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MY OWN OD PROBLEMS FREE WHEELING IN OD

Moss Motors
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mster50 Silver Member Mike Sterling/A
Zanesville, Ohio, USA   USA
To keep my problems separate from the problems of others I have ben advised to start my own topic. I listen to all advise, this will be way less confusing, thank you.

My OD unit and trans are from, I was told, a 1977 MGB. The symptoms are the OD works as explained when cold. In OD, after driving for an hour or driving hard for half an hour the OD free wheels when letting off the gas and has no engine braking. It feels as though it falls out of gear, however if I step back on the gas it will jump about 600 rpms and reengage the OD. I have done some testing and it will do this in 1st and 2nd when cold and as it warms 3rd then 4th.

Any ideas or advice?

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geezer Avatar
geezer Silver Member charles durning
Magee, Mississippi, USA   USA
1958 MG Magnette ZB "Chick Magnette (sold)"
1967 Morris Minor 1000 Saloon (2-door) "Marvin"
1974 MG MGB GT
Please clarify. Sounds like you can you activate OD in 1st, 2nd, and 3rd? On that trans ,if not modified, should only have OD in 4th. Does the freewheeling only occur only in OD?



Who's version of right are we talking about? When you get 10 LBC owners in a room you'll get 12 different answers.

dickmoritz Avatar
dickmoritz Gold Member Dick Moritz
Philly 'burbs, PA, USA   USA
This additional information is helpful. I suspect that your gearbox sat unused for some period of time.

When overdrive is engaged, hydraulic pressure (400-420 psi) pushes against two pistons and the bridge pieces they abut. This pulls the cone clutch off the annulus, where it had been, along with the unidirectional clutch, providing propulsion in direct drive. When overdrive is engaged, this hydraulic pressure pulls the cone clutch off of the annulus and into the brake ring, which stops the rotational motion of the cone clutch. This in turn forces the torque to drive the planetary gears, which rotate both about the sun gear and within the toothed area of the annulus. The result is a gear reduction of about 18 percent, typically observed as a drop in engine speed of 18 percent for a given road speed.

You mention free-wheeling. This can only occur when the cone clutch is partway between engaging the annulus, and partway between the brake ring.

It is very common for boxes that have sat unused for extended periods of time, for the fat o-rings that seal the two operating pistons in their bores to "flat-spot" on the outside diameter. While they can seal when the oil is cold and thick, hot thin oil tends to bypass these o-rings, allowing the two sets of disengagement springs to pull the cone clutch off the brake ring, leaving it in limbo. With higher engine speed, more oil pressure is generated and this is likely why your overdrive re-engages upon acceleration.

All of this is likely what is happening in your box. If this is indeed the case, then the only fix is to remove the engine and gearbox, disassemble the overdrive, replace the o-rings on the operating pistons, and replace other seals, o-rings, gaskets, and any other parts that need replacement, perhaps one or more of the three ball bearings in the overdrive unit...

Dick



Errabundi Saepe, Semper Certi
(Often wrong, but always certain)

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ohlord Avatar
ohlord Gold Member Rob C
North of Seattle, N.W., USA   USA
1957 Land Rover Series I "EYEYIYI"
1971 MG MGB
1971 MG MGB "Bedouin 2"
Start by draining the od,flushing the od with atf, servicing the od all in the car from below. Then refilling with the proper level of 20w50 conventional oil
No need at this early time to jump on pulling the engine and trans for an overdrive build
.



"I'm a long way gone down this wild road I'm on
It's gonna take me where I'm bound
It's a long way around"



"These are the days that must happen to you"

RD2 Radar/ Electronic Warfare Technician
Vietnam 1969-1972

course2kid Jeffrey Johnson
Fountain Valley, CA, USA   USA
1979 MG MGB "Lucy (Lucifer)"
I'd suggest first to check the oil level in the transmission/OD. If it is low, the OD can have some problems generating pressure (maybe the pump cavitates?). Low oil level generally presents a problem getting the OD to engage.

If the oil level is low AND you know that it is currently filled with the correct type of oil (20W-50 or 30W non-detergent), then I'd top it up to see if that cures your problem.

If that doesn't cure the problem OR if you don't know what type of oil is currently in the tranny, then, I would drain it and refill it. Optionally, you can fill it with ATF, drive it for a few hours to flush out the old oil, drain the ATF, and refill with oil. While running with the ATF, the OD may either work better due to it cleaning up some gunk in the OD, or, it may work worse due to it's thinner viscosity generating less OD pump pressure, so withold judgement of its effectiveness until you've refilled with oil and tested the OD.

