ROD ASSEM. 3G8746 - Caterpillar

1/2All Caterpillar Engines Equipped With Speed Tachometer DrivesTwo new self-powered digital tachometers and a signal generator are now available. They permit the use of digital electronic tachometers on engine installations that do not have batteries. The new 7T4677 Tachometer is the basic self-powered tachometer. The new 8T9872 Tachometer is similar to the 7T4677 Tachometer and it also has a backlighting system for night operation. Both of the new tachometers are powered by the 7T6945 Signal Generator. The signal generator mounts on the tachometer drive in the fuel system and produces an AC signal which powers the new tachometers. The new tachometers determine the engine speed from the frequency of the AC signal produced by the signal generator. The new tachometers and signal generator will work on any engine that has a tachometer drive that turns at 1/2 engine speed. If desired, two tachometers (but no more than two) may be connected in parallel to the same signal generator.
Other similar tachometers (3T2159 and 3T4060) are not self powered. They get their speed signal from a sensor close to the flywheel ring gear. These tachometers require a programming board so that they can be used with different ring gears with different numbers of gear teeth. The new 7T4677 and 8T9872 Tachometers do not need programming boards because their speed signal comes from a signal generator.
The 8T9872 Tachometer requires a 9 to 32 VDC power source to operate its backlighting system. Connect the positive (+) wire from the DC power source to the "BATT +" terminal on the back of the tachometer. See the illustration. Connect the negative (-) wire from the DC power source to the "BATT -" terminal on the back of the tachometer. Use an appropriate switch in the "BATT +" circuit to turn the light in the tachometer off when the engine is not in use.
The signal from the 7T6945 Signal Generator must be connected to the two terminals at the bottom of the back of the 7T4677 or 8T9872 Tachometers. See the illustration. If the distance between the signal generator and the tachometer is more than a few feet, use cable that has a grounded shield around the wires to connect the signal generator to the tachometer. This will help eliminate incorrect operation due to radio frequency interference (RFI) or electromagnetic interference (EMI).
Location of terminals on back of 7T4677 and 8T9872 Tachometers.