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MG MGB Technical - OD problem

The overdrive in my '71 always engages perfectly, however, when the tranny is not fully warmed up, it won't disengage normally. By that I mean it goes out of OD, but does not go to normal gear operation. In fact, I can rev the engine with no forward power. It is like free-wheeling. I have to stop and downshift into 1st, it then fully disengages. Any ideas? I just went from ND 30W oil to 20W50, no difference. BTW, when fully warmed, it functions normally.

Mike
M.A. Barrera

Forward drive non-OD uses the locked one-way/Sprague clutch as well as the cone clutch pressed onto the annulus with spring pressure. The Sprague clutch has a series of rollers that each lock into a tapered slot, so that seems an unlikely cause, making the cone clutch or hydraulics more likely. Measuring the hydraulic pressure in direct drive (should be zero) when cold may help. If you have hydraulic pressure in direct drive when cold it could well be counteracting some of the spring pressure enabling the cone clutch to slip. When the oil gets warm and flows more freely the pressure drops just enough. Could be a partially blocked relief valve, I've been speaking just in the last few days who had his relief valve completely blocked, so not only did OD engage all the time but the pressure developed was so high it bent some of the components. This could be fixable from under the car. Could also be one or more broken springs, or glazed clutch linings, which would require removal and dismantling.
PaulH Solihull

Some people have reported using highly detergent ATF has solved o/d problems. Just a comment, not recomendation. :-)
Stan Best

Mike-
This is a good time to service the hydraulic system of your Overdrive unit. If you wish, prior to any disassembly you can add a pint of Dexron 3 Automatic Transmission Fluid to the transmission oil in order to act as a cleaning agent. It is highly detergent and will clean out any sticky deposits. I should point out that I have heard of people using nothing but Automatic Transmission Fluid for a while in order to cure a problem. I have also heard of people claiming that after the Automatic Transmission Fluid had cured their problem, they went back to using engine oil and the problem returned, so they use nothing but to Automatic Transmission Fluid in their transmission. In such cases there is still a problem within the Overdrive unit that the use of Automatic Transmission Fluid is merely masking, and they are running the risk of eventually developing an even more serious problem.

Drain both the Overdrive unit and the transmission. Be warned that Overdrive units are very sensitive to dirt. Clean the sump cover and the area around it so that dirt and grit do not get inside when you remove it. Unscrew the sump cover securing bolts, taking great care with these bolts as their threads can be easily damaged by rough handling; always use the correct sized socket and apply even pressure. Remove the sump cover on the sump filter screen, clean all of the metallic particles from the two magnets that are fitted inside of the sump cover, and then clean both the sump cover and filter screen with carburetor cleaner. If needed, a new combination gasket and filter screen can be obtained from Moss Motors (Moss Motors Part # 466-360).

Now you are ready to tackle the relief valve. It is located in the top left hand corner. Remove both the ½” BSP relief valve plug and its sealing washer, and then withdraw the relief valve assembly. In the following order, remove the filter screen, the spacer tube, the low-pressure valve assembly, and the relief valve spring. Keep them together in that order and in their original orientation. Now, remove the relief valve plunger. Examine both the relief valve plunger and its seat for pitting, scoring, and excessive wear, and then replace any worn or damaged parts. Examine the O-rings of the relief valve body for signs of deterioration and replace if necessary (Moss Motors Part # 290-930 and 290-925). Check the relief valve spring for signs of collapse or weakening. Its free length (uncompressed length) should be 3 cm. Clean the relief valve filter. If the spring is fouled by dirt, or has become weak, the pump will not generate the 400 PSI of pressure that is necessary in order to operate the Overdrive unit. Reassembly is the reverse of the above order. Make sure that the relief valve is installed in its correct order and that its parts are in their correct orientations. I once put a piece in upside down and it took me hours to figure out why the Overdrive unit would refuse to engage.

Now you are ready to check the Solenoid valve. Unscrew the four screws securing the solenoid cover (the name plate), and then remove both the solenoid cover and its gasket. Withdraw both the solenoid and the operating valve assembly by carefully pulling on the solenoid lead. Withdraw the solenoid rod and the operating valve assembly from the solenoid housing. Keep them together in that order and in their original orientation. Press the solenoid coil and the base cap from the housing. Remove both the operating valve plunger and ball by shaking them off of the solenoid rod. Examine the valve ball and its seat for pitting and scoring. Replace all damaged or suspicious parts. The ball may be reseated by lightly tapping it onto its seat by using a wooden dowel rod as a drift punch. Inspect the O-ring seals (Moss Motors Part #’s 290-935, 290-940, and 290-9450) for signs of deterioration and replace if necessary. Reassembly is the reverse of the above order. Be aware that the wire from the solenoid needs to be aligned correctly in the slot of the unit in order to permit the refitting of the cover plate.

Finally, you are ready to tackle the pump and the non-return valve. Remove the O-Ring (Moss Motors Part # 462-620), and then unscrew the pump retaining plug. It is the one with two holes in its face. Remove both the spring and the ball of the non-return valve. Remove the pump body, the pump plunger spring, and the plunger. Keep them together in that order and in their original orientation. Taking care not to damage the bore of the pump body, use a suitable drift to separate the seat of the non-return valve from the pump body. Examine the O-ring seals (Moss Motors Part # 290-915) for signs of deterioration, and replace them if necessary. Examine the ball of the non-return valve and its seat for pitting and scoring. Replace any and all damaged or suspicious parts. The ball may be reseated by lightly tapping it onto the seat with a wooden dowel rod. Carefully reinstall the non-return valve seat into the pump body. Insert the pump into the casing, ensuring that the flat side of the plunger is towards the rear of the Overdrive unit. Reassembly is the reverse of the above order.

When reinstalling the sump cover of the Overdrive unit, tighten the sump bolts bit by bit in a logical alternating-side sequence in order to avoid warpage. I have not seen a specific torque figure for these machine bolts, but the standard figure for 1/4" UNC/UNF threads is 8 Ft-lb to 9 Ft-lb (11 Nm to 12 Nm). Do not exceed that amount of torque or you could either strip the threads or warp the sump pan.
Stephen Strange

To expand on Stan's comment I've heard of people using ATF *temporarily* as a cleaning agent, then reverting to engine oil (4-cylinder cars) and that was all that was required. However others have said that theirs would only work if ATF is left in *permanently*. Fair enough, if it avoids a rebuild, but it does indicate there are still problems in the unit which could get worse later on.
PaulH Solihull

This thread was discussed between 03/09/2010 and 09/09/2010

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