[Healeys] Overdrive pressure drop

gradea1 at charter.net gradea1 at charter.net
Fri Jul 30 11:04:49 MDT 2021

Simon-yes, there are two balls which control the pressure in the unit.
In the operating valve which is engaged by the action of the solenoid,
dirt is a possible issue. There is a very tiny hole drilling in the
stem of the valve which allows the oil to circulate. That opening can
plug with debris and stop the flow. Oil from the accumulator goes thru
this valve and lifts the ball allowing the operating cylinders to move
forward on the clutch producing the overdrive. When the ball is seated
(solenoid off) oil does not pass and is dumped into the OD sump.
You can see that this tiny drilling is very important and must be kept
clear. Often rebuilders suggest to enlarge the hole to 1MM. It may
change the dwell time of the OD action, but its has a better chance of
staying clean...a theory not proven.
The other ball is on the oil pump and that is the one that Del
suggested needed a new spring or extended plunger to enhance
pressure-another theory.
As you measured no pressure when out of OD, that is due to the ball
properly seating on the valve and reliving the pressure that operates
the two pistons. If you move the lever or engage the solenoid you
should get 470 lbs. again.
I would suggest cleaning the valve-use a magnet to get the parts out
and don't use a shop rage to clean them (no lint). If all is ok in the
valve body you need to look further into the accumulator and o-rings
and piston rings. I suggest you contact David at OD Spares for some
additional advise. Hank

	-----------------------------------------From: "Simon Lachlan via
To: "Healeys"
Sent: Friday July 30 2021 7:32:48AM
Subject: [Healeys] Overdrive pressure drop


	I’ve been having issues with my OD. Similar to someone else quite

	The OD would engage and disengage properly at high revs but once the
car was cruising along at low revs, the OD would somehow act like it
was on the verge of dropping out &/or dropping back in. 

	It _may_ be electrical, but I doubt it. The solenoid goes in with a
nice robust click. 

	I was thinking that it would be pressure related. So…………… 

	I’ve bought a gauge from OD Spares here in UK and got the car up on
axle stands. 

	I did the test procedure much per the Del Border article which I sent
out the other day. 

	* Pressure is good, at the top end of 470 to 490 with gearbox oil
cold. ie 470 to 490 psi at 1500 rpm in 4th and then same in 4thOD
after a momentary drop. (40wt non-detergent).
 	* When I go out of OD, brake and switch off, the pressure drops to
nil at once. Which is, apparently and according to the article, WRONG.

	I am now leaning towards something in the so-called “ball valve”,
items 33 to 37 on page 2 of attached. 

	Does the above seem like a good place to start? 

	Could, say, a worn spring or a piece of dirt in that area cause these

	The names of these valves, “Operating valve” at top and “Ball
valve” at bottom seems to vary with every article I read. 

	The “non-return valve” and the “check valve”….are these the
same thing and are they what BMC calls the “Ball valve”, item 37. 

	Thanks, hopefully! 


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