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MG MGB Technical - Loss of drive

I have 1980 MGB GT with overdrive.

Three times yesterday with a drive of 40 odd miles I had what I have called "loss of drive" - no power to the wheels.

1. I changed down to 3rd and accelerated but nothing happened for half a second.

2. I changed from 1st to 2nd; when I tried to accelerate, the same thing happened for a moment.

3. I was about to back into my drive, put it in reverse, raised my foot and nothing happened then it jerked into motion.

I have experienced "slipping clutch events" years ago; yesterday's happenings were something quite different.
Geoff_MGB

when was the last time your clutch fluid was changed, flexi-hose, cylinder, etc. checked
Nigel Atkins

Slave cylinder and flexi-hose replaced in July, about 1500 miles ago.
Geoff_MGB

Is there enough oil in the gearbox? It could be the overdrive slipping.
jim soutar

I regularly check the oil level in the gearbox. The garage thought of checking it today and it was fine.
Geoff_MGB

I did once use a 20/50 oil intended for use in combined engine and gearboxes i.e Mini"s etc. Overdrive slip was severe until I changed back to Penrite 30 gearoil. No further problems in the last 6500 miles
jim soutar

Geoff,

from what you have said, it looks like there could be a problem with your overdrive. Have a look at this University Motors video on Youtube, which may help you.

http://www.youtube.com/watch?v=XPSgUeHAy8E

It is in 3 parts.

Hope it helps.

Andy
Andy Robinson

Thanks Andy, I'll have a look.

My garage man wondered if it was possible to have a "neutral" in O/D.
Geoff_MGB

Definitely sounds like the overdrive cone clutch slipping. It happened to me a couple of year ago, whenever i would change gear and accelerate hard (in hill climb competitions especially) the overdrive cone clutch would slip - i would back off then accelerate slowly and all would be good until the next time. Note this happen when overdrive was not engaged (no hydraulic pressure required, cone clutch engaged by spring pressure only). Solution - remove gearbox and fit new cone clutch!
Interestingly this created a new problem - overdrive engages but does not disengage easily (sometimes i need to resort to backing up with the overdrive switched off to disengage the clutch from the brake ring). Be wary of replacement cone clutches - Overdrive Repair Services advised me that while they make the cone clutches to original specs and tolerances they sometimes get "stickers" and said it will wear in with use! I am still waiting - anticipate i will have to remove the gearbox and sand down the cone clutch external lining a fraction (new brake ring fitted with cone clutch and spring tensions checked and set).
Mike
Mike Ellsmore

With overdrive disengaged the friction clutches are not used when travelling forwards, the one-way Sprague clutch provides all the drive. All other situations, i.e. reverse, and OD engaged, use either the 'disengaged' or the 'engaged' clutch surfaces.

You say it slipped in 1st and 2nd, and did not drive in reverse, as well as slipping in 3rd and 4th although you don't say whether OD was switched in or not in 3rd and 4th.

Unless the one-way clutch and the friction clutches both have a problem, then it has to be something else. One possible cause could be the slave flex hose delaminating internally, and acting as a one-way valve, preventing fluid from going back to the master to allow the clutch to engage.. I note that was only changed 1500 miles ago, which should preclude that, but these days replacement parts especially containing rubber are pretty poor. If it is something to do with the hydraulics, or the clutch itself somehow, then I'd expect to feel a difference in the pedal.
Paul Hunt

I'm with Paul, personally I wouldn't trust a modern rubber hose because of all of the p*ss poor rubbish rubber products I've suffered on my cars for the last 8 years or so

latest one for me is a Gates fan belt two years old and showing signs of crazing
Nigel Atkins

Another vote for overdrive over-run linings on the OD clutch. Most of the time it's glaze on the linings brought about by years of disengageing OD whilst on overrun (the D type OD unit had a vacuum switch to prevent this.)

Confirm diagnosis by observing the following:

Does it happen in direct top or overdrive or both?
(If it happens in OD it can't be the over-run linings)

Check for any signs of partial free-wheeling when in direct top - do the revs drop away a bit more than the road speed (a bit like an auto can). This is what would cause the cat-up pause you mention, as the engine speed gets back up to the matching road speed.

Does it get worse after a long run? (6 miles or more) Hot linings exacerbate it.

If you do find this is the problem, make sure you fit the uprated overrun springs done for the MGC and V8, part no 37H1928. Makes a huge difference. Available from Overdrive Spares, possibly others.

Mike - your sticking in symptom is what can happen if the centre bush in the housing of chrome bumper LH units has dropped its roll pin and spun. The oil feed hole in the centre of the bush no onger aligns with the one in the housing, and so stops the pressure building up. There can be other causes, but that one can be a bit crafty to track.
Paul Walbran

Thanks for all of your replies.

This has only happened three times - EVER - as desrcibed in my first post. I don't think O/D was engaged on the first occasion and certainly not on the other two.

The garage man drove it for 15 miles yesterday and I drove a similar distance today. In neither run was there a problem.

I'll try the free-wheeling thing that Paul W suggested tomorrow.
Geoff_MGB

Is overdrive removal/replacement possible without taking the engine out?
Geoff_MGB

No!
Michael Beswick

You can get the gearbox out without removing the engine if you remove the remote and twist it round.
But then you can't get the remote back on when trying to reverse the process ....
Paul Walbran

OK thanks again.
Geoff_MGB

I didn't think Goeff was asking if he could remove the gearbox with the engine in place. It sounded to me like he was asking if he could remove the overdrive. I've never tried it, but I thought I had read here that the overdrive could be pulled off the back of the trans with the engine and trans in place.

Want to clarify the question Goeff?

Charley
C R Huff

Charley, your interpretation is correct -

I was asking if "the overdrive could be pulled off the back of the trans with the engine and trans in place."

... and put it back on again of course.
Geoff_MGB

No! Bolt access. And re-aligning/attaching the o/d to the gearbox shaft took some doing with the g/b sitting tail up on the floor!
Michael Beswick

Thanks Michael!
Geoff_MGB

Exactly as Michael says, aigning the two sets of slipe inside the OD can be a touch business at the best of times, but if you haven't done it before it's usually an extremely frustrating experience even with the gearbox out of the car.

To align the two splines, I have found it useful to have to Sprague clutch splines a few thou out of line such that if the clutch centre turns (as it usually does) while the OD is being lowered over the mainshaft, the splines come into line. If you start off with the splines exactly aligned, then any shuffle of the clutch will cause them to lose alignment - and being a one-way jobbie that means they won't radomly shuffle back.
Paul Walbran

"If you start off with the splines exactly aligned, then any shuffle of the clutch will cause them to lose alignment - and being a one-way jobbie that means they won't radomly shuffle back."

That's exactly what happened to us, with the same 'fix'.
Paul Hunt

"aigning the two sets of slipe" (sigh) ... more late night typos ....
Paul Walbran

This thread was discussed between 22/09/2014 and 27/09/2014

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