Manual Transmission Service
A standard transmission under normal driving conditions should last for the entire life of the vehicle without disassembly. Failure of transmission generally results from maladjustment of linkage, overloading, or from forcing a gearshift. Linkages that are not adjusted correctly may cause the transmission to slip out of gear or cause hard shifting. Transmission over-loading can result from sudden clutch engagement with the engine running at high speeds and the vehicle at rest. The inertia of the crankshaft, flywheel, and clutch can put very high instantaneous torque loads on the transmission gears, which can break the teeth and, in some cases, even the transmission case. Forced shifting can damage the synchronizer blocking rings and pins. When this happens, shifting becomes difficult and the transmission may slip out of gear when it is under load. These conditions can only be repaired by completely disassembling the transmission.Abnormal transmission noise usually indicates a problem, requiring disassembly. It may be caused by excessive counter gear end play, a loose synchronizer hub, damaged or worn gears, or rough or pitted bearings. Faulty parts must be replaced to correct the problem. The general disassembly procedures are similar for all standard transmissions, with the exception of some details, which are different. A recommended service manual should be followed when disassembling a transmission. Before removing the transmission lubricant is to be drained, and the drive shaft speedometer cable and shifting rods are to be disconnected. With the engine supported, the rear mount, cross member and then the four attachment bolts are removed. The transmission is then free and is carefully pulled straight back until the clutch shaft is free of the bell housing.
Fig. 25.30. Dynamic spiral-thread seal action.
Fig. 25.31. Dynamic lip-seal action.
Transmission disassembly starts with the removal of the cover plate, which gives access to the transmission interior. Most transmissions can not be disassembled with the counter gear in place, hence it is dismantled first. The loosened counter gear is allowed to rest in the bottom of the case while the input shaft and main shaft are removed. These two shafts are separated at the main shaft front roller bearings located at the back of the clutch shaft. Gears and synchronizer drums are held on the main shaft with retaining rings so that they do not fall off. The extension shaft and main shaft are then removed. Gears, synchronizers, and bearings are removed from the main shaft by removing the retaining rings.
Generally, all needle bearings, thrust washers, gaskets and seals are replaced with new parts. Rough operating ball bearings are also replaced. Any gear or synchronizer with a chipped or damaged tooth is also replaced. All the parts should be checked for condition and correct fit before reassembly. The parts should be thoroughly cleaned and lubricated with transmission lubricant during assembly. The procedure of assembly is the reverse of the disassembly procedure. After the transmission is installed it is filled with the proper grade transmission oil up to the level before it is operated.