
TM 9-1829A, April 1944
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2021-07-07
Chapter 1: Introduction
1. Purpose and scope
a.
The instructions contained in this manual are for the information and guidance of personnel charged with the
repair and rebuilding of speedometers, tachometers, recorders, and related drive equipment. These
instructions are supplementary to those in the field manuals and technical manuals prepared for the using
arms. This manual does not contain information which is intended primarily for the using arms, since such
information is available to ordnance maintenance personnel in 100-series TM’s or FM’s.
b.
This manual contains a description of, and procedure for testing, disassembly, cleaning, inspection, and
assembly of all makes of speedometers, tachometers, recorders (including hourmeters), flexible shafts, and
drive joints.
c.
For instructions for removal and replacement of instruments, etc., on vehicles, refer to pertinent operator’s
manual 100-series TM’s.
Chapter 2: Magnetic-type speedometers and tachometers; description, operation
identification and testing
2. Description and operation
a.
Description:
All makes of magnetic-type speedometers and tachometers are similar in outward appearance,
having a case, bezel, glass, pointer, and face dial (where the bezel and glass are assembled in the instrument
panel, these parts are not attached to the speedometer itself).
(1)
Speedometer:
A speedometer (fig. 1) is used to indicate vehicle speed in miles per hour as shown by the
pointer and face dial, and to record distance traveled by means of an odometer. A speedometer is driven
through a flexible shaft connected to a set of gears in the vehicle transmission (fig. 2). These gears are
designed for the particular vehicle model and take into consideration the tire size and rear axle ratio. The
flexible shaft, which connects the transmission driven gear to the speedometer, consists of an outer casing
and the inner drive core (fig. 3). The total odometer usually records up to 99’999 miles before it
automatically returns to zero. The trip odometer usually registers up to 999.9 miles, but by means of the
trip reset it can be reset to zero or any intermediate figure desired. Some speedometers are not equipped
with a trip odometer. These units have an extra wheel or tenth dial at the right of the total odometer.
Figure 1: Speedometer
(2)
Tachometer:
A magnetic-type tachometer (fig. 4) is similar to a speedometer, except that the face dial
indicates in revolutions per minute (instead of miles per hour) and is ordinarily used to indicate vehicle
engine speed. A tachometer is driven through a flexible shaft which is usually connected to the vehicle
generator shaft, crankshaft, or distributor shaft by means of a drive joint or adapter. An odometer is often
incorporated in a tachometer to record total revolutions.