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7.4 - Accessing the Overdrive Top Switch ( Michael B,  September 25, 2003 )

There indeed is a possibility to get access to the top gear switch
from above: youíll have to remove the isolation (felt?) to find the
screws holding down the lid to the tunnel. The then revealed
opening is almost as large as the gearbox itself. The switches (top
gear and reverse light) are right on top of the gearbox and are
easily accessible!

7.4.1 - Information On Non-Working Overdrive Units ( Members,  )

From GNB on July 15, 2004:

Hi Tom, the most likely cause of your difficulty with the
overdrive unit may be the one way return valve is slightly worn or
dirty. Barring this lesser problem the ''O'' rings on the operating
pistons may need replacing or the accumulator piston rings may be
getting tired. Get an oil filled pressure gauge 0-to-1000 psi hook
it up and take a drive see what it does and keep good notes.
sometimes it is as simple as readjusting the pull in solenoid

From Barry Woolcott July, 15 2004:

Tom, your problem is almost certainly a hydraulic one and as
someone has said, it'll be either the hydraulic accumulator or the
operating pistons which control the sliding member which in turn
activates the sun and planet gear system. You can get at the
accumulator from outside, with the unit in situ, and this will
allow you to check the condition of the rings which are just like
small piston rings. At the same time you could check the length of
the accumulator spring(s) which loads up the piston to produce the
pressure in the system. I once spent many happy hours trying to
fix a non-operating unit only to find, eventually, that the spring
was a bit short with the result that the relief hole was being
uncovered, with the result that the unit wasn't getting to the
required pressure. Check all this first and with any luck that'll
be the problem. If not then the o-rings on the operating pistons
would be next to check and this means removing the unit, which can
be done with the gearbox in place in spite of what the book says.
It would also be worth checking the setting of the operating lever,
but if by any chance you find the unit in overdrive and you can't
get in out, DON'T GO BACKWARDS. That would result in the worst
type of internal hemorrhage. A pressure test will help with the
diagnosis. Good luck.
Barry Woolcott

From Chris Burdo, Fri, 16 Jul 2004

You may want to try ,
(also you'll see it progresses into more in-depth reading). for some more info on
the overdrive unit. Also try One point
mentioned was that as time goes on and wear and tear on the O/D increases you
may find that the factory adjustment won't work anymore and the adjustment
needs to be altered so that the O/D engages. Also make sure your filter for the O/D
is cleaned properly as this will cause pressure drop as well. Make sure you are using the
proper weight oil for your O/D as well. The later O/Dís use 80/90W while the early O/Dís
use 30wt. Using the incorrect weight gear oil will cause the O/D to not work.

Dave Symington, Fri, 16 Jul 2004

I had similar problems with my OD in 2002, Seemed to drop out of engagement
after a while. (about 20 miles or so). . . Paul's ideas are spot on. Make
up a gauge as per the Austin Healy web site and test the pressures in the
OD. I did that and found my pressures, while good at first were quite low
after about 20 miles. I checked and set the linkage and checked the
solenoid operation. Both OK . . . I suspected the pump but that turned out
to be OK. I ended up removing the OD from the car and sending it to a
rebuilder named Duane Hoberg. It turned out that the accumulator seals were
leaking and not holding pressure. The OD was rebuilt and returned for about
$200 . . . I put it into the car, installed the gauge again and tested it.
Worked like a charm and I haven't had any problems with the OD since then.
Oh yes, I also followed Paul's suggestion to use the Redline synthetic and
it seems to work perfectly.

Greg Bernier 15 Jul 2004

I had similar issues with the o.d. on my Mark IX, but not to such an
extreme. After trying both 30wt and Redline synthetic oil I am now using
75W-85 hypoid gear oil in the transmission, and it has helped a lot. I still
have to back off the throttle a bit to drop down into o.d. when accelerating
in 4th gear, but I just treat it as an idiosyncracy, not a problem. It does
shift up more quickly before the oil gets warmed up, probably indicating
that the pressure is falling off when the oil thins. I really do enjoy the
way the o.d. gives the car the ability to cruise at relaxed r.p.m.s while
still having the lower rear end ratio to give good acceleration and a fourth
gear that you don't have to shift out of all the time, when taking slower
corners on country roads.

Gary Odell, 15 Jul 2004

There is a pressure port you can check the pressure with (500+ lbs)
while driving if you find a high pressure gauge with a few feet of hose. My
guess is, if the OD box is old, a seal is failing; if the box was rebuilt or
is fairly new (less than 10 years maybe?) and someone else has worked on it,
the pressure regulator could have been damaged or set wrong. The pressure
gauge would show this - should certainly be in spec when cold, if not it's
the regulator. When hot, I'd say the hydraulic piston seals.

From Paul Saltwick, July 15, 2004

If the gearbox leaks the level may have dropped after your long trip, causing
this symptom. You need to raise both ends of the car to accurately check the
level, I usually measure off the bottom of the doors.

The first thing I would do is spend $15 and fill it with Redline MT90
synthetic gear oil. While it may look thinner when you pump it in, it maintains
viscosity better at temperature, lowers the gearbox temperature and improves the

If you want to test the pressure on the road, you need to first make a
fitting from an old valve plug on the top of the box. This plug holds the spring
against the valve ball, which needs to be in place for the test. Sounds like
you have a spare and you can drill and tap it for an NPT fitting. You need to
remove the console and gearbox cover and test the pressure from the interior.
Pressure should be 550 psi. Try

Barry Woolcott added on July 15, 2004:

Incidentally, you can do a pressure test in the garage by raising
the rear of the car (wheels off the ground) and running the car.
This allows you to check the pressure without the distraction of
avoiding running into others, and without the need for a ''helper''.

7.4.2 - Overdrive Solenoid ( Laurent/Waugh,  August 20, 2003 )

From Laurent August 20, 2003

The usual failure mode of the solenoid is an improper actuator set up.
The stroke has to be correctly adjusted (according to the manual, with
reference to the actuating lever located on the opposite side of the OD
which has to be centered with a pin inserted in a purposely placed hole in
OD casing). If not, the plunger does not move far enough as it should to
switch current from the actuating coil to holding one. The actuating coil
remains powered, overheats and burns the solenoid. Nothing works anymore

From Andrew Waugh, August 19, 2003

The solenoid is actually 2 coils, an actuating
one and a holding one. There is an internal contact that
changes the applied voltage to the holding coil.
To test it properly you'll need an ammeter. The coil
should draw something in the order of 2A. Tests must be
done with the overdrive assembled

7.4.3 - Overdrive Reverse Lockout Information ( Pual Saltwick,  April 23, 2003 )

There is no relay or reverse lock-out on the factory 420/S OD wiring.
The solenoid on the gearbox gets its power from the same source as
the reverse switch (before contacts), but it is irrelevant to the circuit.
The power goes from the reverse light switch terminal, through the 4th
gear switch (if closed by being in 4th gear), through an in-line fuse,
through the column mounted OD switch, and onto the solenoid.
It would have been easy for the factory to wire a relay through the reverse
switch, but they chose to just print a warning "Don't reverse with the OD
engaged " in the manual. Make sure you put an in-line fuse between the 4th
gear switch and the column ("ON") switch, the solenoid is a two coil device
that can draw 20 amps when it fails.


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