Usage:
784B (5RK) Tractors;
785B (6HK),
789B (7EK),
793B (1HL) Trucks
A correct procedure is used for transmission and oil change refill and startup. There is a possibility of inadvertently damaging the transmission clutch through the use of incorrect transmission refill and startup practices.
At oil change, the filters, tank and both transmission compartments are drained, but only the tank is refilled before startup. When the engine is started, the transmission pressure pump, filter and controls pressurize very quickly followed by the lube system. In addition to refilling the pressure system, approximately 15 gallons of oil disappear into the transmission compartment (sump(s)) (while the truck is at low idle with cold oil). This uses up all the initial fill of oil in the tank and causes the pressure pump to suck air unless the engine is shut-off before the tank drops to that level. If the pressure pump is allowed to suck air after the filter and controls have been pressurized, the air will stall the pump with only 20 psi (approximately) being maintained. It may take several minutes for this air to clear the pump and for normal pressure to be restored, after oil has been added and the pump suction port covered.
Twenty psi is likely to let the number 3 (neutral) clutch slip and lack of lube pressure can allow the number 2 and number 3 clutches to self engage due to a lack of oil in the balance piston chambers. Clutches 1, 5 or 6 can also be damaged if the transmission is shifted into forward or reverse while the pressure is up. Then dropping the pressure to 20 psi will let those clutches slip and be damaged. At 20 psi, the transmission cannot be shifted because 20 psi won't move the rotary actuator.
The possibility of clutch damage may be avoided by following the given procedure:
1. Fill the transmission tank as full as possible. Start the engine and run at low idle just long enough for the tank oil level to drop into the lower sight glass. Then stop the engine.
2. Refill the tank to the top sight glass and repeat the steps until the oil level stays where it should be with cool oil. The transmission shift selector MUST be left in neutral until the system is full.
If the tank is filled with a pressurized "quick fill" system, don't assume it can keep up with pump demand. It probably cannot. The 793B pumps require approximately 29 GPM at 700 RPM.
If only one service person usually completes this operation, use a second person, on one occasion, to time how long it takes for the oil to drop out of the lower sight glass and use that to time the engine shutdown on subsequent occasions.