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MG MGB Technical - slow O/D engagement

The O/D on my 1968 roadster is slow to engage when I switch it in. Somewhere between 1 and 2 seconds. It disengages instantly. I shall be having the engine and gearbox out this winter and want to know what part of the O/D will need looking at.
Steve Church

Both of mine are slower if that helps!
rachmacb

Oh, and mine goes in fast enough, but sometimes a while to dis-engage. Mike
J.M. Doust

Steve,
You might check oil level - a little low and it can do that. Also, cold oil can slow reaction time. Mine varies a bit, both in and out, unless level is right and the box is warm. Still works well enough though so I don't worry about it.
Roger T

The shims on the relief valve are used to adjust the sharpness of engagement, if it's always been like it. If suddenly changed then it'll be something else. Shims can be accessed with the gearbox in-situ, maybe even without draining, although I was doing an oil change when I accessed mine.
PaulH Solihull

Sounds like I may be worrying over nothing then. Paul, it has always been like that but then I have only had the car for a year. I shall have a look in that area when the gearbox is out of the car.
Steve Church

My car had been doing the same thing for a couple of years. When the solenoid was beginning to fail, I took it apart and found that the solenoid plunger was very poorly machined from the factory. It had several ridges on the perimeter which were causing it to bind and hang up on the inner wall of the solenoid. I smoothed it down, a little bit at a time, until I was satisfied that it wouldn't drag on the solenoid anymore. After re-assembly, it shifted almost instantaneously, both into and out of overdrive. Can't hurt to check it out. I almost overlooked it when I had the system apart. RAY
rjm RAY

Steve , change the selenoid. Fixed a Jaguar recently after a specialist garage said it's normal. Also check that you have the correct oil from the factory specs for the year of the car. The selenoid can be checked by watching it as it's switched on, should snap across.... Sean
S Sherry

I used 20W50 oil in the transmission/OD for 25 years and it was always slow to engage. After switching to non-detergent 30W oil, it engages and disengages instantly. The lack of detergents eliminates foaming of the oil and increases the efficiency of the hydraulic system. RAY
rjm RAY

Ray, does using straight 30W oil compromise the gearbox in any way? I thought it should have 20/50 as per the engine.
Steve Church

While on the subject of oils for gearbox and overdrive. When I bought the GT 1972. all was and is working reasonably fine albeit it the slow dis-engage of the od, still, earlier this year I changed the oil in the gearbox and filled with 20/50. I also took the opportunity to place a gearbox additive, gulp, I hope I have not mucked things! Anyway, all working fine and actually gear selection is easier and smoother and less noisy, but has this had an adverse effect on the overdrive? It is still doing what it did before a slow dis-engage. So I figured no worse. It is one of the Nulon products. But as Ray has said a straight gear oil, we used to put this in our other cars, my old TR6 had Hypoi 90 in its gear box.And don't forget. our ambient temperature here is not going to reach the extremes of Europe or North America, ( unles you are in the interior.)So the range is not as critical? Mike
J.M. Doust

The gearbox on the 4-cylinder car is *supposed* to have the same oil as the engine, but that could be anything from 10W/40 to 20W/50 for 'temperate' and warm climates, 10W/30 to 10W/50 for temps between 10C and -20C, and 5W/20 or 5W/30 for temps consistently below -10C. Same for the carbs, but as we know there are very many opinions on what is 'right', including ATF and special gearbox oils. The point is that the OD *should* work satisfactorily with the recommended oils, but with 100 and 200k miles under their belts it would be hardly surprising if things were no longer at their original tolerances and functionality. I've know people use ATF for a while which corrected a problem, others have continued to use it because it only works properly with that. As long as you bear in mind that it *is* indicating there is an issue with your OD, it's certainly easier to carry on with it rather than dismantle the OD hoping to find something wrong, and cheaper than replacing the OD then and there.
PaulH Solihull

These OD units were designed to work on anything from ATF to 90W gear oil. I've worked on TR6s, that came factory equipped with 90W oil, that shifted in and out of OD instantly. John, of Quantum Mechanics who is an expert on these units, did tests using various oils and observed the pressure, created by the pump, on a gauge. The pressures ranged from 400 psi, with ATF, to 700+ psi using 90W. He had to free up the sliding member, with a hammer, after the test with the 90W, due to its locking up with the brake ring. The non-detergent 30W oil works very well because it lacks the foaming agents that come with regular engine oils as part of their detergent package that is designed to keep your engine clean after combustion takes place. The transmission/OD doesn't see this kind of contamination of its lubricant since no combustion takes place inside of it. After his tests, John recommended the use of non-detergent 30W oil in all of his rebuilt OD units. RAY
rjm RAY

"ATF to 90W gear oil."

