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MG MGB Technical - Have I ruined my overdrive?

I set about replacing the ballbearing that sits atop the solenoid plunger today. I couldn't find the ballbearing that I had removed previously so I put a bigger one in as an "it'll do" measure, thinking that even if it gets stuck on it wont be the end of the world. But of course I totally forgot that it would be on in 1st, 2nd and Reverse! I drove out of the workshop anyway to test drive it, and indeed it was stuck on, but silly me, I reversed and there was a horrendously loud whine from the gearbox as I did so. So I went back up to the workshop after getting the right size ballbearing (3/16"), praying I didnt damage something and installed it instead. It was only when I saw the 3/16 ones that I realised how much smaller they actually are to the one I used.

So anyway now whats happening is there is a sort of slippage type effect occuring, particularly in first as I am taking off and there's sometimes a jolt as well. The whine is still happening as I reverse, and the overdrive isn't engaging in 3rd or 4th.

So whats the verdict? Very bad news? Would I be better off resigning to the fact that Ill need a new overdrive? How easy (or difficult) are they to fit? Does the gearbox have to come out?
Ross Kelly

I think your sprag clutch may not have survived. Probably worth trying thre correct size BB before spending a fortune as I have heard of ODs surviving being reversed.
Stan Best

Actually mine seems to be hanging in there. Its not that its not engaging as I thought, but in fact now its not disengaging, even in reverse. I took it for a drive this evening when the traffic died down and up onto the motorway so I could be sure, and yes the overdrive is on cause instead of the engine doing 4500rpm it was only doing 3000. Its obviously the overdrive clutch slipping in first cause it cant take the torque when Im moving off. I really need to get this sorted now. I can move off gently in second but its difficult to get by without reversing at all. Can anyone confirm what size the ballbearing should be?
Ross Kelly

When you are reversing you are forcing either the overdrive clutch to slip or the sprague clutch to slip or a combination of the two. If you continue you will likely rip the clutch lining off the sliding member and/or ruin the sprague clutch rollers and the end of the third motion shaft in the trans, this would be the shaft that sells for a little over $1000 US and the sprague clutch is not cheap either. You may also be spreading clutch lining bits and metal fragments throughout the trans. Time to pull it out.
J Heisenfeldt

I'm not sure if you are saying you knowingly drove it with the lockout switch bypassed, or whether the over-sized ball bearing caused the hydraulics to act as if the solenoid were operated when it wasn't - I can imagine that happening.

In reverse (or when travelling forwards with OD disengaged) both halves of the Sprague clutch should be turning at the same speed as OD should not be engaged. In reverse the drive is dependant on the linings, but the forces in reverse tend to push them together harder which resists slip. In a forward non-OD gear you get generate high torque when accelerating and even though some of the drive is through the linings if they tend to slip it pushes the Sprague clutch into its non-slip position which effectively gives as solid a drive as gears.

If OD is engaged when you reverse i.e. the solenoid is operated and you drive fast enough to build up enough hydraulic pressure the effect is that the Sprague clutch is forced into its non-slipping position which can cause it to jam solid. What happens than is that in a forward gear in overdrive it can't slip as it is supposed to, so it may well cause the OD linings to slip instead.

Why is it permanently on anyway, with the correct sized ball-bearing? Is the manual switch bypassed as well as the gearbox switch so the solenoid is engergised all the time? If not, i.e. you are saying OD is still engaged when there is no power to the solenoid, and you have the correct sized ball-bearing, and you have slipping in the lower gears and reverse, then you have indeed wrecked the OD.
Paul Hunt 2

Well, did you byepass the reverse lockout switch then, Mr. Kelly?
vem myers

No I certainly did not! I rectified the problem today. I assembled the solenoid wrong. I put the ballbearing in the wrong place and the overdrive couldn't disengage cause the ballbearing was blocking the port through which the oil is supposed to flow. The solenoid itself was jammed as well, to make matters worse. Basically what had happened is I understood instructions wrong and put the ballbearing atop the solenoid housing in the sort of dish with the hole in it, instead of under this, pushed by the solenoid plunger, which is where it should have been. Ill draw a diagram tomorrow and explain it better. My overdrive seems to be working hit and miss now anyway, which is what it was like anyway before I ever went near it so I think I got away with my error. Worst case scenario I shaved some life off the OD unit I think. I think now my oil pump is stiking, which is causing it to not engage when its supposed to. Is there any oil additive that I could use to free it without having to dismantle it? Ill be changing the oil again soon anyway cause of what I did to remove any metal shards that may have been caused.
Ross Kelly

