[TR] TR3A Overdrive Switches

[Randall]

> What should the operating pressure be?

It varies with which OD you have, and which engine it's behind.
Unfortunately I can't lay my hands on the factory spec for a TR3A offhand,
but several sources give it as 350-370 psi so that's probably correct.  The
early TR2 unit (OD only on 3-4) took more pressure (420-445) as it had
smaller operating pistons, and the TR6 unit took more because of the
increased torque it had to handle (450) and its smaller accumulator.

> I understand that if you attached a
> gauge to the overdrive unit, you can measure the pressure.

True, but it takes a special adapter.  It's also best to use a liquid filled
gauge, as the pressure pulsations caused by the pump will eventually damage
a standard gauge.  There's a fellow that sells the gauge and adapter on eBay
occasionally, but I don't recall his name offhand.  Be sure you get the
A-type version, as the J-type is different.

> If the pressure is down, is it the sealing o ring where the gauge goes
> that may be leaking off some pressure?

I'm not sure what O-ring you are talking about here, Ibsen.  There is a flat
fiber washer where the gauge attaches (to an A-type), but if that were
leaking, you would see the oil running out from it.

The most common leaks that cause low pressure in 2nd gear, IMO, are the pump
non-return valve; and the piston rings on the accumulator and operating
pistons (which were metal on the original TR3A units).  But there are other
possibilities, like the ball in the operating valve not seating into the
casting with the unit disengaged.

There was an article written by "Mr. Finespanner" (Del Border) about the
non-return valve; which you might still be able to find on the web.  However
it seems to be missing from its original repository on the Team.Net server.
If you can't find it, let me know and I'll email you a copy.

Randall