If you want to know all about how it works, you should really read the
description in the Bentley manual very closely. You can check for movement on
the solenoid to make sure it is good, but you can't tell if the unit is good
without running it, or if the solenoid is adjusted correctly until you run it
under load. The solenoid pushes a plunger that changes the path of the oil
pressure which makes the OD engage or disengage (see section Fa2 - description
on
page 160).
Here is what I would check while you have the unit on the bench - the layshaft,
clustergear, and it's bearings. The OD units are pretty robust, and as long as
the lining is in there, and the solenoid moves and is adjusted correctly, it
will
work. The layshaft/cluster gear/bearing arrangement, on the other hand, is
known
to go out at about 100,000 and pretty faithfully. The car will still run and
move for quite a while, but will pop out of gear and send metal all over inside
the gearbox. Also, DO NOT USE A DETERGENT OIL IN THE GEARBOX. It is hard to
find but get a non-detergent. Supposedly (I have not experienced it myself, and
I had detergent oil in it at one time) the detergent oil will destroy the
overdrive unit. Make sure you hook up the 3-4 gear lockout (if you are
following
the diagram, you almost have no choice, other than shorthing them - don't do
that) because if you back up you will destroy the OD unit in reverse.
The other listers are correct on the function of the vacuum switch - it protects
the OD and car. For some reason the thought people have is that when you stomp
on the gas with OD on, the vacuum switch will automatically kick it into normal
drive - that is NOT how it works. When you turn overdrive from off to on in 3rd
or 4th gear, it will go on (assuming it is adjusted correctly). No question
about it. When you turn overdrive off, it will only go off if you have the gas
pedal depressed somewhat (i.e. not drawing a huge vacuum). If you turn it off
with your foot off the gas, the OD will stay engaged until you give the car some
gas, then it will disengage (section Fa12-page167).
Good Luck!
Phil Bates
Mgbbob@aol.com wrote:
> Thanks for the advice Phil.
> The conversion seems a little less daunting than it did a few days ago.
> As you noted, all the parts are available from the big three with the
> exception of the vacuum (throttle) switch.
>
> You can bore me with you're explanation of the vacuum switch after I've had a
> shot at it.......since it overrides the manual overdrive switch, I figure
> it's designed to keep the solenoid from engaging (or energising) if the RPM's
> are too low. It could have been redundant if the operator was instructed to
> engage the OD only at 2000 rpm's or higher....no?
>
> Are there any easy ways to inspect the linkages internal to the OD? I drained
> the oil and was gratified to see it fairly clean. The mag rings around the
> filter needed to be cleaned, but I am very encouraged.
>
> I was talking to a trained Leyland mechanic(?) today that told me to throw
> the unit away if it had been crashed. I've been looking WAY long for this OD
> and I'll be damned if I won't rebuild it. How tough can it be and how can I
> tell if it's been crashed?
>
> Can't I just apply 12 volts to the coil to see if everthing works? Can't I
> engage the solenoid and rotate the input shaft....shouldn't I get a response
> at the output shaft?
> Is it more complex than I imagine?
>
> Bob Stahlbush
> 66 MGB GHN3L 78708
> 60 TR3A TS 81398L
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