SPEEDOMETER AND TACHOMETER SPEEDOMETER

Figure 11 Removing Bulb and Bulb Socket From Rear of Cluster

TM-5-3820-256-24-4 Drilling System Well Rotary Truck Mounted Air Transportable 600 Feet Capacity Model LP-12 NSN 3820-01-246-4276 Manual
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S-SERIES INSTRUMENTS SPEEDOMETER AND TACHOMETER SPEEDOMETER The speedometer used on this vehicle indicates miles and kilometers per hour. Distance traveled is recorded (in miles*) on an odometer. Speedometer is driven by a semi-rigid cable connected to a set of gears in vehicle transmission. Specified gears match the particular truck model and take into consideration such other factors as tire size and rear axle ratio. The semi-rigid cable which connects transmission driven gear to the speedometer head consists of an outer case and an inner core. Odometer records up to 999,999 miles* and returns to zero. School bus models record 99,999.9 miles.* (*Total metric speedometers record kilometers.) Speedometer Adapter Should the need arise for changing a truck axle ratio or tire size, a corresponding change must also be made to the speedometer drive. This is accomplished by changing the speedometer adapter and in some instances the driven gear in the truck transmission. Information for finding the correct adapter and driven gear can be found in the Speedometer Section of the Parts Catalog. TACHOMETER An electrical or mechanical tachometer (not standard for all vehicles) is also located in the instrument cluster to record engine RPM (Revolutions Per Minute). This instrument enables driver to keep engine speed within an efficient operating range. The mechanical tachometer is driven by a flexible shaft connected to an adapter or driving unit located on engine (distributor shaft for gasoline engines or air compressor injection pump, auxiliary shaft, etc. for diesel engines). The electric tachometer reads the same as the mechanical unit but receives electrical impulses by wire instead of rotating cable. These impulses are received from a sender unit mounted on the engine and using the mechanical tachometer drive take off. SPEEDOMETER AND TACHOMETER CABLES A semi-rigid type cable is used for both speedometer and mechanical tachometer on S-Series trucks. This cable consists of two segments: (1) An outer case assembly with nut and ferrule at lower end and a quick-disconnect ferrule at upper end. (2) A wire-wound flexible inner core assembly with squared drive at upper end and floating tip (Figure 14) at lower end. The floating tip which is detachable from cable allows core to float and thereby minimize thrust on speedometer or tachometer head. (Speedometer cable assemblies are sometimes made in two sections for assembly purposes.) Figure 15 - Semi - Rigid Cable Outer Case The cable outer case is formed from an inner nylon liner, an intermediate ply of wire braid and an outer polypropylene or polyethylene case (Figure 15). Figure 14 Semi-Rigid Cable End Details CTS-4140Y Page 6

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