The third step is a bit more work, but, easily done in a few hours and can be done without doing steps 1 and/or 2. Drain the tranny, remove the OD cover, remove and clean the OD filter screen and the OD pressure relief valve (see attached John Twist videos where he is fixing an OD with your same symptoms). You can also remove the Solenoid valve cover, remove its plunger assy and clean it and you can also remove the OD Pump and clean it. Since the soleoid is such an easy job, I'd recommend cleaning it up while you have the tranny drained. The pump can be a bit more tricky as John Twist mentions. Personally, I could not get my pump out despite my best efforts using a scribe to snag it. However, with the car in neutral, I was able to rotate my driveshaft and see the pump moving and, since my OD was working fine before I started the OD service, I just skipped pulling the pump. Before you begin this mini service, you'll want to have on hand a new filter screen, solenoid cover gasket, relief valve screen, and O-rings for the pressure relief valve, solenoid, and pump. You can get all those parts for about $25 plus shipping from Moss.

After this mini OD service, refill the tranny and test the OD. If the original problem isn't remedied, then you will probably have to do the full service as suggested by Dick Moritz.




mgbanthony Avatar
mgbanthony Platinum Member Anthony Henderson
Gananoque, Ontario, Canada   CAN
1962 MG MGB
1962 MG MGB
1970 MG MGB GT
1974 MG MGB    & more
Try the ATF trick for a hundred miles or so. It tend to soften seals which have hardened from disuse, and if there is crud, or something sticking it will flush it out.

Laycock recommended ATF in very similar Volvo units, so it isn't going to hurt anything. I've had a high success rate in solving OD issues by using it.

mster50 Silver Member Mike Sterling/A
Zanesville, Ohio, USA   USA
Additional information;

OD will engage in any gear. I was told I could use it in 3rd and 4th so I did. And it works until it gets hot.
I have forgot it was in OD and down shifted all the way to first and stopped, then taken off in 1st OD.
Accelerating, it will go through all 4 gears without slipping as long as you are on the gas. If I let off the gas
it will free wheel. This is in 1st and 2nd only when cold all gears when hot. When it does any of it's slipping
or free wheeling it is on deceleration and you can step on the gas, it will jump up 600 rpm or so like you revved
it at idle, hit that correct rpm, snap into gear and accelerate.
The trans and OD and engine came from a car in a shop waiting a V-8 transplant and is a 35k mile car.
It must have been sitting a lot of the time.
I only forgot once, reverse and OD will just slip and only move the car about half speed of what it should.

Does this pin it down closer or confirm your suspicions?

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andy1965MGB Avatar
andy1965MGB Silver Member Andy Lincoln
San Diego, California, USA   USA
1965 MG MGB "Blue Jay"
1967 MG MGB GT "Cardinal"
I'm sure that Dick Moritz and others will chime with with more expert knowledge than I... But it isn't correct for your OVD to operate in all gears. The solenoid is powered by 12V (ignition-switched). This +12V goes to an operator controlled OVD switch and then it should connect to a switch on the transmission. The switch should be closed in 4th gear (or perhaps 3rd & 4th) and open in all other gears. I think this switch was left out of the installation, so that 12V is only controlled by the operator switch. The danger of leaving it alone is that the engine makes a lot of torque in first gear, more than the OVD is supposed to handle. Also, if there is no transmission switch, it's possible to forget and end up with OVD engaged in reverse, which will cause serious damage.

Unfortunately, it is very difficult to get at the transmission switches with the engine & transmission still in the car.

Andy

dickmoritz Avatar
dickmoritz Gold Member Dick Moritz
Philly 'burbs, PA, USA   USA
Your cone clutch or its control mechanism is FUBAR. That may well be the result of abuse of the overdrive unit due to improper wiring. When wired correctly, the overdrive isolation switch will prevent the engagement of overdrive in first, second, and reverse gears. Your cone clutch may well look like that in the photo.

If it's slipping and free-wheeling, you can replace all the o-rings in the world, you can perform every John Twist service he's ever filmed, you can say a Novena, throw chicken bones and eye of Newt onto the campfire, do anything you want, but that box has to come out and apart...

Dick



In reply to # 3606915 by mster50 Additional information;

OD will engage in any gear. I was told I could use it in 3rd and 4th so I did. And it works until it gets hot.
I have forgot it was in OD and down shifted all the way to first and stopped, then taken off in 1st OD.
Accelerating, it will go through all 4 gears without slipping as long as you are on the gas. If I let off the gas
it will free wheel. This is in 1st and 2nd only when cold all gears when hot. When it does any of it's slipping
or free wheeling it is on deceleration and you can step on the gas, it will jump up 600 rpm or so like you revved
it at idle, hit that correct rpm, snap into gear and accelerate.
The trans and OD and engine came from a car in a shop waiting a V-8 transplant and is a 35k mile car.
It must have been sitting a lot of the time.
I only forgot once, reverse and OD will just slip and only move the car about half speed of what it should.

