Tachometer; Emission Maintenance Indicator Lamp; Instrumentation Diagnosis; Test Equipment - Jeep J-10 Series 1982 Technical & Service Manual

expand. Current flows from the sending unit (connected
to the cylinder block) through the heating coil to the
CVR(B+).
Tachometer
Tachometers are optional instruments available for
CJ vehicles only.
Tachometers are wired in series between the ignition
switch terminal and the ignition coil negative terminal.
The current flow through the coil is turned on and off by
the ignition system and the tachometer integrates the
interruptions. The engine rpm is directly proportional to
the integrated voltage amplitude.
Coolant Temperature Gauge
A coolant temperature gauge is standard equipment
for all vehicles.
All temperature gauges operate on regulated voltage
provided by the constant voltage regulator (CVR). The
temperature gauge system consists of a gauge, a vari
able-resistance sending unit, appropriate wiring and the
CVR.
The gauge pointer is attached to a bimetallic coil that
reacts to temperature changes. A heating coil wrapped
around the bimetallic coil provides heat that causes the
bimetal to expand. Current flows from the sending unit
(connected to the engine) through the heating coil to the
CVR (B +). The sending unit has high resistance at low
engine coolant temperature and very low resistance at
high engine coolant temperature.
Emission Maintenance Indicator Lamp
The emission maintenance indicator lamp is used only
with four-cylinder engine vehicles (CJ) manufactured
for sale in California. This lamp is illuminated at 30,000
miles (48 280 km) to indicate required service for the
oxygen sensor (C4 System). Refer to Maintenance-
Chapter B and Exhaust Systems—Chapter IK for addi
tional information
After performing the service, the emission mainte
nance switch must be reset by turning the reset screw on
the switch body. The switch is located under the hood
between the upper and lower speedometer cables on the
left side of the dash panel. Turn the spring loaded reset
screw approximately 1/4 turn counterclockwise to the
reset detent position.
INSTRUMENTATION DIAGNOSIS
General
Improper operation of electrical gauges or meters can
be usually traced to either faulty electrical wiring conti
nuity (including printed circuit boards), improperly cali
brated components or high resistance caused by loose or
corroded connections.
ENGINE INSTRUMENTATION 1L-3
A common diagnostic procedure is to bypass a sus
pected component, wire, printed circuit, or connection
with a jumper wire. If the gauge or meter functions
normally with the jumper wire installed, the problem
usually is within the bypassed printed circuit, wire,
connection or component.
Test Equipment
Several gauge tests require the use of Universal
Gauge Tester J-24538. This instrument provides a wide
range of variable resistance. If the tester is not avail
able, a suitable substitute can be constructed with an
accurate ohmmeter and a spare fuel gauge sending unit.
(1) Attach one ohmmeter test probe to sending unit
terminal.
(2) Attach other ohmmeter test probe to sending
unit ground wire.
(3) Refer to applicable Sending Unit Resistance
(Ohms) chart for resistance values that apply to gauge
being tested. Charts are included in Specifications. To
calibrate, move float arm and mark appropriate resist
ance values on sending unit case.
(4) Remove ohmmeter probes. Attach jumper wire
to sending unit terminal. Tester is now calibrated and
ready for use.
Printed Circuit Board Test
The following procedure is used to locate suspected
breaks or short circuits in the conducting foil.
(1) Remove instrument cluster from vehicle and re
move all bulbs, gauges and meters. Refer to Instrument
Cluster Replacement for procedure.
CAUTION: When using ohmmeter, disconnect power
from circuit being tested. Othenvise ohmmeter internal
circuits will be destroyed.
(2) Connect one ohmmeter test probe to applicable
pin terminal for circuit to be tested. Trace each circuit
from pin terminal to bulb, gauge or meter terminal in
circuit with other test probe.
NOTE: Set ohmmeterfor the low resistance scale (0 to
10 oh ms) and zero meter pointer.
(3) Test for continuity at each uncoated position in
circuit. Ohmmeter should indicate zero ohms at each
position.
NOTE: When circuit tracing, time can be saved by
starting at the middle of the circuit. This will isolate
circuit defects to one halfof the circuit.
(4) Trace circuit leading away from bulb or gauge
terminal to ground terminal pin or ground screw.
(5) Connect one ohmmeter probe to ground termi
nal pin and other probe to cluster metal case. Ohmmeter
should indicate zero ohms.
(6) Replace printed circuit board if ohmmeter in
dicates other than zero ohms on any test.