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MG MGB Technical - Strange Clutch Slipping Problem

I have a 1973 mgb roadster with overdrive. When I first bought the car 9 years ago, it had been neglected. It had a lot of trouble shifting out of overdrive. I changed the oil and put in a lighter 10W40 oil and changed the filters. Been working great ever since. I had the clutch rebuilt 3 years ago and have had no problems with it either.

Until now.

So here is the weird thing. I have probably only put 10,000 miles on it in the last 3 years, if that. But I recently had some unusual clutch slipping (I suspect).

When in gear running around 3000 RPM, if I take my foot off the gas, while the car is in gear (no overdrive engaged) the tach drops to idle speed (as does the engine by the sound of it). As soon as I hit the gas, the car slides back into gear almost unnoticably and I can accelerate normally.

The last time the clutch went, I lost third gear, then first. This seems like a different monster.

So, do I have the clutch rebuilt again? Or is there something else I can look at?

Thanks for any help!

-C
Christopher Hoddinott

Could be a slipping OD. Have a look in the archives, lots on it. Does it happen when OD is engaged or not? If it doesn't this could suggest a worn cone clutch in the OD on the direct drive side.

Iain
I D Cameron

<< if I take my foot off the gas, while the car is in gear (no overdrive engaged) the tach drops to idle speed (as does the engine by the sound of it). >>

Chris,

This is text-book worn OD inner clutch lining which I've suffered myself.

In direct drive the drive is first transferred by the inner cone clutch lining then locked by a one-way roller clutch. When the cone clutch lining wears it slips so you still get forward drive via the one-way clutch but not on the over-run (as the roller clutch is one-way); you get the free-wheeling you describe.

When it gets more worn still you get slip when reversing.

In direct drive the cone clutch is engaged by the return springs, the hydraulics or electrics aren't involved, so really it's almost certainly a case of rebuilding the OD.

Here's the BMC fault chart,

http://homepage.ntlworld.com/richard.maile/MGB/ODFaults.gif

Rich

It may be worth checking that you have the right oil in the O/D. I had a similar problem and discovered that the PO had used hypoid oil instead of 20/50 engine oil. The O/D doesn't like Hypoid oil at all and after I had flushed the gearbox out, cleaned the filters and replaced the oil the problem disappeared. Worth a go as it's cheaper than removing the gearbox!
cheers
Paul
Paul eades

Rich,

Thanks for that document. Very useful for looking at solutions. It seems to confirm that my OD box has a worn cone clutch, but I'll try oil change, rings and filter anyway!! Worth a try for £20 and a morning under the car!

Iain

I D Cameron

You're welcome Iain. As Paul says an oil change may be worth a go. If you don't know the history it could have had incorrect oil in the past. Stuff like Molyslip or Slick would be disatrous, not sure how long it would take to get rid of or if the lining would glaze up first. Non-solid friction modifers might be easier to get rid of.

It's worth noting that on the over-run the inner clutch cone alone is responsible for turning the engine. It gets additional pressure applied to it by the construction of the drive arrangement but friction on the lining is essential.



Rich

John Esposito/Quantumechanics is an excellent source in your area.

quantumechanics.com/
Phil collura

This thread was discussed on 25/07/2005

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