TM 9-1829A, April 1944
13/100
2021-07-07
Figure 14: Testing for tight mechanism
c.
Test hair spring and magnet:
If the instrument mechanism turns freely, an initial fast spin of the short test
cable should swing the pointer over from zero to about half scale register. From half scale the pointer should
quickly return to zero. This indicates reasonable hair spring and magnet performance.
d.
Test for calibration:
Providing the instrument is not “bound up” (test b), place on the calibrating machine
(T-170645) and run at various speeds. Note instrument readings obtained as compared with the master head
(when checking tachometers it is necessary to know ratio of speed indication to shaft speed. This ratio is
sometimes stamped on the case. A speedometer or tachometer may be checked on the vehicle by using the
special flexible shafts supplied with the calibrating machine). The cause and remedies of any irregularities
can be found by referring to subparagraph g following. Refer to chapter 13 for further instructions.
e.
Test odometer:
While checking the unit on the calibrating machine, note if the odometer is functioning. If it
is not, the unit must be disassembled and repaired.
f.
If instrument passes tests b, c, d, and e above without showing defects, then it is in proper working order, and
the trouble must be in flexible shaft, drive gears, or fittings on vehicle.
g.
Probable causes and remedies of defective Instruments:
Following is a list of common troubles some of
which may not apply to all makes of instruments. For complete information on each make instrument refer
to applicable chapter.
(1)
Pointer fluctuates or wavers.
a)
Broken, worn, dry, or dirty front or rear jewel:
Replace, clean, or lubricate jewels according to
manufacturer’s recommendations.
b)
Excessive end play in magnet shaft:
Reflare collar on end of magnet shaft.
c)
Dirt, grease, or foreign matter on magnet and speed cup:
Clean speed cup and magnet with cloth.
Care must be taken to prevent damage to the speed cup and hair spring.
d)
Speed cup staff rusted or corroded at jewels:
Clean ends of staff. If too badly rusted or corroded,
discard.
e)
Worn main frame magnet shaft bearing:
Replace main frame or magnet assembly as required.
f)
Rear jewel out of line:
Line up rear jewel bracket with main frame. If bracket is bent, discard.
g)
Bent speed cup staff:
Discard and replace.
h)
Field plate not positioned properly:
Relocate field plate. Clearance between speed cup and field
plate must be uniform.
(2)
Inadequate zero bank (pointer does not return to zero).
a)
Weak, broken, or improperly adjusted hair spring:
Replace or adjust hair spring, or replace hair
spring and speed cup assembly (as specified by manufacturer).
b)
Front jewel too tight:
If there is insufficient play in speed cup staff, adjust upper jewel.
c)
Dirt or grease in mechanism:
Disassemble and clean.
d)
Pointer improperly set:
Remove pointer and reset at zero mark on face dial.
e)
Broken or distorted hair spring regulator:
Replace regulator.
(3)
Incorrect speed indication.
a)
Dirty or grease-filled mechanism:
Disassemble and clean.
b)
Out of calibration:
Recalibrate.
(4)
Instrument shows correct speed indication over approximately one-half the dial, and incorrect
reading over balance of dial.
a)
Hair spring coils touching:
True hair spring with tweezers.