How to Shift a Mack Truck Transmissionby K.K. Lowell
Mack trucks use transmissions made by several different manufacturers so truck buyers can more match the truck to the job. Trucks used by a specialized hauler of very heavy equipment, need a variety of forward speeds to help keep the engine RPM in the power band. The general technique to shift any transmission is similar, so learning to shift any of them will enable the driver to learn the basics of shifting others.
Start the engine and allow the engine to warm while the air pressure builds to operating levels.
Make sure the range selector is in the "Low" position. Attempting to drive off with a transmission in "High" range can damage the transmission or the drive-train.
Depress the clutch pedal fully and shift the transmission into the first gear position. This position will be denoted by a "1" on the gear chart or on the shift lever decal which may be on the top of the shift lever knob. This is not "Low" or "Low Low". These gear positions are only used to start off with extremely heavy loads.
Hold the brake pedal down and push in the parking brake release knob. When the parking brake is fully released slowly release the brake pedal and the clutch pedal simultaneously. Do not press down on the throttle pedal. The engine has enough torque to start the truck moving without any increase in the throttle position as long as the clutch is not released too fast. It will not be necessary to feather the clutch, releasing it should be a fluid motion. Start pressing the throttle to increase the engine speed as the truck starts to move.
Watch the tachometer and listen to the engine as the truck starts moving. When the engine speed has reached approximately 2,000 RPM, release the throttle while pressing in on the clutch. Do not press the pedal any further than is necessary to disengage the clutch.
Move the shift lever to the neutral position and release the clutch. Immediately press the clutch pedal down again while moving the shift lever to second gear. Don't allow the engine RPM to drop below 1,300 rpm.
Repeat the steps to shift the transmission through fourth gear. Before shifting out of fourth gear raise the range button to the "High" position. After raising the range button shift back to the "First" gear position using the same double clutching technique. This is now fifth gear.
Shift the transmission through the remaining gears while always keeping an eye on the tachometer to maintain the proper operating speed range.
Save wear on the truck brakes and maintain better control by downshifting through the gears when slowing or stopping. Maintain the RPM in the same range as when up-shifting.
- These trucks are very heavy. Driving a large truck in traffic is a very responsible position. Do not attempt to drive anywhere but an empty parking lot until you are very familiar with the truck and comfortable with your abilities. Pressing the clutch pedal down too far causes the clutch brake to be applied, which will be rapidly destroyed on a moving vehicle. Do not exceed 2100 RPM at anytime.
K.K. Lowell is a freelance writer who has been writing professionally since June 2008, with articles appearing on various websites. A mechanic and truck driver for more than 40 years, Lowell is able to write knowledgeably on many automotive and mechanical subjects. He is currently pursuing a degree in English.