https://www.truck-manuals.net/mack/ OPERATION 195 MACK T31821, T318L21 and T318LR21 Shifting Instructions
CAUTION Make sure air pressure is at least 90 psi and unit is at operating temperature before making range shifts. Always start in LO range. This also applies to dynamometer testing. When the truck is stationary, do NOT shift into HI range and then start moving the truck. Otherwise, damage to the synchronizer may result. Note: When making a SPLIT SHIFT from gear to gear, the splitter switch must not be actuated, either up or down, until the main box is in NEUTRAL. When making a SPLIT SHIFT in the same gear to a preselected gear, especially under a heavy load, the splitter will not shift until the driveline torque is relieved. This can be done by depressing the clutch pedal and releasing it, or by backing off the throttle and reapplying it. The splitter switch must not be actuated, either up or down, until IMMEDIATELY before the driveline torque is relieved. When making a RANGE SHIFT, it is important to preselect the range shift. This means that the range selector must be moved to its next position (up or down) BEFORE the gearshift lever is moved. Then move the gearshift lever through NEUTRAL to the next desired gear position. As the lever passes through NEUTRAL, the range shift will be completed. Upshift — For purposes of explaining the shifting procedures through ALL gears, assume that the vehicle has a full load and will climb a slight grade. To prepare for moving the vehicle, position the shift lever in NEUTRAL, select LO-split using the splitter switch, and select LO range using the range selector. Refer to the shift pattern and move the shift lever to LO speed gear (LO range, LO-split). Apply accelerator and engage the clutch, then accelerate to governed speed. Select HI on the splitter switch (preselect) and release the accelerator pedal long enough for the split shift to complete. Reapply the accelerator. Shift through first LO-split, first HI-split, second LO-split, second HI-split and so on, until you reach fourth HI-split. Remember to double-clutch between gears. To continue while in fourth gear HI-split, flip the range selector up to HI range (preselect), and as you move the shift lever through NEUTRAL to fifth gear, move the splitter switch from HI to LO. As the lever moves through NEUTRAL, the range shift to HI will be completed. You are now in fifth gear LO-split (HI range). Continue shifting to fifth HI-split, sixth LO-split, sixth HI-split and so on, until you reach eighth HI-split. Remember to double-clutch between gears.
