SHAFT-DRIVE 2520031 - Caterpillar

"hour" meters have no direct correlation with actual clock hours. Mechanical "hour" meters are usually driven by the fuel pump drive and operate at one-half of engine rpm. This device is more correctly called a service meter. It displays service meter units (SMU) because its increments are based only upon engine revolutions. The more total engine revolutions that occur, the greater the SMU reading and the sooner a given service interval is reached. For example, an engine idling at 600 rpm for one hour would show 1/3 the SMU reading that a similar engine running at 1800 rpm for one hour would show.
Present electronic hour meters are solid state devices that display actual clock hours with an accuracy of 2%. The 6T7337 and 9G6762 Meters are examples of current electronic hour meters. These are very reliable meters that operate on 12 or 24 Volts DC. They have a gear wheel in the center of the meter face which indexes every one second. This feature makes it easy to verify that the hour meter is working. The electronic hour meter is usually activated by the oil pressure switch. The article "New Service Meter Kits Used For Service Replacement" in the May 12, 1986 Service Magazine contains more information about part numbers for electronic hour meters.
Electronic hour meters are the standard of the industry for measuring operating hours. They are more reliable and less expensive than an SMU device. Electronic hour meters can be easily mounted in the dash panel where they can be easily seen and conveniently read.