MG-Cars.info
MG MGB Technical OD trans slipping BBS discussion at MG-Cars.info
MG-Cars.info
Welcome to our Site for MG, Triumph and Austin-Healey Car Information.
Recommendations
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 - OD trans slipping
| I have a peculiar problem, that I assume is related to my OD trans on my '71 roadster. Here's what it's doing: Cruising along, in most any gear, but more usually 2d or 3d gear, when I take my foot off the gas pedal, the engine and trans appear to de-couple. Specifically, the engine RPM drops off immediately...about 700 RPM or so...and waits for the rest of the car to decelerate to that particular RPM...at which point engine and trans are coupled again. Sometimes there's a slightly audible 'clunk' at the point the engine and trans begin turning at the same speed. This doesn't happen all the time! Most of the time, engine and trans are locked together, as you'd expect. This problem has materialized since the weather has turned cool here in sunny Florida. I've checked/added trans fluid (Penz 10W-40), but the problem is still with me. I should also mention that the problem occurs with the OD 'off'. I haven't been able to get it to duplicate with the OD 'on'. My first thought was that I have some clutch slippage going on. But I really don't think this is it. I've performed the standard test for clutch slippage: hard acceleration in 2d gear on a slight incline. Also, I can easily 'chirp' the tires in a 1st to 2nd shift. This problem only occurs on deceleration. I can only think that the OD piece in my trans is the culprit, but don't have much experience with the innards of this hydro/electric mechanism. Anyone had a similar problem or know what I'm up against? Thanks in advance! David Tye '71 Roadster Florida, USA |
| David Tye |
| I've had exactly the same thing on my V8 recently, but only on one trip. The connection between wheels and engine was making and breaking about once per second, I could feel the car slowing and then not slowing so much, and see the revs going up and down, and it was having switched OD out coming up to a junction. Foot clutch slippage is normally seen in 4th gear, or even more in 4th OD, before seeing it in lower gears, as that is when the torque on it is greatest in normal driving. Conversely OD slippage is more likely in lower gears, again because that is when the torque on it is greatest. When the engine is driving the wheels with OD disengaged drive is going through the one-way Sprague clutch in its locked position, so it isn't dependant on the OD clutches. But on the overrun drive from the wheels to the engine *is* dependant on the OD clutches, but the mechanical thrust forces are tending to press these harder into together anyway. So in theory slipping on the overrun in direct drive can be caused by worn or glazed linings, or possibly the hydraulic pressure failing to release. The later should be detectable on a pressure test. In my case it could well because I'm a DP(resent)O, not having changed the oil in the gearbox since I got the car 13 years and 75k miles ago (nor in the roadster 17 years and 25k)! I'm in the process of rectifying that and changing the oil in the diffs as well. |
| Paul Hunt 2 |
| Paul, Thanks for this info. Frankly, something like this is what I was afraid of. Why do I always get the odd problems??? That's a rhetorical question. Here's some additional info that might be relevant: The trans was overhauled, by a trans shop that is familiar with little British car transmissions, about 5 years ago. I just reviewed the parts list from the overhaul, and I don't see OD clutch plates listed. I haven't put too many miles on the car since...about 5K miles? The fluid in the trans isn't ancient, as you can see, but is perhaps due for a change due to the time interval, if nothing else. I'm using Pennzoil 10W-40 in the trans, and just noticed that the bottle has a notation "with PureBase". Don't know what that stuff is, but perhaps they've stuck in some sort of anti-friction additive, which would be a bad thing. On to the questions: - I wonder if I could 'cleanse' the trans and OD unit of old fluid by running some sort of solvent through there? then draining and replacing the fluid. - You mentioned a pressure test to determine if OD hydraulic pressure is releasing properly. I'm assuming this is a bench check? And if it isn't, is the fix possible without removing the engine/trans? If so, it might be more cost/labor effective to make the 'fix' just to see if it works. - I wonder what harm I'm doing, if any, by driving the vehicle as is? It still 'goes' and 'stops', and no slippage is apparent on acceleration. And the slippage in question doesn't occur all the time, but does occur, typically, during any particular errand trip. Anyway, those are my thoughts at this time. Any other observations or thoughts are much appreciated! Dave T |
| David Tye |
| Slip in reverse or freewheel on over run is due to either glazed or worn linings or a broken circlip on the sun wheel - all mean the OD unit has to come out! |
| Chris at Octarine Services |
| Well, that's just great (not). Appears I need to pull the engine/trans...not an unfamiliar task. I won't want to delve into the OD itself, though I should be able to pull it off the trans and re-install. Anyone with knowledge of where I should ship the OD unit for overhaul in the USA (preferably Southeast)? I appreciate the response, Chris, if not the bad news. Dave T |
| David Tye |
