Hover over a wire to confirm the colour
- The Technical College taught me in a Workshop Manual fashion, without the advantage of many years post-production knowledge. It was therefore never explained that the majority of problems occur in the engaging process, and that there is often a simple solution. Late, LH model Overdrive. Basic description.
- 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.
LH type with ignition relay - UK (77-on):
LH type with ignition relay - North America (77-on):
This picture shows the yellow wire from the manual switch coming out of the main harness, joined to a yellow/red going into the gearbox harness. Insert the fuse here. (Image by David Farrar on the MGOC Forum)
Laycock lh overdrive service manual Some STERLING Truck Manuals & Maintenance PDF above the page. In 1951, Sterling merged (until 1953 still.
Note: For a few months from late 76 to Feb 77 the original gearbox Overdrive switch operating in 3rd and 4th was used plus an additional TCSA (Transmission Controlled Spark Advance) microswitch operating in Reverse, 2nd and 4th. Wiring these two switches in series allowed the TCSA i.e. vacuum advance to be enabled in 4th gear only whilst Overdrive was still available in both 3rd and 4th. In Feb 77, possibly due to unreliability or cost considerations, the microswitch was deleted and the Overdrive switch arrangements changed to operate in 4th gear only, feeding both the TCSA and Overdrive. With this arrangement the output of the gearbox switch fed both the Overdrive manual switch and the TCSA solenoid directly, so OD was only available in 4th gear.
Is this (A) the TCSA switch? Certainly a microswitch as per the original description I had read, and it closes in Reverse, 2nd and 4th as the gear change shaft (B) moves forwards in those gears:
The location of the gearbox Overdrive and reverse light switches. This is a UK rubber bumper 74.5 to 76, but the switches are the same on all four-synch gearboxes:
Showing the large access panel on top of the 3-sync tunnel (left), this should be enough to get at both the OD and reverse light switches: Image from Moss Europe
The main reason for this picture was to show how the cover is moved backwards when a 4-synch gearbox is fitted to the earlier car, but you can see the screws in the forward section, and the holes in the rear section which were for the remainder of the screws originally, which has been tack-welded in place, with a filler-strip:
4-synch: Showing the smaller access panel on the 4-sync: Image from Moss Europe
Incidentally this shows the 4-synch removable panel (HZA1431) correctly with the hole for the gearlever towards the rear, it is shown the wrong way round by Moss. Replacement panels do not seem to come with the fibre-board ring that the gaiter screws down onto, instead a full size foam seal AHC188 is shown for under the panel by various suppliers but the Leyland Parts catalogue only shows this under the auto selector unit.
By removing the bolts from the removable cross-member and lowering the tail of the gearbox as far as it will go, and pulling the gearbox towards the right, I can get my hand in, although care needs to be taken not to damage the speedo cable. The reverse light switch should be easier to get at from below with channel-lock pliers, as it is lower down and faces sideways. If you have a 72 and later car with the centre arm-rest and cubby, cut the carpet around this access panel as well as round the gear lever hole, which means you don't have to pull the tunnel carpet back to remove the panel:
Keep the carpet section from over the panel, and drop it back in place for noise reduction before re-fitting the arm-rest:
Duct-tape over the edges of the hole to protect hands:
Switch removed, wires tied back to the gear lever to stop them dropping out of sight:
Original switches (left) have the hex smaller than the terminal end, so you can only get an open-ended spanner or grips on them from the side, which isn't possible with the 4-synch OD switch with the gearbox in-situ. However replacement switches (right) have the hex as the widest part, so I'm hoping with Vee's replacement switch (the old one is intermittent) I can get a socket or box spanner on it to make sure it is tight.
Solenoid assembly ...
... disassembled.
The spacer lifts the coil up, so the plunger is pulled upwards when the coil is energised. Without it the plunger is as likely to be pulled downwards as upwards. However without any of the case fitted, there is almost no magnetic force on the plunger when the coil is energised and it doesn't move at all. With the outer case and bottom cap fitted the plunger moves up slightly as shown. But when the top cap is fitted there is a very strong attraction upwards. You can pull the plunger most of the way out of the bottom, and powering the coil will pull it all the way in and against the top cap with a real smack. It has to resist 400-420 psi of oil pressure in the 4-cylinder OD, and 510-530psi in the V8.
Ball-seat inside top cap
Plunger and ball, small O-ring arrowed, which prevents oil escaping down the inside of the solenoid, and leaking from the cover. The slot in the plunger prevents air-pressure or leaked oil pressure resisting the movement of the plunger. When the plunger presses the ball against its seat oil cannot flow from the inlet to the outlet of the top cap, the pressure rises at the inlet, and OD is engaged.
Top cap. When OD is engaged and oil is not flowing from the inlet to the outlet, the medium O-ring prevents oil escaping past the ball when on its seat, which would result in pressure loss. The large O-ring prevents oil escaping down the side of the solenoid, which will leak past the cover.
In the released position the ball and plunger should be pushed back so releasing oil pressure. With a sound small O-ring the plunger is unlikely to fall back under its own weight, which is why - apart from possibly the first time the coil is powered after the car has been driven - you won't hear any noise from it, unlike the 3-synch solenoid. If the plunger doesn't move back far enough there may be enough residual pressure to prevent the operating pistons and clutch sliding member moving fully back from the annulus to the outer casing so it can't fully engage direct drive. While the clutch sliding member is between the two there is no engine braking (you still have drive as until OD is fully engaged the one-way clutch is bypassing the slipping clutch) and you get a distinctive 'pulsing' sensation. However under normal circumstances the plunger comes back at least 2mm when the solenoid is released, giving a clear path through the valve, as can be seen here.
O-ring set (MS&C)
Solenoid coil, with earthing spring. Flat on Bee's removed coil on the left, whereas opposite sides should be bent up to form a spring, as with the new one on the right.
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 youare 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.
Laycock Lh Overdrive Manual Download
Next Test
Laycock Lh Overdrive Manual Online
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.