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MG MGB Technical - Overdrive not working when warm

When cold the overdrive on my '73 MGB GT works flawlessly, however, once it warms up well (15 miles or so) it starts playing up. It feels like it is coming in and out of its own accord in 3rd and 4th gear. It is very noticable when slowing for a roundabout and then accelerating again. The overdrive switch does nothing and it cannot be engaged or disengaged using the switch.

I have changed the oil and cleaned the filter, the oil level is correct.

Has anyone heard of similar issues? I want to drive down to Goodwood next month and must try to sort this first. It is a good 100 mile drive from South Oxfordshire (not Australia!).

Thank you in advance, David
David JM

Cannot be engaged and disengaged with the manual switch? Then it has been bypassed. Does pulling the gear lever around in 3rd and 4th cause it to disengage and engage? If so it is probably wear on the switch, these are usually fitted with two spacer washers, try removing one, or it may just be loose. The problem is that the switch is tricky to get at even when lowering the rear of the gearbox and cross-member on a jack and removing the arm-rest cubby, tunnel carpet and access plate.

OTOH my V8 has had a couple of instances where on *disengaging* the manual switch I can feel the OD slipping and catching when slowing from a relatively high speed for a roundabout. It's only done it on two occasions, a year apart, when I was carrying the same passenger, who I hadn't carried at any other time. I blame them.
Paul Hunt

David

Has this started since you changed the oil?

What oil did you use as these new oils have additives in them that make the overdrives slip.

If you have the new type oils in drain them off and get the correct type of oil with no additives in it.

Steve
Ste Brown

It may be an internal leak in the hydraulics for the overdrrive or a worn oil pump for the OD. It is loosing too much pressure when the oil thins out. By all means try an oil change as recommended above first. You may also want to try using ATF instead of motor oil.
John H

No, the problem was there before changing the oil. I thought it may have been resolved, but it turned out I just hadn't taken it on a long enough run. The run to Silverstone in June brought it out!

The car is a GTV8(I know there is a page for V8s, but the MGB technical page is far more active!), so I am using the recommended 90EP gearbox oil. No additives have been used, the oil is Castrol.

The manual switch for the overdrive works fine for 20 miles or so, seemingly until the car has got well and truly warm.

As Paul described very well, it is a slipping and catching feeling when slowing and accelerating away again, coupled with the fact that the overdrive no longer responds to the switch.

David
David JM

I agree with Paul in moving the gear lever around while driving in 3rd and 4th with the OD on to see if it creates the intermittent fault. You can also do a simple test by bypassing the switch that deactivates the OD in 1st & 2nd by connecting the manual switch direct to the OD solenoid.
I had a similar problem also caused by this switch. I bypassed it until I got round to my engine rebuild. I then serviced the switch and got it operating properly.
I have also had a wire coil wire broken off inside the solenoid where it is soldered. This was difficult to see and only tugging gently on the wires showed the fault up. This had also caused an intermittent fault similar to yours. It would run great for a while, and then has a mind of its own.

Richard.
RH Davidson

The standard MGB gearbox and overdrive uses engine oil, not 90EP gear oil.
Is there something different about the V8 gearbox?

David
David Overington

Change all the O rings in the OD accessable behind the filter and valve, and the ball bearing that is part of the electric valve.
werner haussmann

Yes David, on the V8 gearbox EP90 is recommended rather than engine oil. Possibly due to higher torque loads or something similar. There is a lubrication chart on the V8 Register website you can find it at the following link http://www.v8register.net/subpages/V8NOTElubricationchart.htm
I use standard engine oil in my midget, Sprite and did in the 4 cylinder mgb roadster before I put a V8 in front of a Supra gearbox.

David
David JM

david

When you say supra box do you mean a toyota one, If so wont that be diff oil to a V8 box?
Ste Brown

Ste,
Yes it is a Toyota Supra box. I can't remember what oil it uses, a gear oil of some sort. I was just making the point that I used engine oil in my MGB 4 cyl gearbox before the conversion. In Australia we tend to use Supra boxes rather than Rover boxes. There is a company in Sydney called Dellow http://www.dellowauto.com.au/main.html who make bellhousings to put Supra boxes behind Rover V8s. The Supra box is strong, has a variety of ratios and will fit without any bodywork changes!
David
David JM

This thread was discussed between 22/08/2008 and 24/08/2008

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