Issue link: http://read.dmtmag.com/i/44411
STANDARD VS. AUTOMATIC BELT TENSIONER SYSTEMS When purchasing a new or used truck, you will have to choose between a standard, adjustable accessory mounting system and an automatic tensioner system. Traditionally, belts were adjusted as an accessory was rotated, typically with the square head of a socket drive. The accessory, such as an alternator, would be mounted on a hinge at the bot- tom and a slotted lever at the top. Once the hinge bolt and upper bolt were loosened, the belts could be properly tensioned. FIGURE 2 An improvement to this system came with "pad-mounted" accessories, where the alternator, power steering pump or AC compressor was bolted at four points and held tightly to the block, killing damaging vibrations. Pad mounts definitely trump a mount where the accessory rotates to adjust the belt, so smart operators always look for pad mounting systems. The next development was the automatic tensioner, a natural outgrowth of pad mounting systems that called for some sort of movable tensioning pulley. Automatic tensioners use "ser- pentine" type belts rather than traditional v-belts. They are flat on the outside with grooves for friction on the inside. The tensioner typically works by putting pressure on the outside of the belt and normally has a wheel with a flat outer surface. FIGURE 3 An automatic tensioner is normally preferable to a standard, traditional belt tensioning system. New belts installed to the right tension and then checked shortly after and readjusted as neces- sary will normally run for some time and need only an occasional inspection and adjustment. However, the automatic tensioner just about eliminates such maintenance, as it will adjust to compensate for all but the most extreme level of wear, meaning a nearly maintenance-free belt system for the life of a belt. The only disadvantage of the automatic tensioner systems is that the tensioner will eventually wear out. While normally simple to replace, operators need to know how to inspect them and can expect an associated parts cost ($100-$200) that does not apply to the traditional, simple sys- tems unless vibration damages one of the brackets. FIGURE 4 Recheck any new belt after 3,000 miles. TO CHECK TENSION WITH AN AUTOMATIC TENSIONER Start the engine, raise the hood and get as close as you safely can to the belt tensioner. Any howl- ing sound could mean the device's roller bearing is worn and should be replaced. Shut the engine down, and remove the keys from the cab. 1 2 3 Depress the outside of the belt with your thumb as shown above. If there's a problem, you'll feel a significant loss of tension. There's a tension marker line on the tensioner and two more on the block. Make sure the single line stays between the other two. Note which way the tensioner FIGURE 5 rotates to tighten the belt. In the setup shown (figure 4), it would be counter-clockwise. 4 To replace the belt, first note the routing of the belt around the crankshaft, various accessories and the tensioner (figure 5). Draw a diagram if the routing is com- plicated. Use the socket drive to rotate the tensioner away from the backside of the belt, to reduce ten- sion to the point where the belt can 5 Rotate the tensioner away from the back of the belt using a square socket drive and long handle, unhook the belt, and then slowly rotate the device back and forth between its zero tension position and the maximum. If you feel any roughness, the device's pivot bearing is worn and should be replaced. FIGURE 6 be pulled out of the groove in the tensioner's pulley, and pull it out. Then, rotate the socket drive so the tensioner will return to its neutral position. Finally, remove the belt from around the remaining pulleys. 6 To put in the new belt, first route the belt around the crankshaft and accessory pullies. Then rotate the tensioner in the direction away from the normal location of the belt, and route the belt next to the tensioner pul- ley (figure 6). Finally, allow the socket drive to rotate to allow the tensioner to restore tension and then remove it from the tensioner. Check you've gotten the correct belt by ensuring that the mark on the tensioner is between the two on the block. NOVEMBER 2011 TRUCKERS NEWS 29