[TR] Controversial subject...

Randall tr3driver at ca.rr.com
Wed Mar 26 17:52:28 MDT 2014


---- John Macartney <john.macartney at ukpips.org.uk> wrote: 
> Laycock themselves set the primary pump pressure appropriate to the make and model
> of car to which the O/D would be fitted.

I believe that is true, tied mostly to engine displacement (hence maximum torque) and gear ratio in the lowest gear with overdrive available.  There is even a S-T service note that says "Pressures vary according to installation".

> Allen Hess says his unit engages
> with a 'bang' and Allen, it shouldn't.

IMO it depends on which OD is involved, and what throttle position.  The early A-types did have a rather harsh engagement, when engaged at part or trailing throttle, and I believe that was by design.  Somewhere, I've even got an advertisement that describes it as "kicks like a mule".  Later A-types, for cars with IRS, were modified to ensure a more gradual engagement, apparently to reduce the shock on the differential mountings.  Even later than that, the J-type (like the one you drove in Uncle Jack) does shift more like an autobox.

> Once the pressure was adjusted to suit
> the oil we were using, everything was fine. 

The actual viscosity (resistance to flow) of a "straight weight" oil varies a lot with temperature; typically 20:1 or more over the range from 20C (a cold start on a mild day) to 100C (roughly normal operating temperature).  The overdrive's pressure regulation system has to be able to deal with this wide variation; which is much more than the difference (at any given temperature) between say 30 weight and 90 weight (which is only roughly 1.5:1).

So, while I don't doubt that low pressure was your problem, I doubt that the oil used was to blame for the problem.  Much more likely that the spring in the pressure control dashpot was weak from heat and old age, IMO.  Thankfully it's much easier (and safer) to adjust on a J-type than an A-type.

Randall


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