TACHOMETER
DESCRIPTION
A tachometer is standard equipment on all instrument
clusters. The tachometer is located to the right side of
the instrument cluster, just right of the speedometer.
The tachometer consists of a movable gauge needle
or pointer controlled by the instrument cluster circuitry
and a fixed 210 degree scale on the gauge dial face that reads left-to-right from “0” to “7”. The text “RPM X 1000”
imprinted on the cluster overlay directly above the hub of the tachometer needle identifies that each number on the
tachometer scale is to be multiplied by 1000 rpm.
The tachometer graphics are black against a white field, making them clearly visible within the instrument cluster in
daylight. When illuminated from behind by the panel lamps dimmer controlled cluster illumination lighting with the
exterior lamps turned On, the white gauge dial face appears blue-green with the black graphics silhouetted against
the illuminated background and the red gauge needle still appears red. Gauge illumination is provided by an integral
electro-luminescent lamp that is serviced as a unit with the instrument cluster. The tachometer is serviced as a unit
with the instrument cluster.
OPERATION
The tachometer gives an indication to the vehicle operator of the engine speed. This gauge is controlled by the
instrument cluster circuit board based upon cluster programming and electronic messages received by the cluster
from the Powertrain Control Module (PCM) over the Controller Area Network (CAN) data bus.
The tachometer is an air core magnetic unit that receives battery current on the instrument cluster electronic circuit
board through the fused ignition switch output (run-start) circuit whenever the ignition switch is in the On or Start
positions. The cluster is programmed to move the gauge needle back to the low end of the scale after the ignition
switch is turned to the Off position. The instrument cluster circuitry controls the gauge needle position and provides
the following features:
•
Engine Speed Message - Each time the cluster receives an engine speed message from the PCM it will
calculate the correct engine speed reading and position the gauge needle at that relative speed position on the
gauge scale. The cluster will receive a new engine speed message and reposition the gauge pointer accord-
ingly about every 88 milliseconds. The gauge needle will continually be repositioned at the relative engine
speed position on the gauge scale until the engine stops running, or until the ignition switch is turned to the Off
position, whichever occurs first.
•
Communication Error - If the cluster fails to receive an engine speed message, it will hold the gauge needle
at the last indication for about three seconds, or until the ignition switch is turned to the Off position, whichever
occurs first. After three seconds, the gauge needle will return to the left end of the gauge scale.
•
Actuator Test - Each time the cluster is put through the actuator test, the tachometer needle will be swept to
several calibration points on the gauge scale in a prescribed sequence in order to confirm the functionality of
the gauge and the cluster control circuitry.
The PCM continually monitors the crankshaft position sensor to determine the engine speed. The PCM then sends
the proper engine speed messages to the instrument cluster. For further diagnosis of the tachometer or the instru-
ment cluster circuitry that controls the gauge, (Refer to 8 - ELECTRICAL/INSTRUMENT CLUSTER - DIAGNOSIS
AND TESTING).
For proper diagnosis of the crankshaft position sensor, the PCM, the CAN data bus, or the electronic message
inputs to the instrument cluster that control the tachometer, a diagnostic scan tool is required. Refer to the appro-
priate diagnostic information.
8J - 74
CLUSTER
ND