GMC 4000 Series Service Manual page 476

to frame. Start engine and check for leaks and
gauge operation.
DASH GAUGE
Refer to wiring diagram shown on "Fuel Gauge
System Trouble Diagnosis Chart."
The dash fuel gauge is an electromagnetic in­
strument which visually indicates quantity of fuel
in tank when the ignition switch is turned to the
"ON" or "ACCESSORY" position.
The dash gauge consists of a permanent mag­
netic armature and spindle assembly to which a
pointer is attached. Surrounding the magnetic arm­
ature are two coils wound perpendicular to each
other. These coils provide the magnetic field which
deflects the armature and pointer. The intensity
and direction of the resultant magnetic field of the
coils is dependent on current flow controlled by re­
sistance of the tank unit rheostat. When the posi­
tion of the slider arm varies, a change in rheostat
G M C S E R V I C E M A N U A L
resistance occurs which results in a proportional
current change in the dash gauge coils. This a l­
lows the armature (and pointer) to align itself ac­
cording to the resultant magnetic field produced by
the coils. The total angular travel of armature and
pointer from "EMPTY" to "FULL" is 90 degrees.
stat resistance is approximately zero ohms when
the fuel tank is empty (float arm is at its lowest
position). As the tank is filled, the tank unit float
arm will rise to increase rheostat resistance which
in turn, will result in the pointer in the dash gauge
to deflect towards the "FULL" position.
placed as an assem bly. The dash gauge fixed cal­
ibration resistor shown on the "Fuel Gauge Wiring
Diagram" is connected across the two external
wiring terminals of the gauge. Refer to CHASSIS
ELECTRICAL AND INSTRUMENTS (SEC. 12) of
this manual for illustrations of dash gauge as posi­
tioned on various types of instrument clusters.
WARNING: Conventional cab models
have a 90 ohm dash gauge and a 90 ohm
tank sending unit. All steel tilt models
have a 30 ohm dash unit and a 30 ohm tank
sending unit. It is important that dash
gauge and sending units not be interchang­
ed between conventional and ti It cab model s
If wrong fuel gauge components are in­
stalled, inaccurate fuel quantity readings
will result. To be certain that mating re­
sistances between dash gauge and tank
unit are maintained, be sure to install cor­
rect parts as listed in current G M C M as­
ter Parts Book.
FUEL TANK A N D EXHAUST
As mentioned previously, the tank unit rheo­
The dash gauge is not repairable and is re­
Sec. 8A
Page 465