Does this pin it down closer or confirm your suspicions?



Errabundi Saepe, Semper Certi
(Often wrong, but always certain)

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Attachments:
MGB overdrive cone clutch failure.jpg    42.5 KB
MGB overdrive cone clutch failure.jpg

ohlord Avatar
ohlord Gold Member Rob C
North of Seattle, N.W., USA   USA
1957 Land Rover Series I "EYEYIYI"
1971 MG MGB
1971 MG MGB "Bedouin 2"
His od interlock could be eliminated or bypassed .
I'd test and service before being drastic.
No harm no foul.



"I'm a long way gone down this wild road I'm on
It's gonna take me where I'm bound
It's a long way around"



"These are the days that must happen to you"

RD2 Radar/ Electronic Warfare Technician
Vietnam 1969-1972

dickmoritz Avatar
dickmoritz Gold Member Dick Moritz
Philly 'burbs, PA, USA   USA
I think I just said that... "That may well be the result of abuse of the overdrive unit due to improper wiring."

Forward propulsion with overdrive disengaged is provided by the friction of the inner friction material on the cone clutch squeezing the O.D. of the annulus, and is supported by the unidirectional clutch. Overrun eliminates the function of the unidirectional clutch such that decel is impacted only by the cone clutch on the annulus. Thus, if his OD is free-wheeling on decel, it can only be because the cone clutch is not properly engaging the annulus...

Dick

In reply to # 3606955 by ohlord His od interlock could be eliminated or bypassed .



Errabundi Saepe, Semper Certi
(Often wrong, but always certain)

Ex-Calif Avatar
Ex-Calif Gold Member Dan D
Cincinnati, Ohio, USA   USA
1968 MG MGB GT "Bart - Yellow And Naughty"
1977 MG MGB "Red Betty"
2012 Jeep Liberty "Tank"
2014 Hyundai Accent "Skate"
It's usually a bad idea to bet against Dick, especially on OD, and I think he may ultimately be right.

The ATF stuff probably can't hurt and is worth a try but...

The image below for freewheeling (yes it's a thing) is from the LH dealer manual (link attached - page 6)

As Dick says there probably isn't any magic stuff for this.

I would be willing to come up and have a look but couldn't get there till Friday but really I don't think I am gonna teach you anything new.

I am pretty confident you need an OD overhaul - Dick is a great guy to send it to should you choose to.

Click Here --> MGB Tech Manuals



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Attachments:
LH OD Freewheel.JPG    24.8 KB
LH OD Freewheel.JPG

mgbanthony Avatar
mgbanthony Platinum Member Anthony Henderson
Gananoque, Ontario, Canada   CAN
1962 MG MGB
1962 MG MGB
1970 MG MGB GT
1974 MG MGB    & more
In reply to # 3606955 by ohlord His od interlock could be eliminated or bypassed .
I'd test and service before being drastic.
No harm no foul.

Exactly. I had one with the identical symptoms in my 3rd MGB back about 1974. A wise old Irish mechanic of recent immigration told me to try the ATF trick. It "fixed" it. I sold the car to my cousin about 1980, and almost 40 years later that overdrive still works just fine with something like 275000 mikes on it.

Not to say that it will fix it, but prior to spending big bucks with a professional...

GMB3 Avatar
GMB3 George Badger
Soquel, CA, USA   USA
1968 MG MGB "1QUICK B"
If the OD can engage in all the gears, there is a good chance it was left engaged in reverse a couple of times which at Dick points out the cone clutch could be FUBAR.



---
GEO

course2kid Jeffrey Johnson
Fountain Valley, CA, USA   USA
1979 MG MGB "Lucy (Lucifer)"
Like Dick said, without the interlock switch working, it is likely that the OD is FUBAR from inadvertently running in reverse with the OD engaged.

Also, if you have OD engaged in 1st or 2nd gear, the torque levels may have been too high and damaged the OD.

On the other hand, in 1st and 2nd gears, the OD input shaft isn't spinning nearly as fast as in 3rd or 4th gears and, consequently, the OD pump will be producing less pressure. This may well explain why you're seeing the OD freewheel in 1st and 2nd.

Since ODs are fairly expensive and because the job of replacing one seems fairly daunting to me, I think I'd try changing the oil/flushing with ATF, and cleaning the OD hydraulics just to see if you have gotten lucky and not damaged the OD clutch. Make sure to only use OD in 3rd and 4th gears while testing, and then, if the OD works without slipping, fix the 3rd/4th gear interlock switch/wiring.

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