Automatic Transmission Service (Automobile)


Automatic Transmission Service

Faults and Adjustments.

All manufacturers provide suitable test procedures for the diagnosis of a wide range of faults. Some simple faults are listed in Table 25.4. Regular servicing normally prevents problems enclosed in Table 25.3.

Table 25.4. Gearbox faults.

Area Cause Fault
Fluid Low level Starvation of the pump causes improper operation of the hydraulic system so that faults ranging from non-engagement of gears to erratic operation may arise.
High level Governor strikes oil surface, so speed changes are affected.
Poor quality Rough gear changes. If oil is contaminated with grit particles, then a valve may stick causing problem in the part of the system controlled by the valve.
Selector lever Incorrect adjustment Some gears, particularly those at the end of the travel, do not select. Lever should be adjusted when it is at mid-point of selector range.
Throttle cable Incorrect adjustment Road speed at which gear changes take place becomes either too high or too low.
Starter inhibitor Incorrect adjustment Starter becomes active in positions other than P and N.

Test Sequence. If a particular fault is not identified from the Table 25.4, then a test programme with the following sequence is carried out.
(i) Road Test. This test should follow the procedure suggested by the manufacturer.
(ii) Stall Test. This test checks the torque converter for stator slip and the brake bands and clutches for slip.
(Hi) Pressure Test. Pressure take-off points are provided to check the pressure in the lines under given conditions using a pressure gauge.
After a specified running period of the vehicle, the transmission should have a routine service, consisting of an oil change, band adjustment, and visual inspection. A pressure test is conducted to ascertain if the correct fluid pressure exists at the proper time in the shift cycles. When the fluid level is correct, low pressure can be caused by leaking seals, a clogged filter, low pump output, or a faulty regulator valve. Pressure that builds up at the wrong time or to the wrong valves is the result of leaking seals or sticking valves.
For carrying out minor transmission services, the valve body is made accessible by draining the transmission fluid and removing transmission pan. With linkage control disconnected, the entire valve body can be removed from the transmission. If the spool control valves in the valve body are faulty, they can be serviced without further transmission disassembly. The band servos are also accessible with the valve body removed. They can be adjusted without further transmission disassembly if improper adjustment is the cause of the problem. In most automobile applications, the rear transmission extension housing can be removed without pulling the transmission from the automobile. The propeller shaft is removed. The transmission must be supported and the rear mount removed. In some cases, the frame cross member is required to be removed. The extension housing flange bolts are removed. The extension housing can then be slid back of the transmission output shaft. This exposes the governor and parking brake pawl. Repairs are made and new seal is installed with a new flange gasket between it and the transmission case.

Transmission Removal.

If the problem is not in the valve body, servos or extension housing, the transmission is required to be removed to attend repair. After the transmission fluid is drained, the selector linkage, throttle linkage, speedometer cable, vacuum hoses, and electrical wiring are discon­nected from the transmission.
The propeller shaft is removed and the rear mount disconnected. The engine is supported by a fixture. The frame cross member with the mount is removed for an access. The splash plate is removed from the lower front of the transmission bell housing after marking their relative positions, to expose the attaching bolts between the drive plate and the torque converter housing. The torque converter is disconnected from the drive plate. A transmission jack is positioned and then the bell housing flange bolts are removed. The bell housing is carefully separated slightly from the engine, and then the torque converter is pushed away from the drive plate. When the torque converter front hub is clear of the crankshaft pilot, the transmission can be carefully lowered from the automobile and placed on a bench for further service.
In case the front seal leakage is the problem, the torque converter is removed to expose the front seal in the transmission pump. The seal is removed with the seal puller and the new seal is installed using a seal driver. If no further problem is there, then the transmission is reinstalled following a procedure, reverse of the removal procedure.

Transmission Overhaul.

There is a lot of work involved in the removal of the transmission so it is usual practice to overhaul the transmission while attending major faults. The general procedure used during transmission overhaul is to disassemble the transmission into major subassemblies. The major subassemblies are then individually disassembled, cleaned, inspected, repaired, sealed, and reassembled. When all the subassemblies are overhauled, the transmission is reassembled. In this way there are minimum number of parts to keep track of, and so there is less chance of improper assembly.
Disassembly starts with the removal of the pan and the valve body. This is followed by the removal of accumulators, servos, and regulators depending upon the type of transmission. Externally removable modulators and governors are than removed. The extension housing is removed next, which exposes the speedometer drive gear, parking pawl, and governors. These are removed as sub-assembly. The transmission pump is unbolted and is removed using slide hammer puller. The stator shaft is part of the pump housing. After the pump is removed, the turbine shaft is exposed, which is part of the front drum subassembly and this can be lifted from the transmission. Bands, clutches, discs, and drum assemblies are lifted from the transmission in the proper sequence. Transmission overhaul continues with the overhaul of each subassemb­ly. All the parts in each subassembly is thoroughly cleaned, dried, and inspected for signs of excessive wear, scoring, overheating, chipping or cracks and required parts are replaced.

Transmission Assembly.

When all of the subassemblies are overhauled and all required parts are at hand, the transmission can be reassembled. Before reassemble, the case should be thoroughly cleaned and all passages blown out. All parts are coated with the proper type of automatic transmission fluid as they are being assembled. Bands should be soaked in the fluid for 20 minutes before they are installed. Transmission installation is best done by supporting the transmission in the vertical position with the bell housing up. The weight of the parts hold them in place and they stay centered until the pump is secured in front of the case.
Parts are lowered carefully into the case in the proper order making sure that the clutch plates and band linkages are properly engaged. Thrust washers are thoroughly lubricated as they are installed. Sealing rings should be centred on their shafts so they slide into their bores without breaking. All assembly bolts should be coated with automatic transmission fluid to prevent galling and seizing, especially as they are turned into the threaded holes in the aluminium case. After completion of the assembly, it is filled with the correct type of automatic fluid and is examined for leaks. The automobile is road tested after installation of the transmis­sion.

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