Given what has happened to it I doubt the latest problems are being caused by the oil pump suddenly deciding to stick. But it's worth a punt before removal and dismantling - try draining and refilling with ATF. Some say that has cleared hydraulic problems they have experienced, then they have refilled with engine oil and it has been fine. Others have said they had to stay with ATF as going back to engine oil caused problems, but those are certainy cases where the ATF seems to be getting round internal physical problems.
Paul Hunt 2

I know what you mean, but this problem existed long before I ever went near the overdrive. It then eventually stopped working completely but that in the end turned out to be an electrical fault. The switch had become disconnected. I rectified that fault but by then I had already disassembled my solenoid by accident and didnt know where the ballbearing was supposed to go so that had to be put right before it would ever work again. Now Ive rectified that but the intermittency still exists, and it seems to be just when everything has heated up. I'll try the ATF in the gearbox. Will this run the risk of doing damage anywhere else in the gearbox? And another thing... Where is the damn filter? The reason I took out the solenoid by accident was because I thought the filter was in there. Is it under the bigger plate beside the solenoid and drain plug? I dont want to remove that and have 30 different parts fall out and have no way of putting them back in.
Ross Kelly

The filter is under the bigger plate as you stated. ATF will not hurt the gearbox, it is formulated for transmission applications and is likely better for it then engine oil as so many additives have been removed from engine oil. It is used in manual gearboxes by many manufactures and GM uses it in there 4x4 transfer cases.
John H

There is also a cylindrical filter as part of the relief and low-pressure valve assemblies, accessed by undoing the large hex plug by the solenoid plate. There are quite a few parts above that plug, having just changed the oil and cleaned the filters in both my gearboxes I have documented it at http://www.mgb-stuff.org.uk/wn_gearsframe.htm and click on 'oil change', you might like to have a look at that before you start. Putting them back in is easy, it is putting them back *correctly* that is the challenge :o) Two parts are shown in the manuals that didn't come out of either of mine. That could have been because they were retained by something else, which may just have been stiction (I did have to tease the filter and assemblies out with a pair of long-nose pliers on both cars). This is why I strongly recommend that you have a clean and empty container under that hole when you are away from the car with the plug out, just in case something else does drop out and rolls away or gets lost on the ground.

As far as intermittent operation goes you will have to determine whether it is electrical or something else first, and the only way you can do that is with an ammeter in series with the circuit. If, with the manual switch on and in an OD gear the OD isn't working, the ammeter will show if you have electrical continuity or not. If you don't, it is electrical, and you will need to do further electrical tests to determine where the break in the circuit is. If the ammeter still shows current flowing (about 800mA on the LH-type in 4-synch cars) then the problem is mechanical or hydraulic inside the OD.
Paul Hunt 2

ok I did up a diagram of what I did wrong. Hopefully it will serve to prevent anyone from destroying their overdrive from this simple mistake. The reason I did this was because the top of the valve had a rubber seal around its base and to me it looked like this was actually the solenoid plunger. It was only after I found it couldnt move that I tapped it a couple of times with a screwdriver handle and the top came away, exposing the plunger, and where the ball should have been. The ball blocks the hole in the top of the valve and prevents the fluid from flowing through to the holes in the side. I was told the ball should sit on top of the plunger, and because I mistook the valve body for the plunger, I permanently blocked the hole in the top by putting the ball on top of it, instead of underneath it. Anyway look at the pic if you dont understand me...



Ross Kelly

That figures. On top the oil pressure will press it onto the hole sealing it, which simulates the solenoid being operated and pressing the ball onto the bottom of the hole. "Whadda mistaka to maka".
Paul Hunt2

Well I did call myself a numpty when I saw what I did. I need a new haynes manual. The one I have is for an earlier car and really doesn't cover much useful information at all. Everything relevant is different. It detailed the D type overdrive, whereas I have the LH type. That and it speaks about the HS4 carbs, the 18G engine, 3 synchro transmission and a generator instead of alternator, amongst other things. SO tell me, whats the stock horsepower of my engine? Its an 18V engine. Is it 95bhp? What are these engines good up to? I mean is 140bhp achievable without forced aspiration and massive mods?
Ross Kelly

This thread was discussed between 25/04/2008 and 01/05/2008

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