| David, I'm going to John Twist's (University Motors, Ada, MI) transmission rebuild seminar next month. It will be my third trip up there. It's a long drive, but a fun experience. Drove through a blizzard last year. Take your OD transmission and rebuild it yourself in two days - just a thought. You might also consider sending your unit up to John Esposito (Quantum Mechanics, Monroe, CT) for a rebuild - that might take a month, though. |
| Steve Buchina |
| Thanks for the input, Steve. Don't think I want to invest in that long a trip. Would be fun, but simply don't have that kind of time. I'll be following up on the Quantum Mechanics lead, though. Thanks very much! Dave T |
| David Tye |
| You should be able to do a pressure test in-car. Normally this is done to check pressure with OD engaged - 400 to 420 psi at 30mph - and zero pressure with OD switched out. There are often reports of people successfully using pure ATF in the gearbox to flush the OD hydraulics and cure problems, worth consideration before removal perhaps, cleaning filters before refilling. I'd then go back to engine oil - cleaning the filters again, but there are one or two who claim that brought the problems back again, so they run ATF permanently. To me that just says it is covering up a real problem, and while ATF may be fine in the OD it may not be as good for the main gearbox. |
| Paul Hunt 2 |
| Thanks Paul. I will give this a shot...drain, flush, drain, and re-install new oil. It was suggested that I try non-detergent oil for the final fill, which I will do Dave T |
| David Tye |
| There is nothing wrong with leaving ATF in the trans. Several manufactures use it in their standard gearboxes, GM also uses it in the transfer cases of 4X4's. With all the additives that have been removed form the engine oil ATF should be better as it is made to run in a transmission and has better shear properties. |
| John H |
| Thanks for the input, John. I understand there has been some discussion on this issue...ATF vs motor oil, and seems there has not been a concensus reached on the subject. I haven't delved into it much, and certainly don't have the cred to weigh in on the subject with any authority. The fact GM is using it, as you stated, gives strength to the argument. One fellow I've corresponded with (stateside), who specializes in British ODs, recommended switching back to oil after the ATF flush. I don't think he has a specific reason other than motor oil is what has been used successfully in the past and ATF is a relative unknown in British transmissions w/ OD. Based on your thoughts, though, I'm open to further input on the subject...maybe some evidence that ATF has been used over xx miles with success. Anyone else have any thoughts on the subject? Dave T |
| David Tye |
| David, There is no doubt that ATF works fine as a gearbox oil - Rover changed to it in the LT77 & R380 manual gearboxes used on the SD1 and RV8. The only problem is that it is highly detergent (but has anti foam additives) and will seek out every leak! The OD unit itself is designed to run on any oil between ATF and Hypoid. However, if the problem is glazed or worn (even sheared) linings or a broken/displaced circlip, then changing the oil will do nothing. |
| Chris at Octarine Services |
| David, my daughters Mazda went over 200k miles with no trouble from the trans. ATF was stock from the factory. Since an auto transmission is nothing more than a lot of planetary gears and hydraulic clutch packs the ATF gets along fine with the OD clutch. |
| John H |
| David, my daughters Mazda went over 200k miles with no trouble from the trans. ATF was stock from the factory. Since an auto transmission is nothing more than a lot of planetary gears and hydraulic clutch packs the ATF gets along fine with the OD clutch. Chris is correct though, if you have a leak it will find it. |
| John H |
| Looks like my finger stuttered, |
| John H |
| chris at octarine services. i was interrested to read what you said about these overdrives are designed to run on any oil between atf and hypoid what sort of hypoid are you talking about thank you. andy |
| andy tilney |
| Thanks for the responses! Chris, I feel like I need to give the fluid change a try since its easy and 'might' yield a result. I feel confident that ATF is a suitable fluid based on the inputs and will give it a shot. I'm concerned about the 'leakage' issue, because I've always had a hard time getting the plate on the bottom of the OD unit to seal properly. It leaks a bit, but its pretty manageable, a drop or so every other day or so. I know the ATF will likely make it worse. I drained the existing oil today, from both the main trans and OD units, and it looks practically new. The OD screen contains absolutely no debris. Once the new filter/gasket assy arrives in the mail, I'll install with some ATF fluid and see how it goes. Thanks to all for the discussion. David T |
| David Tye |
| Andy, Anything up to an EP140 as long as it is GL4 or lower - GL5 is not suitable as it will attack the bronze. |
| Chris at Octarine Services |
Mine has been doing this for quite some time. To clarify, there is neither slip in reverse nor freewheel in overdrive, but rather, it accelerates in its non-od ratio and decelerates in its od ratio, and there's this weird space in between where it seems 'decoupled.' I've been able to go on driving: If I wait until everything is thoroughly warm before using the overdrive, all is well. Simple solution, then, I just warm it up before using it. I'll deal with the problem when the box has to come out for some other reason. I'm running 30wt non-detergent motor oil. |
| Sam Good |
