear cross-member on a jack and pull back the tunnel carpet to get at the switch. If regular indications but still intermittent operation then it looks like mechanical problems inside the unit.

Paul Hunt 2

Certainly, a faulty lock-out switch can cause this problem and if you experienced "an occasional type of clutch slip" in reverse, I would run, not walk, to your nearest MG parts supply and get a new switch. I agree with the answers posted above, but I would be a lot more emphatic about it. I experienced this on my '73 GT: just once it "slipped" in reverse, and sure enough it shifted into O/D in 2nd gear. Lock-out switch had shorted itself out (stayed "on" all the time).

If overdrive is intermittent in 3rd or 4th, pushing/pulling the gear lever "a little further" into 3rd or 4th will generally make it go into O/D, and the fix might be accomplished by tightening the switch and/or removing one of the washers. But I don't believe the opposite is true. If it is staying "on" all the time, I don't think loosening the switch will fix it because I don't think the ball at the end of the switch is in contact with anything unless 3rd or 4th is engaged. Somebody correct me on this if I'm wrong.

I would not drive the car at all until this is resolved - certainly not in reverse. Somehow, both of us got by with that once. If reversing didn't tear up the one-way clutch inside your O/D, consider yourself lucky. If you must drive it, disconnect the yellow overdrive wire first! The switch CAN be replaced with the transmission in the car, but be prepared for a serious struggle. In addition to releasing the transmission crossmember to lower the rear of the transmission as far as it will go (until the drive flange rests on the permanent crossmember), you will need to remove the gear lever, gaiter, console, tunnel carpet and the sheet metal "surround" that screws to the tunnel around the gear lever opening. From there, you can just barely reach the O/D lock-out switch. In case you give up, I've thought of wiring the yellow wire to a relay such that the secondary is OFF when energized, the primary side being connected to the reverse lights. This would not prevent O/D from engaging in 2nd but it would prevent it from engaging in reverse. At least as long as the reverse lights are working!

While you're replacing the switch, replace all the "O" rings on the pump and the solenoid, and carefully check the wire on the solenoid as well as the rubber grommet where the wire exits the overdrive unit. I've seen a number of these that weaken to the point of breaking at this point. If the insulation breaks first, the lead could short: very bad news if you haven't added an in-line fuse to the yellow.

Make sure your transmission oil is topped up - wityh 20-50.

At this stage, if the wiring passes Paul's test but the O/D still doesn't work, I guess you'll have to look internally.

FWIW,
Allen
Allen Bachelder

Wow, thanks to all for their advice. The car is parked up at my mother in laws and will only have to be driven half a mile back to ours for the work to be done.

As I have a spare engine and box from another parts car of the same year, I'll swap the units and get the box refurbished (probably professionally).

As a point of note, the overdrive switch on my Jubilee is the annoying column mounted stalk, something I may change over to the gearstick top switch whilst I am doing it. The redundant column stalk may then be reused for a rear wiper kit that I have seen previously on a GT.

Thanks again.

Andrew.
A I McGee

The gearstick top switch is not a nice to use as the column or dashboard switches - it is also prone to the wires leading up the gearstick chafing and potentially shorting out, if you go this route make sure you add a fuse to the power supply.
Chris at Octarine Services

This thread was discussed between 05/12/2005 and 08/12/2005

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