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OverDrive operation


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Hi Folks,

the more avid readers will know that I put my Overdrive in for repair at the beginning of the month.

 

Three years ago the OD stop functioning and when opened up it was found that a large circlip had broken and allowing the gears to float around - willy nilly.

 

So after a delay by the garage and a rocket sent up their rear ends from myself they jumped into action opened the OD and found

that the same circlip had broken again.

 

OK it has all been put back together - with a new circlip - but!! what is causing the circlip to break?

Has anybody had the large circlip fail?

 

The garage does a lot of Austin Healey and Jag GB's and OD's and have rarely come across this.

They suggest dipping the clutch during OD changing.

 

So what is the best way to engage/disengage the OD?

 

I've always tried to match the rev's and it is always smooth.

 

Roger

 

 

 

 

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I have always dipped the clutch.

 

Still do on the Vitesse, which has a D type.

 

Over the Triumphs I have owned fitted with overdrive the J type was the smoothest.

 

Dave

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I don't dip the clutch, but back off the throttle when engaging O/D, & blip the throttle when disengaging. always a smooth change, never had a problem.

 

Bob.

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My j type overdrive has been used ( and abused) for 20 years since it was installed and always works.

 

Obviously its a different type Roger but surely there must be an underlying cause for your two circlip failures, beyond driver input?

 

How is the circlip failing?, is it a manufacturing defect or is there soemthing else amiss in your gearbox....... ( thats not a metaphor...)

 

Steve

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Hi Steve,

as you know the A type OD is pulled in much stronger than the J type - this doesn't help.

 

I can't see the circlips being duff.

I have just had a look at a picture of the assembly and note there are two quite large circlips.

I can't see a reason for spontaneous disintegration.

 

I shall try and dip the clutch but I can't see me doing it every time.

If it fails again I'll take it apart myself and see what is going on.

 

Roger

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Roger, circlips are used to retain stuff so perhaps there is too much movement in the shaft/gear that is being retained and the circlip is getting hammered or the channel that the clip sits in has a problem and is allowing the circlip to twist and snap. I recall the problem with UJ circlips and variations in thickness preventing some circlips from engaging fully in some yokes.

 

Re OD operation, I'm with the group that backs off before engaging. I use an OD controller so most of the time my OD disengages automatically as I change gear. When I do switch it off manually I do nothing but flip the switch. Never had an issue with 30+ years of my TR6 J type, never had a problem with 5 years of badly abusing my 1972 TR6 A Type when I was young an foolish. Not had enough seat time in the TR3 to experience the A type OD other than noticing it is a lot more aggressive than the J type.

 

Stan

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Keep throttle on both ways up and down. I have had overdrives for more than 40 odd years and never broken one yet. Funnily enough though Roger Chris that does my boxes has just had the same broken circlip on his 4a box too and he doesnt drive his particularly hard.

Stuart.

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Hi Stuart,

that is very interesting. I don;t drive my car that hard.

 

Thanks Neil & Stan,

that is roughly what i do.

 

OK, I'll drive the car as normal and see what happens.

If it fails again, I'll strip down and see what is going on.

 

Thanks for all the replies

 

Roger

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Many years ago when I had a whole bunch of ex US big Healeys to restore, all the overdrive units were knackered one way or another which was mostly down to abuse/bad operation. In fact one had all the planet gears welded up!

Stuart.

Edited by stuart
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I engage/disengage under throttle most of the time. Works fine, knock wood.

 

Roger, I wouldn't eliminate the possibility of duff circlips out of hand. Is there a chance the two that broke came drone the same box (production lot)?

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Roger, Sound like something else is causing the failures. I use the clutch because it generally reduces wear on the whole of the transmission.

 

Personal preference.

 

Dave

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I agree with Stuart - I use full throttle when engaging overdrive, and a small amount when disengaging. If braking whilst disengaging, one can heel and toe to avoid the shock on the transmission system.

The logic overdrive unit saves a lot of bother when changing gear, and avoids the embarrassment of overdrive engaging when changing from 1st to 2nd gear, having left the OD switch in the ON position!.

Ian Cornish

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I don't dip the clutch, but back off the throttle when engaging O/D, & blip the throttle when disengaging. always a smooth change, never had a problem.

 

Bob.

Hi Bob,

much the same as you. I wonder why my circlip is breaking.

 

Roger

+1

Always just lifted the throttle off a touch

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Way back when I was told always have some positive drive (not trailing throttle in other words) when operating o/d - in or out. No need to de-clutch . blip throttle etc. This gives just a change in engine note/tach with no jerking. Very satisfying smooth operation.

Back then had no problems with o/d on 3a then 4 both used in club rallying and everyday long commutes.These days I ease off hard acceleration on point of going into 2 o/d . Full right welly seems too brutal on 50+ years old transmission.

 

Mike

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Roger,

Those are the only two Circlips in there. Repeated failures could indicate a problem with the thrust ring assembly or the end of the clutch where the second circlip fastens. The first holds the bearing into the thrust ring. The second secures the bearing to the clutch.

Having rebuilt over 300 A-types, I have never seen the failure of those circlips.

 

Dave

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Surely if the clip is broken then there must be too much load on it, the bearing must surely be a reasonably be a tight fit, and the clip is just there to locate it, perhaps the groove is not right in some way.

 

There are 2 ways to fit a circlip, one correct the other will allow it to pop out under load.

 

If you look at a circlip ( bigger ones are easier see ) they are stamped out of a flat sheet, so there will be a rounded edge, when the stamp comes down, and the other side will have a sharp edge, so the sharp edge should always be fitted away from the bearing, so the sharp edge will bite into the groove.

 

It also helps with the circlip pliers not slipping off on the rounded side.

 

John

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