TM 9-1829A, April 1944
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2021-07-07
14. Calibration
a.
Check pointer position:
At this time pointer must already be assembled to staff so that it comes to rest at
exactly zero or “no bank” as it is often called. To check this “no bank” position, bend pointer stop arm so that
pointer can go two or three miles below zero, then hold speedometer in same position as when it is installed
in the vehicle, and rotate it to right and then to left, enough to move pointer two or three miles off zero
position. Hold speedometer still; if pointer does not come to rest at exactly zero, remove pointer (par. 11 c)
and install in correct position (par. 13 k). Be sure this adjustment is correct before proceeding.
b.
Magnetizing:
(1)
Connect the magnetizer (846294) to a 12-volt direct current source. Clamp the magnetizer poles over the
speedometer field plate and press the magnetizer button firmly for two or three seconds.
(2)
With the speedometer mounted in the calibrating machine (T-170645) run the calibrating machine at
1,000 revolutions per minute (60 miles per hour) and the speedometer should indicate at least 70 miles
per hour. If it does not indicate this speed, the magnet is insufficiently magnetized, and the magnetizing
procedure (par. 14 b (1) above) should be repeated. It may be necessary to connect the magnetizer to a
24-volt source of direct current. On SPA-type speedometers, it may be necessary to remove odometer
lock plate to ensure sufficient magnetization and demagnetization.
Note: Watches and any other
electrical instruments which may be affected by magnetism should be kept at least 48 inches from
magnetizer during this procedure.
Figure 35: Charging magnet, using Magnetizer (846294)
c.
Demagnetizing and calibration
(chapter 13):
(1)
When the speedometer is magnetized so that a speed of at least 70 miles per hour is indicated when
master head indicates 1,000 revolutions per minute (60 miles per hour), then it should be demagnetized
down to an indication in accordance with the specifications (par. 14 d). With the magnetizer connected
to 12-volt direct current source, demagnetize by holding one pole of the magnetizer near (not touching)
the speedometer field plate, and pressing magnetizer button (with the calibrating machine running at 60
miles per hour). It may be well to hold one finger between the field plate and the pole presented to it to
prevent the pole from being pulled in contact with the field plate. Take care that the speedometer is not
demagnetized below the desired speed. If speedometer is demagnetized below this speed it is necessary
to again magnetize and demagnetize as outlined above.
(2)
Run the calibrating machine at 30 miles per hour and note speedometer indication. Turn the speedometer
180 degrees by twisting the calibrating swivel joint connection, and check the speed indication. If all
parts in the speedometer are balanced, the speed indication will not change. If the speedometer reads fast
in new position, correct by placing a small amount of white lead in the balancing cup on top of the speed
cup 180 degrees from the pointer. If there is no balancing cup, place white lead directly on speed cup,
being sure to place it where it will not interfere with free operation. Note: White lead is used because it
hardens by oxidization without losing weight.
(3)
Recheck speedometer reading in both positions, and also by varying only 90 degrees from original
position. When correctly balanced, speedometer indication will not vary from that shown in original
position.