TM 9-2320-273-20
PANEL MOUNTED INSTRUMENTS.
5-66. TACHOGRAPH MAINTENANCE (Continued).
A. READING TACHOGRAPH DISK.
TIME
The outer ring of the disk has markings for 24 hours. Any events shown on the disk by
markings happened at the time shown on the outer ring of the disk. Also, each time the
tachograph is opened, a small tic mark is put onto the disk by each stylus. (There is a
stylus for vehicle speed, engine speed, and distance).
SPEED
Next to the outer marks for time there is a band about 3/4” wide. The stylus moving in
this area records vehicle speed at any moment. You can see at a glance if an operator has
been speeding.
RPM
After the speed band, there is a narrower band (about 1/2”). Another stylus moves in
this area recording engine RPM. You can see if the engine has been overrevved. You can
also determine whether the driver warmed up the engine before moving out. To see this,
compare RPM reading with vehicle speed. The disk should record the RPM for the proper
number of minutes before vehicle speed is recorded.
DISTANCE
A third stylus moves in the narrowest band, closest to the disk’s center. This stylus moves
away from the center for 5 miles and then toward the center for 5 miles.
In the disk shown, the vehicle did not move fro midnight to about 5:37 AM. Then the engine was
started, rewed momentarily to about 1800 RPM, then idled at 600 RPM for six or seven minutes, rewed
to 2100 RPM momentarily, then shut down at about 5:46. At 7:10 AM the engine was started again,
idled for 22 minutes. At 7:32 it moved out, attaining a speed of about 28 mph, then stopped and idled
for eight minutes. Then it moved out, accelerated to about 45 mph and did stop-and-go driving for about
32 minutes, accelerating from 0 to about 45 mph three times and traveling about 15 miles, At 8:11 it
stopped, having traveled about 15 miles, it idled for about 48 minutes and then was shut down at about
9 AM.
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