Tackling the Triumph Overdrive

The Laycock A-type overdrive is a two-speed hydraulically controlled planetary transmission. Direct drive is always controlled by spring pressure. That is, the spring pressure you see when you put the gearbox and overdrive units together, created by eight springs. The overdrive shift is always controlled by hydraulic pressure. Unlike a regular automatic transmission, which works on relatively large volumes of low-pressure oil in the order of 50 or 60 psi, the Laycock unit operates on a very small volume of extremely high pressure oil at 400-500 psi. It tolerates very little internal leakage; consequently, it requires very close control on the oil level in the gearbox.

Normally speaking, if a car slips while it is in direct, it generally means that the spring pressure, for one reason or another, is gone (by the way, this is an extremely unusual situation), because the mechanical advantage always favors the direct drive, and consequently, the higher pressure and operating the higher pressure and operating conditions are needed for overdrive, or at least to energize overdrive.

The overdrive is energized by the simple fact of oil being picked up from the pump in the bottom through the screen, pumped through the system, through the control valve, and into two pistons about 1″ in diameter that are behind a device called an actuating bar. When the pistons move forward, the actuating bar pulls two rods from the back and pulls the cone clutch in the opposite direction, activating the overdrive. So 99% of the time, if you can hear it energizing and it is slipping, it is something in the hydraulic system. So let us do a quick rundown on the things you can test, some of them not requiring you to have to remove the gearbox.

First Test

One of the big problems we have with the Laycock overdrive is that people will fill the gearbox initially and not realize that it is somewhat difficult for the oil to work its way back in the overdrive unit. With that idea in mind, they do not realize thai once the gearbox has been filled and the thing has been run for a while, part of the oil has run back into the overdrive. That simply means that both units are now low on oil.

What we generally do is put oil in the gearbox and jack the car up from the front, let it run back into the over drive for 10 minutes or so, then sit the car down with the plug out and a tray underneath and allow all the excess oil to drain back out of the gearbox. But first thing, check the oil level, as it is entirely possible you have an inadequate supply of oil.

Second Test

This can also be checked out from under the car. If low oil is indeed not the problem, then the next thing to do is to drain the overdrive from the large bronze plug in the rear and look at the condition of the filter inside. If it is completely covered with trash, it may be that the filter is prohibiting a satisfactory amount of oil from finding its way into the overdrive and consequently not allowing it to function properly.

Next Test (same location)

In the center of the cavity that the filter came from is a plug with a head of about 9/16″ or 5/8″. Removing that plug will give you access to the overdrive oil pump, at least to where you can see the plunger in the pump. With the car in neutral, have someone hold one rear wheel and have someone turn the other, then check to see what kind of pump travel or plunger travel you have on the pump.

It is possible that when the units were put together, the roller activator that runs off the eccentric may not have completely found its way under the roller, and it may be caught on the shoulder. This would give you only a very, very short stroke on the pump and consequently extremely low oil pressure. If you see this is not the problem, you can now put the plug back in the bottom, put the oil filter back in, and put the bronze and plug on.

At this point, you will have to refill the transmission with gear oil, as the next test requires that you have gear oil in it. Take up the transmission tunnel and take out the plug that is on the upper right-hand side of the overdrive unit. This gives you access to a spring, a ball, and the operating valve. Leave the operating valve in place and take the bull and spring out.

With this test you have to be a little careful, because occasionally transmissions become air locked. Normally when they are air locked they just do not function. What you are going to do is determine whether or not it is pumping oil.

With the car jacked up in the back, start the engine, put it in fourth gear, let out the clutch, and be careful, because if the oil comes out of there, it will come out like an absolute tornado. What you are doing is testing for oil at the vent on the right-hand side of the gearbox up high. If there is no oil coming out, then that is not the problem. Replace the ball, spring, and plug.

Now the thing is getting a little tricky. On the left-hand side of the overdrive, down low behind the solenoid, is a plate that has to come off. In order to remove it, you must first remove the lever, then the two screws which secure the solenoid can be removed, followed by the solenoid itself. You should now see there are three cap screws and two nuts, the nuts having fairly long threads on them to evenly release the tension and the heavy spring holding the accumulator piston in place. Remove it very carefully.

Looking inside the cavity, you will see the heavy spring holding the accumulator piston on the bottom, and, to the right, a plug which looks like the one on the upper right of the transmission. The plug is known as a “drain back” or “check” valve, which could cause a leak sufficient to reduce oil pressure. If this turns out to be your problem, you need look no further. Just clean the valve carefully and replace.

If the hydraulic problem persists, then there is yet another option to investigate, that of leaking rings on the accumulated piston. Explained briefly, the accumulator holds only a small amount of oil, but does require a moderate amount of oil to activate the pistons. Therefore, built into the system is the accumulator piston in a closed cavity with a heavy spring behind it. As you drive the car, the overdrive pumps oil behind that piston, slowly but surely forcing it to the back of the cavity and against the spring, where it maintains pressure upon it and at the same time maintains a reserve supply of oil.

This extra supply of oil is necessary to keep the unit functioning in a situation where you flip from overdrive back to direct drive several times in quick succession, which could conceivably deplete the oil supply necessary to keep the pressure on the piston. This condition is an extremely important factor in road racing, but of less severe impact in street operation.

To test for leakage around the accumulator piston, replace the solenoid cover with a piece of stout metal bar (about 1″ to 1 1/2″ wide and 1/4″ thick) placed over the two long studs. Start them back in their original holes to hold the spring in place. Now, start the car again, and, while in fourth gear, look for a profuse amount of oil leaking around the accumulator piston under pressure. While you are at it, you might check and see whether or not there is even enough oil to force the piston back against the spring.

Next Test

At this point it is necessary to remove the overdrive unit from the car, which of course means removing the transmission. Separate the two units and remove the same plug where you were previously looking for oil on the top. By leaving the ball in place and removing the spring and cap, you can apply air pressure to the system, which should force the two pistons out against the actuator bar. If you hear a great deal of air hissing, you could indeed have bad rings on the applied pistons.

This system is a little unique in that two types of piston rings were used. Earlier cars usually have interlocking cast iron rings, while later cars tend to have rubber O-rings. After many years (and rebuilds) though, Lord knows what may be in there! At this point, you have now exhausted all possibilities for hydraulic malfunctions.

'Tackling the Triumph Overdrive' have 2 comments

  1. December 9, 2019 @ 4:58 pm Mike Soltez

    I read the beginning of this article and couldn’t get any farther than that. In direct drive going forward the springs aren’t in play at all. Torque goes from the mainshaft through the unidirectional clutch to the output flange. In reverse the spring pressure on the clutch provides drive. The unidirectional clutch only engages in forward. Also there are only 4 springs!
    Who wrote this?


  2. November 8, 2020 @ 9:09 am Micky Mouse.

    The check valve is on the left, not the right side when you look in the spring cavity.


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