MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

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 - The occationaloverdrive

The OD still seems to work when it darned well wants to. It will drop the RPM's when the switch is thrown but only engage every now and then. If I let it keep decelerating and flip the switch OUT it locks up in non-OD mode with no problem. Sometimes after it does decide to engage it'll drop out by itself, and it may or may not decide to let me re-engage it. I've done all the o-rings, new non-return valve kit, both new balls, seated the solenoid ball, oil pump feels tight and is in correctly, new screen (the old one was clean). Blasted it clean with 2 or 3 cans of carb cleaner and air pressure. Tried 30W non- detergent oil, RedLine MT-90, a 32 viscosity mineral oil, and currently 20W-50 oil.

Before I tear it out and look at the clutch material on the sliding member, I have concerns about the relief valve. I've attached a photo of the valve in exploded, semi-assembled and assembled state along with it's measurements. The sleeve (with the holes in it) covering the valve itself does not show up in anyone's catalog!! The assembled valve seems to be about 0.052" from reaching the end of that sleeve.

I hope: someone has the measurement of the assembled valve OR the correct length of the spring, OR both. Would really make my day. All I'm coming up with is that it's possible the top end of the valve assembly is not sealing against the housing on the inside top. I'd spend the $90 to buy the new valve assy, but I still wouldn't be SURE that it was the correct length.

Thank you very much,

Tom

currently sucking down a Miller Lite and swearing at the OD unit.

T Lea

I just went through the same grief. After extensive testing and replacing the usual suspects, I found the solenoid coil was working intermittently. I replaced it and the overdrive works just like it did when it was new. RAY
rjm RAY

Wouldn't disagree with you, but I can feel and see the RPM's drop when the switch is turned IN and feel and see the R's increase when OUT is selected. I can see and feel this every time.
T Lea

I had exactly the same problem with my roadster when I first rebuilt it. Sometimes the overdrive would not engage but then come in without warning. Turned out to be the switch on the dash. Try substituting this with a simple toggle switch and see what happens.
Iain MacKintosh

T

One of my club members has a hobby business rebuilding OD units. May want to drop him an e-mail.

Kenneth Quade quadtech at athenet dot net.
Bruce Cunha

You must confirm you have electrical continuity before you do anything else. Depends on the year, but if you insert an ammeter in the OD circuit either at the dashboard switch, or if you have the column switch where the yellow wire goes from the main to the gearbox harness by the fusebox, of if you have the gear-lever switch in the white wire there. You need to drive it and switch OD on, and confirm you have about 800mA flowing (4-synch gearbox, 3-synch is a bit more complicated), before you can conclude it is something mechanical or hydraulic.

The biggest cause of intermittent electrical problem is wear on the lockout switch on top of the gearbox, for which the cure is to remove one of the spacer washers.
Paul Hunt 2010

THANK YOU Bruce. Contacted Ken and he was VERY helpful. I think the problem is narrowed down to the relief valve and I'm on the road to fixing it.

Thanks again

Tom Lea
T Lea

Problem solved thanks to Bruce Cunha who referred me to Ken Quade. Ken provided the necessary measurements and it was determined that the relief valve was not sealing at the top due to the spring getting shorter over time. Shimmed it and we're BACK IN BUSINESS.

A HEARTY thanks,

Tom Lea
T Lea

Great to hear that you didn't have to pull the trans to repair the overdrive. RAY
rjm RAY

Ken is very good with all kinds of Lucas OD units. He has a small shop in his basement with mini lathes and other tools.

I don't know how many a year he is still rebuilding, but if you can get him to do it, he is great. He is out of Appleton WI.
Bruce Cunha

Bruce, I didn't know that Lucas made overdrives. Perhaps, you meant to write Laycock De Normanville. After all the work that you've put into the wiring system on your car, I fear that you have Lucas on the brain. A common affliction. RAY
rjm RAY

This thread was discussed between 05/07/2010 and 09/07/2010

MG MGB Technical index

This thread is from the archives. Join the live MG MGB Technical BBS now