Overdrive does not engage
Low oil level
Faulty solenoid or wires
Low hydraulic pressure
Does not disengage
Sticking solenoid valve
Cone clutch sticking
Control orifice between solenoid valve
and relief valve clogged (clean only with
Faulty circuit in electrical system
Cone clutch slipping in
Low oil level
Low oil pressure/sticking solenoid
Worn or burnt clutch linings
Slow disengagement, free wheeling on
overrun, slip in reverse gear
Sticking relief valve
Sticking control valve
Control orifice blocked
-Hayes Spitfire Manual
Probably 90% of overdrive problems that are not
electrical faults are traceable to either the gear
oil level being too low, or an improper adjustment
of the solenoid, so we'll skip these and
concentrates on the others.
An overdrive that fails to engage can sometime
be traced to dirt between the check bail and seat
in the pressure regulator valve, or sometimes to a
badly worn pump. Both are quite rare, though
possible, so don't overlook them. Worn or broken
rings on the operating pistons or the accumulator
piston can make for a lazy-shifting or slipping
However, the real winner is a partially or
completely blocked bypass port in the operating
valve. The valve, which is activated by the
solenoid through the operating shaft and lever, has
a small hole bored in it (about .018') which is
very easily blocked by dirt, etc. The results of
blockage can be really fun to find. First, partial
or intermittent blocking results in slow
engagement, but most noticeable is that there is no
compression-assisted slowdown. The unit appears to
hang between overdrive and direct drive and feels
like it is not in gear. Basically, what is
happening is that the partially blocked valve will
not allow the oil to return to the sump fast
enough, and the pressure build-up above the valve
tries to engage the overdrive while the unit's
springs try to engage directly, and there we hang
between the two. The fun begins when the bypass
port becomes completely blocked. Above a certain
speed, the pressure above the valve becomes high
enough to engage overdrive, no matter what you do
or where the switch is.
Fortunately, the valve blockage is easy to
repair. With the transmission tunnel removed, the
access to the valve is on the right side of the top
of the overdrive unit and is under a 7/16" plug.
Under the plug is a spring and check ball assembly
which can be removed with a small magnet. After it
is cleaned, It can simply be set back in place,
hole-in end facing up, followed by the ball and
spring assembly. it Is very straightforward and
easy to remedy once you find it.
-Ken Gillanders, Temple City, CA
"Lots of overdrive faults are due to switch on
top of the gear shift lever not making proper
-from a magazine I read recently
It would be a good idea to point out that the
problems listed here are mostly only for D types
(pre 74) - J types have their own problems,
invariably solenoid or unidirectional clutch."
(Type D) - Operating Lever
1. If the overdrive does not engage, or will not
release when it is switched out, providing the
solenoid is not at fault, the trouble is likely to
be that the operating lever is out of adjustment.
Adjustment can be made without removing the
2. Undo the three bolts and washers holding the
solenoid cover plate in position, to give access to
the operating lever and solenoid plunger.
3. Procure a short length of mild steel rod of
3/16 in (4.76 mm) diameter. Switch the ignition on,
put the car in to gear, and flick the actuating
switch to the overdrive position.
4. If the 3/16 in rod can now be passed through
the hole A in the operating arm into the hole in
the casing, adjustment is correct.
5. If the solenoid does not move the arm far
enough for the rod to be inserted into the hole in
the casing, or if it moves the arm too far, hold
the solenoid plunger from turning by means of the
two flats machined on its shank, and pressing the
plunger tightly into the solenoid, screw the self
locking nut B in Fig. 6.26 in or out until the 3/16
in test rod can be pushed fully home into the hole
in the casing.
6. Operate the switch several times, checking
with the test rod to ensure adjustment is correct.
Measure the current consumed by the solenoid which,
with the operating arm correctly set, should be 2
amps. If a reading of about 20 amps is obtained
this will show that the solenoid plunger is not
moving sufficiently to switch to the holding coil
from the operating coil.
7. Continuous high current will cause premature
8. With the solenoid again de-energized,
re-align the setting holes and insert the test rod.
Hold the solenoid plunger against the blanking plug
and check that dimension A is between 0.150 and
0.155 in (3.81 and 3.937 mm).
9. To obtain this dimension, alter the thickness
of the blanking plug washer (on early models) or on
later units turn the adjuster screw.
Here is a set of wiring diagrams for
A GT6 Overdrive and gearbox rebuild with many
Type Overdrive Rebuild
Another great site explaining a J-Type
to Top 10 Problems Page