The D-Type Overdrive

Common Problems and Some DIY Cures

(Different part numbers show if the unit is fitted with a 5/16 or a 3/8 bolt rear flange)

Before you do anything check the overdrive is not stuck in or coming in automatically as it gives the impression of not working. (The car will also 'slip' in reverse). This can be very bad solenoid adjustment or more usually a bit of stuff in the operating valve, remove valve as per workshop manual and clean it. Check for black hairy bits as per 5 below.

A lot of overdrive faults come from an electrical fault, what should happen is the operating switch should operate a relay which then activates the solenoid by pulling a plunger back, this plunger then operates a set of points in the back of the solenoid so that the initial 30 amps necessary for pulling the plunger is reduced to a couple of amps to hold it. If the points fail to open then the 30 amps leads to smoking wiring, and often a burnt out solenoid. Checking its operation is simply a matter of running the car with tunnel cover and solenoid side plate removed (the one that says Laycock de Normanville overdrive etc. in blue or black printing), and engaging the switch (remember the car needs to be in 3rd or 4th) and check the plunger pulls the arm back so that the hole in it lines up with the hole in the case (check with 1/8 drill). Waggle the gearstick about to make sure the cut-off switch at the front end of the gearchange is working properly. Running 12 volts straight to the solenoid will show if any fault is electrical or not.

So after ascertaining the solenoid is functioning correctly and the arm is pulled back the required amount, the overdrive can then either -

  1. do nothing,
  2. come in but not hold when trying to accelerate,
  3. make funny noises or vibrations,
  4. work correctly when cold but not hot,
  5. possibly even work correctly all the time.

If everything appears to be OK then all you can really check is the filter, this is located on the LHS under an oblong plate and can be removed and the filter and magnetic cardboard rings cleaned. Lots of grey slime is normal, small black hairy particles are not and show that the clutch or sliding member is breaking up, at some point the overdrive will start to slip and the clutch will need replacing, no special tools are required for this and with care the overdrive can be removed on its own from the car. If you happen to have a 0 to 1000 PSI (that's pounds per square inch) pressure gauge and can screw it into the testing point (the plug that hold the operating valve in) then the running pressure can be checked. Excessively low pressure can be upped by putting shims (washers) into the bottom of the right hand bottom plug, a bit awkward in the car, and don't lose any of the other bits that fall out. Pressure above 400 PSI is adequate. The higher the pressure the quicker the unit will work but the more strain on the clutch lining.

1. Nothing Happens

See if there is any operating pressure at all by running the car with the operating valve plug loose, oil should ooze out (even without the overdrive engaged). If no oil the unit will have to be removed as the fault could be worn or missing pump cam, stuck/damaged pump (most likely), missing pump ball bearing. If there is pressure then it may be possible to get the unit working by checking the relief valve as this could have stuck open. In a car that has stood for a long time the clutch can stick, drive the car, operate the overdrive and whack the brake ring to try and free it.

2. Comes In, But Won't Hold Properly

The most likely cause is a worn clutch, (refer black hairy bits). It can be low pressure due to wear or a worn cam but this is rare. The adapter plate could be at fault, (yes really). The four studs that can be seen when the unit is removed move forwards when the unit is operated, sometimes the adapter plate is not deep enough to allow this to happen (and it can occur as a clutch wears). Drill big relieving holes in the adapter plate, there will be marks where the studs have been hitting.

3. Making Funny Noises or Vibrations

It's knackered and will probably need a rebuild. If a unit has been recently stripped and is working apart from vibrating, the planet gear assembly has been fitted incorrectly (refer to a workshop manual)

4. Works Fine Cold, But Not Hot (May Slip or Not Hold Properly)

Causes and solutions as per 2. As a temporary measure the pressure can be raised by fitting a 7/16 flat washer into the relief valve plug, don't overdo it without a pressure gauge.

Other Dos and Don'ts

  • Hit the plugs with a hammer before trying to loosen them.

  • It is not necessary to remove the drain plug (LH underneath).

  • When removing the overdrive always leave the adapter plate on the gearbox.

  • Never use gear oil additives - they can contaminate the clutch.

  • Change the gear oil at least every 50000 miles, it is working very hard.

  • Expensive (often synthetic) gear oils are well worth it, lower viscosity oils can be used in very cold climates.

  • And finally, there is no easy way to get at the bottom solenoid screw.


John Kipping