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Hydraulics Dismantling Now the all important hydraulics. As we have already seen, gears are changed by varying the application of clutches and a band. These applications and releases are controlled hydraulically, and to ensure the right amount of sophistication there are 18 valves to give gearchange timing to suit the drivers requirement, smooth, precise gear changing and differing pressures to clamp the clutches under varying torque requirements. The all important valve block is accessed by removing the sump and the filter. 74 Then remove the reinforcement plate, and the brake band servo cover, arrowed. You can now release the selector mechanism detent spring and the bolts which secure the valve block assembly, and lift it ear.

Look out for this boost valve ball which is now loose. If you lose it reverse gear and manual 1 would be affected. You will need special tool 18G 1415 to compress the brake servo piston and remove its retaining circlip. Use this spanner 18G 1418 to remove the modulator vacuum unit from the rear of the housing. Then remove the modulator valve assembly. Many of the valves in the valve block are held in position by a roll pin. Take sure the edges of the holes are deburred before attempting to remove the valve. If you do not, the valve will be scored and must then be placed. And it goes without saying that complete cleanliness is essential when working on any components. Any dirt or damage will affect the correct workings of the valves.

Pressure Test Now that we have seen what the valves do, next some points about pressure testing. First make sure the transmission fluid is at the correct level and is at normal operating temperature, then stop the engine. Remove the blanking plug on the left hand side of the transmission and fit this adaptor 18G 677B. You can now connect the pressure test equipment 18G 677ZC. Chock the road wheels, start the engine, apply the hand and foot brake and select 'D'. Make a note of the pressure at idle, then kickdown and note the increased pressure. Repeat the test in '1' at idle and at full throttle. If the boost valve is working correctly, these pressures will be nearly twice as high as those recorded in 'D'. In both tests, if the idle pressure is as high as full throttle pressure, suspect a fractured vacuum pipe or split modulator diaphragm.

Hydraulics Rebuild Before we end this section, we will look at the points to note during the rebuild of the hydraulics. We have already mentioned the importance of cleanliness, but the correct sequencing of valve components into the valve block is also vital. To refit the servo, use the same tool as for dismantling to compress the servo spring and fit the circlip. Be very careful to align the servo and the tool with the bore in the casing, which is not at right angles to the casing face. You will find two dummy studs very useful to align the gasket and valve block and make sure the manual valve operating link rod is fitted correctly, with the longer end into the manual valve.

Test and Fault Diagnosis Static Tests In the final section of this programme, we will look at testing and fault diagnosis. Always carry out a series of static checks before road testing. First check the fluid level in the way we have already described. If the level is very low carry out a thorough inspection for oil leaks, and do not forget the oil cooler and its pipes. Next check the vacuum lines and fittings. A leak here will give incorrect modulator valve operation and delayed upchanges. Then check the inhibitor switch adjustment. Make sure that the engine will only start in the 'P' and 'N' positions, and that the reverse lights only come on in 'R'. Next the selector rod adjustment. Apply the hand and foot brakes, select 'N' and start the engine. Move the gear lever to the 'R' position and check that reverse is engaged. Then slowly move the lever back towards 'N', checking that 'R' disengages just before or exactly when the lever locates the 'N' position. Repeat the procedure, this time moving from 'D' to 'N'. If disengagement is correct; then the selector rod is properly adjusted. Use 18G 677ZC test equipment to carry out a stall test and a pressure test as we have already described.

Kickdown Linkage Adjustment If the kickdown change speeds are incorrect, adjust the cable like this. First slacken the kickdown cable lock nuts. Next turn the cable adjuster locknut until the kickdown lever is just touching the lever stop, arrowed. Then tighten the locknuts. Incidentally, for photographic purposes we have disconnected the modulator vacuum pipe. Make sure that the accelerator pedal can be fully depressed without the cable acting as a throttle stop, then test drive the car again to check that the kickdown changes are now correct. Incidentally the procedure for the Rover 3500 and 2 litre are different, and details are in the Repair Operation Manual. Whenever you test an automatic transmission, make sure you carry out all the tests we have described. You can then refer your findings to the diagnostic data given in the Repair Operation Manual. That completes this part on the GM 180. At first sight fault diagnosis and rectification may appear quite daunting. But in fact most problems are as a result of dirt getting into the hydraulic system, lack of routine maintenance or incorrect adjustments.