Slipping Overdrive when hot

[Gary Flinn]

I have had a problems with my Standard 'A' Type overdrive which slips out when hot, normally after around 15 miles or so, it becomes slow to engage and slips with any power or acceleration, but will stay engaged whilst coasting?

 

However coming back from Drive it day on Sunday, the overdrive worked perfectly cruising at 70mph on the A38 Dual carriageway for 35 Miles or so.

 

I have been advised by an Overdrive specialist the cone clutches may be worn, but surely it would slip all the time if this where the case.

 

Any advice welcomed?

[stillp]

Have you checked the oil level?

 

Pete

[Gary Flinn]

Not for about 6 Months, it was fine then and no significant leaks since?

 

It normally only slips when hot, but was fine on Sunday.

[TriumphV8]

Would bet its a oil pressure problem.

 

You can check that by fitting a gauge on top.

Oil must be warm to have same situation.

 

Would look at the presure collector for the culprit.

Often there are deep scratches inside the cylinder

and/or piston rings are broken.

 

For a short time swapping to oil 75W-140 might help.

[AlanT]

I would suspect that this is simply the adjustment of the operating lever. The traditional adjustment technique using a 3/16in drill through the lever end and into the casing may have been OK when new but with wear in the holes and the shaft bearings rarely succeeds now.

 

The best way to do this is with a dial-indicator or a depth-micrometer on the end of the operating valve shaft.

You will then get a true measure of the lift. The adjustment is fiddly and "walks-about" as you tighten the lock-screw.

 

This can all be done from the top with the tunnel out and is well worth checking. Dial indicators can be had for only a few pounds these days and have other uses like tappets, end-float adjustments of all kinds.

 

WATCH OUT FOR TINY PIECES when you take out the plug. Maybe lay some light coloured cloth around to catch them.

 

Other troubles in this area are deficient balls and seats. If you replace the balls get them from a good bearing supplier who do list "loose balls".

 

Oh and on a long run the expansion evens out a bit and you stayed in. At 15minutes some parts are much hotter than others.

 

The reason it fails when hot is that the setting you have is marginal and expansion alters it. It only takes a few thou to upset this.

Most likely you will find you are well short of the book figure. Many OD's will be set up like this.

 

This is a crude mechanism trying to do a precision job. There can easily be many thou wear in a soft Aluminium castings after all these years.

Edited April 26, 2017 by AlanT

[Gary Flinn]

I Have a Mate who's got a suitable Guage that reads up to 600 PSI and I think he's got a dial guage too, what figure should I be looking at when Hot?

 

Hopefully it will be a simple matter of tweaking the adjustment of the operating lever to counteract the wear, or a new Ball bearing and clean out of the valve

 

Thanks for the advice, will keep you all posted .

[AlanT]

Workshop manual refers. There is an on-line version but its a big pdf and a bit slow and clumsy.

 

I wouldn't do this without a read of this.

[Motorsport Mickey]

Try using the search function, there are 31 results that show when Overdrive operating pressure is typed in and I've found 3 so far with information for you.

 

Mick Richards

[dingle]

According to the Buckeye article (the info came from ORS), the pressure should be 450psi for the small accumulator used on the IRS cars and 360psi for earlier units.

My plan of attack would be to check and set the valve opening to at least .016", and check the pressure. If the pressure is low, shim the accumulator spring(and or replace it. to increase it, but be carefull not to shim it to the point of it becoming coil bound. If the accumulator sleeve is scored it can also contribute to low pressure. If the problem still persists, the linings on the sliding member might be a suspect.

Here is a link to the Buckeye articles- http://www.buckeyetriumphs.org/technical/AOD/AOD4/AOD4.htm

Berry

Edited April 26, 2017 by dingle

[AlanT]

The trick is to search using Google. You put:

 

site:www.tr-register.co.uk 4A overdrive

 

This is much better than a forum search for totally obvious reasons.