Gear oil viscosity is measured on a different scale to engine oil, and 90W gear oil is about the same as 30W engine oil. The V8 always had 90W gear oil in the gearbox and OD, and has never caused me any trouble with either.
PaulH Solihull

My only complaint, with using 90W gear oil, is that the transmission is extremely hard to shift when cold. I think that the excessive pressure, caused by the use of 90W gear oil, also punishes the OD components due to the increased operating pressures involved. RAY
rjm RAY

My V8 lived outside for several years and was used in all weathers, including periods of snow and below freezing lasting several weeks, but didn't have that problem. It *is* the recommended oil for the V8, which operates at higher OD pressures than the 4-cylinder. Mine's on it's third time round the clock, 85k in my hands. I've read of complaints about 90W gear oil from other people in America, maybe it's a different formulation is some way.
PaulH Solihull

OK, great to read a lively discussion from ol friends. So here is my conundrum: just got on the road again with a rebuilt Moss SC engine and fuzzed up OD. I had replaced the solonoid, ensuring- on the bench- the ball bearing piston action was smooth when the unit was activated, the trans grounded, the B+ from the shifter switch test 11.5 volts. Rebuilt the pressure pump, polished the shaft and tube, reinstalled with the roller flat side at the cam lobe facing rearward, eyeballing the action and piston depression from below while on the rack. All checked out peachy. I could verify the solonoid action and click with a hot wire, but it did not shift into OD, from below with the car running in 4th gear at about 2700 rpm. I have not hooked in a pressure gauge yet, although 2-3 years ago it checked out around 425# as I recall. Lowering the car off the rack after fruitlessly testing the OD, the car- trans in neutral- would not roll in reverse anyway at all. I unbolted the driveshaft, ensured the differential rotated freely, then rotated the trans flange in the foreward direction...all free, loose, smooth and good. But, it would not rotate in the reverse direction !!!!: like a bar was blocking the layshaft or some such. Back up the rack, pulled the covers, confirmed again the pump action was there, could see no metal, shards, or obstruction of anykind. Pulled the solonoid and the ppump and replaced the still clean filter screen and covers. The car backed off the rack no problem. Test drive showed it was back in the pink, not a hint of problem. Shifted without a hitch into reverse and backed up under power flawlessly and without noise. Back on the rack, pulled the covers and inspected the ppumb lobe under strong light and mag glass. There was no evidence of any galling, excessive wear, bite marks, etc. Put the covers back on without solonoid or ppump, topped up the oil ( 30 wt engine) and drove up to Lake Tahoe from the bay area to break in the engine. Didn't have a single problem or hitch of any kind. And I didn't have the OD functioning. Any ideas from you all before I try it again? Cheers Vic
vem myers

After some extensive head scratching and 5 disassemblies, I focused my attention on the solenoid plunger. It looked like something that a 12 year old had made up in their basement. It had a number of raised ridges around its perimeter. Very poor machining. I gently filed them flat and used some fine grit sandpaper to smooth the surface as best I could. The result was an OD that responded instantly to the flick of the switch either to engage it or to disengage it. Just something to think about. I had never come across this problem before, but my OD can't be the only one with a poorly machined plunger.Simple enough to check. Remove the 4 solenoid base retaining bolts and the plunger will fall out when the base plate is removed. RAY
rjm RAY

Failure of the output shaft to reverse indicates the one-way clutch was locking-up, which happens when the sliding member is in the operated position i.e. OD is engaged. It could also explain why it didn't seem to be shifting into OD when you operated the solenoid, i.e. because it was already stuck in OD, which unless you compared revs with speedo you wouldn't have known. This can happen with an OD that hasn't been used in a long time or where the clutch hasn't fully bedded-in, the cure being a whack on the brake ring with a mallet, cause and cure being in the Workshop Manual.
PaulH Solihull

All very interesting. When there was a fashion for "low friction" additves, overdrive units were not happy using these!
I phoned up Morris lubricants the other day. They suggested SS 75w90 would be fine in these gearboxes/overdrives. I also remember reading an article by an ex Castrol engineer who was very dismissive of the idea of a gearbox running with engine oil at all, but then again Mini's/1100/1300's ran with the same oil as circulated round the engine!!
Allan Reeling

This thread was discussed between 14/11/2011 and 28/11/2011

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