| Sam, A little different than my symptoms, though I may adapt your interim solution. Mine simply seems to de-couple momentarily upon deceleration in non-OD. OD doesn't enter into the equation at all, unless OD is engaged, in which case the de-coupling simply doesn't occur. I suspect the OD cone linings are slipping (probably not the right terminology) when in the non-OD mode, but they don't slip in the OD mode. Since it doesn't slip on acceleration (in any mode), I don't know what further harm I might be doing, but I suspect 'none'. Still haven't changed the fluid out...still waiting for my OD filter assy's to arrive. David T |
| David Tye |
| Sam & David T - 2 years ago my OD would occasionally overrun (decouple) - while switched off - during decceleration. It was also lazy to engage and disengage until the engine was well-warmed. Switching the OD on momentarily, then off, would make things right for the rest of the day's driving. During an engine rebuild, I put the tranny on the workbench and changed every single O-ring, gasket, seal, and filter, and surgically cleaned every part as well as the the innars of the OD. (...none of this was any fun, by the way). It's been over a year ago and the OD snaps in & out smartly when switched on & off, and the overrun issue has been moot ever since. |
| Daniel Wong |
| Daniel, During your OD rebuild, did you replace the clutch linings? Was the circlip on the sun gear broken? David T |
| David Tye |
| David, the clutch lining is actually two seperate pieces. one face is used for OD while the other is used for normal, so one can slip and not the other. If the lining is bad for normal drive the sprague (roller clutch) will take the load when accelerating. When on overrun only the clutch takes the load. If the clutch is bad you will get slippage in reverse. |
| John H |
| David, You've already gotten advice from some of the best heads on the board. But having said that, $2 worth of "O" rings put in from under the car solved the same symptoms in my LH overdrive. That's a lot easier than pulling the engine/transmission out. At least try it first. FWIW, Allen |
| Allen Bachelder |
| Thanks, John and Allen. Allen, not entirely sure which o-rings you are refering to, but I'll have a look tomorrow or Sunday. I remember a solenoid that sits in a bore, accessable when the OD cover plate (on the bottom of the OD unit) is removed. Perhaps the o-rings you refer to are integral to that. Should be easy to change out, and like you say, worth the effort 'in case'. Thanks! Dave T |
| David Tye |
| David We have a pretty good OD trans rebuilder in our club in Oshkosh WI. A bit of a way to ship, but he has done just about every OD in our club (not just MG's but Austins, Triumps, etc). I can get you his number if interested. BTW. John Twists transmission rebuild class is a great event. Did my TD transmission there a few years back. Great fun. |
| Bruce-C |
| Thanks Bruce. But I think I have a guy in mind in CT...John Esposito of Quantum mechanics. I've corresponded with him already and he seems like a decent sort, also with extensive OD experience. In fact, I believe that's his specialty. Thanks for the response... Dave T |
| David Tye |
| David, If you have a Haynes manual handy, check out the exploded diagram on page 132. The "O" rings are items 20, 21, and 22. on the solenoid and low pressure valve. All are accessible from under the car. Steve and Bruce, My heart is broken (along with my 2nd gear) because I waited too long to register for Twist's gearbox rebuild seminar this year and it's now full! I did it several years ago too - with an early 3-synch D type overdrive; it's not only fun and educational, it may be the cheapest way to rebuild a gearbox. I'm on the wait-list, should there be a cancellation, but that's probably unlikely... 8^( Cheers, darn it... Allen |
| Allen Bachelder |
DITTO Allen, I got the same when I called last Monday - sorry, we're full. Caroline said they have 12 coming (2 over) and many would be bringing OD's. Last year, we only had 5-6 participants. For me, that was a much better experience - more one-on-one with John and shorter lines. I was going to take up a 3-synchro out of a '66 (bad 1st/rev) and a LH/OD unit recovered from a junk yard (unknown condition). My youngest son was going to come along and tackle the OD unit. He attended last year and proved to be quite proficient - far better than me. Perhaps, it's for the best. I have a couple engines to assemble and my son (with heavy work load) really needs to study for his upcoming PE-exam (civil engineer). Besides - perhaps, now is the good time to build some of my own. Thanks to ebay and Moss, I should have all need parts - just need to muster a bit more courage and a good hydraulic press. |
| Steve Buchina |
| David, You did mention temperature in your initial post. I put an overdrive of unknown history in my BGT, and then put about 6000 cross country miles on it with no trouble. Then, a couple days ago, I fired it up in the morning when it was 12 degrees F, and had the over run slipping that you describe. After about 5 miles of gentle driving it worked fine again. So, I think it is likely that oil viscosity is my problem, and could be yours as well. I would have to look at the container to be sure, but I think I have either Castrol 10-40 or 20-50 in the box now. I guess I will either change it to a lighter oil or ATF for the winter. Let us know how your problem gets corrected. Charley |
| C R Huff |
This thread was discussed between 04/01/2008 and 26/01/2008
MG MGB Technical index
This thread is from the archives. Join the live MG MGB Technical BBS now