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Re: [Mgs] LH overdrive trivia

To: "Ed Woods" <>, <>
Subject: Re: [Mgs] LH overdrive trivia
From: "Paul Hunt" <>
Date: Thu, 15 Oct 2009 09:05:07 +0100
I can't directly answer your questions, but in theory it must press the 
ball-bearing onto its seat with a good enough seal to cause the hydraulic 
pressure on the other side of the seat to rise to a minimum of 530 psi which 
is the V8 engaged pressure.  If the diameter of the opening were 1/16" say, 
that would give a surface area of .003sq in.  To resist 530 psi the ball 
would need to be pressed onto its seat with a force of 1.63 lb., which 
doesn't seem unreasonable.

The winding should be about 15 ohms taking about 800mA, which I'd expect to 
deliver quite a strong push/pull.  Unless the connection from the winding to 
the body of the solenoid were faulty I can't see how you can have a ground 
issue with the OD in the car, bolted to the engine as it is.  If there were 
a problem with the engine ground cranking problems are the first thing you 
would notice.  If a replacement solenoid shows the same behaviour then 
unless you are very unlucky it must be something around the solenoid that is 
causing the problem.  Although the solenoid system is quite different on the 
earlier D-type (don't know the A-type) unit what it does is the same i.e. it 
lifts a ball up onto a seat to block oil flow and cause the pressure to 
rise.  Are you sure you have the correct voltage and current on the bench? 
Measure them, this is one of the few places on the MGB where a current 
measurement is relevant.  On-car also measure the current as there are 
several places where a bad connection could reduce current and hence force 
which *would* affect multiple solenoids.  A poor seal between ball and seat 
would also cause low pressure, the manuals recommend a light tap from a 
hammer to get a good one as a matter of course.  But you won't get any 
pressure without a working pump, and even a working pump won't generate any 
pressure if there is some other leak in the high-pressure circuit such as 
the relief valve jammed open or a faulty operating piston seal.

To eliminate the solenoid I'd be packing the space between the end of the 
plunger and the cover with washers or whatever to mechanically force the 
ball onto its seat, but don't drive it like this as reverse will wreck the 
one-way clutch.


----- Original Message ----- 

> I've got an MGB with an LH overdrive that won't engage. No pressure on the 
> gauge. 


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