FOUR-SPEED TRUCK TRANSMISSION The gear shift lever positions shown in the small inset in figure 13-6 are typical of most four-speed truck transmissions. The gear shifting lever, shown in A, B, C, D, and E of the figure, moves the position of the two shifting forks that slide on separate shafts secured in the transmission case cover. Follow the separate diagrams to learn what takes place in shifting from one speed to another. For example, as you move the top of the gear shift toward the forward left position, the lower arm of the lever moves in the opposite direction to shift the gears. The fulcrum of this lever is in the transmission cover. Shifting transmission gears requires the use of the clutch to disengage the engine. Improper use of the clutch will cause the gears to clash and may damage them by breaking the gear teeth. A broken tooth or piece of metal can wedge itself between two moving gears and ruin the entire transmission assembly. When you shift from neutral to first, or low, speed (fig. 13-6, A), the smallest countershaft gear engages with the large sliding gear. Low gear moves the truck at its lowest speed and maximum power. The arrows show the flow of power from the clutch shaft to the propeller shaft. The second-speed position is obtained by moving the gear shift lever straight back from the low-speed position.  You  will,  of  course,  use  the  clutch  when shifting. In figure 13-6, B, you will see that the next to the smallest countershaft gear is in mesh with the second largest sliding gear. The largest sliding gear (shift gear) has  been  disengaged,  The  flow  of  power  has  been changed as shown by the arrow. The power transmitted to the wheels in second gear (speed) is less, but the truck will move at a greater speed than it will in low gear if the engine speed is kept the same. In shifting from the second-speed to the third-speed position, you move the gear shift lever through the neutral position. You must do that in all selective gear transmissions. From the neutral position the driver can select the speed position required to get the power needed. In figure 13-6, C, notice that the gear shift lever is in contact with the other shifting fork and that the forward   sliding   gear   meshes   with   the   second countershaft   gear.   The   power   flow   through   the transmission has again been changed, as indicated by the arrow, and the truck will move at an intermediate speed  between  second  and  high. You shift into the fourth, or high-speed, position by moving the top of the shift lever back and to the right from the neutral position. In the high-speed position, the forward shift or sliding gear is engaged with the constant speed gear as shown in figure 13-6, D. The clutch shaft and the transmission shaft are now locked together, and the power flow is in a straight line. In high, the truck propeller shaft revolves at the same speed as the engine crankshaft, or at a 1 to 1 ratio. You shift to reverse by moving the top of the gear shift lever to the far right and then to the rear. Most trucks have a trigger arrangement at the gear shift ball to unlock the lever so that it can be moved from neutral to the far right. The lock prevents unintentional shifts into reverse. Never try to shift into reverse until the forward  motion  of  the  vehicle  has  been  completely stopped. In figure 13-6, F, you can see how the idler gear fits into the transmission gear train. In figure 13-6, E, you can see what happens when you shift into reverse. An additional shifting fork is contacted by the shift lever in the far right position. When you shift to reverse, this fork moves  the  idling  gear  into  mesh  with  the  small countershaft gear and the large sliding gear at the same time. The small arrows in the inset show how the engine power  flows  through  the  transmission  to  move  the propeller shaft and the wheels in a reverse direction. The   different   combination   of   gears   in   the transmission  case  makes  it  possible  to  change  the vehicle speed while the engine speed remains the same. It is all a matter of gear ratios. That is, having large gears drive small gears, and having small gears drive large gears. If a gear with 100 teeth drives a gear with 25 teeth, the small gear will travel four times as fast as the large one. You have stepped up the speed. Now, let the small gear drive the large gear, and the large gear will make one revolution for every four of the small gear. You have reduced speed, and the ratio of gear reduction is 4 to 1. In the truck transmission just described, the gear reduction in low gear is 7 to 1 from the engine to the propeller shaft. In high gear the ratio is 1 to 1, and the propeller shaft turns at the same speed as the engine. This  principle  holds  true  for  most  transmissions.  The second-  and  third-speed  positions  provide  intermediate gear reductions between low and high. The gear ratio in second speed is 3.48 to 1, and in third is 1.71 to 1. The gear reduction or gear ratio in reverse is about the same as it is in low gear, and the propeller shaft makes one revolution for every seven revolutions of the engine. 13-6