Gauge Diagnosis Details - GMC 1976 ZEO 6083 Maintenance Manual

12- 10 CHASSIS ELECTRICAL
INSTRUMENT CLUSTER
LEGEND CIRCUIT CONNECTOR
A
C
D
E
F
G
H
ILLUMINATION LIPS .
FUEL GAGE SENDER
OIL PRESS . GAGE SENDER
BRAKE -TELLTALE
IGNITION
GEN .-TELL TALE
TEMPERATURE GAGE SENDER
PRINTED CIRCUIT
CONNECTOR RECEPTACLE
Figure 16-Gauge Cluster Printed Circuit
FUEL GAUGE
When checking the fuel gauge circuit, first deter-
mine whether the tank units, wiring, or fuel gauge is
faulty . To check fuel gauge wiring, refer to Figure 19
for appropriate wiring harness connections. Connec-
tion at the gauge head is made through a printed
circuit located at the rear of the gauge head .
The fuel tank selector switch position determines
which tank level is being measured . When the fuel
selector switch is moved to the "FUEL AUX" posi
tion, a coil in the fuel selector valve (figure 17) is
energized . The energized coil closes off the fuel sup-
ply from the main fuel tank to the carburetor, and
opens the fuel supply from the auxiliary tank to the
carburetor. Simultaneously, the main fuel tank send-
ing unit is switched to the auxiliary tank sending
unit.
CAUTION :
When checking fuel tank gauge
wiring, never apply 12-volt current directly
to the fuel tank float unit body feed wire as
this will destroy the units'resistive element.
The fuel gauge shows the approximate fuel level
in the main tank when fuel selector switch is in the
"FUEL MAIN" position, and the fuel level in the
auxiliary tank when fuel selector switch is in the
"FUEL AUX" position . The pointer will indicate
the correct levels, only when the ignition is in the
"ON" position . Since both fuel tanks are intercon-
nected the gauge is designed to read the same (with
the switch in either position) until approximately
60% of the total fuel capacity has been used .
If a condition is present where the vehicle is run
in the "MAIN" position and allowed to deplete fuel
supply on gauge, then switching to "AUX" does not
provide the reserve 7 to 9 gallon amount, it is possi-
ble the wiring connector (figure 17) near the frame
rail and tanks has the wires crossed in connection.
The wires in this connection should be color to color
from each side of the connector. This condition can
also be detected by running the vehicle with the
switch in the "AUX" position, ---- the fuel supply is
Figure 17-Fuel Selector Valve
FUEL GAUGE SYSTEM DIAGNOSIS
ERRATIC FUEL GAUGE READINGS
almost depleted. Switching to the "MAIN" position
provides a gauge reading of a greater amount.
Changing the fuel selector switch position
changes the fuel selector valve and fuel gauge send-
ing unit from the main tank, as it goes empty, to the
auxiliary tank, which will contain approximately 7 to
9 gallons of fuel . The fuel selector valve is used only
when the switch is moved from the "MAIN" to the
"AUX" position . The valve is located inside the left
frame rail, Figure 17, in front of the main fuel tank .
Should the selector valve malfunction, it can be re-
placed as a unit .
The following checks of the fuel indicator system
will determine quickly whether incorrect fuel gauge
readings are the result of an improperly operating
fuel gauge, fuel tank sending unit or circuit wires.
Refer to Figures 18 and 19 and also "Fuel Gauge
Diagnosis Details" for fuel gauge system diagnosis.
NOTE :
The vehicle is equipped with two fuel
tanks so be sure to diagnose the entire system .
After "FUEL MAIN" portion of the circuit is
checked, move the selector switch to "FUEL
AUX." position and repeat the test.
GAUGE DIAGNOSIS DETAILS
Inspect all circuit wiring for damage to insulation
or wire, also carefully check for proper electrical
connection at the following locations:
1 . Terminal connections at two-way connector to
tanks.