Figure 20 - Servicing Auxiliary Gauge
OIL PRESSURE GAUGE
The oil pressure gauge circuit (Figure 21) consists of two
basic components - the cluster-mounted gauge and the
sending unit. The sending unit senses the pressure of oil
in the oil gallery during engine operation and registers
the pressure on the gauge. Operation of the oil pressure
gauge system is similar to the fuel level gauge except
that the sending unit re-sistance is controlled by a
diaphragm instead of a float.
Disconnect wire at oil pressure sender unit (located
under cab floor on drivers side).
Connect one lead of gauge tester to end of sender
wire. Connect second lead to ground. (Gauge tester
now substitutes for sender unit.
Turn key switch "ON" (Be sure there is power to
Set tester for 88 ohms. (Oil pressure gauge should
read at full scale.
Set tester for 47 ohms. Oil pressure gauge should
read at half scale. (Pointer within 2 pointer widths of
Set tester at 1 ohm. Oil pressure gauge should read
at low scale. (Pointer within 2 pointer widths of hash
If oil pressure gauge responds correctly to various tester
settings, gauge and wiring between gauge and sender
unit are OK. Trouble is in the sender unit or the sender
unit is not grounded.
NOTE - Sealant or dirt on threads of sender unit or
engine oil gallery may prevent a good electrical
ground. Check for this condition before replacing
Figure 21 - Oil Pressure Gauge Circuit Diagram
If gauge does not respond to tester:
Check continuity of gauge wiring circuit. Make
sure connector terminals are clean and tight.
Check gauge cluster ground circuit. Make sure
circuit is grounded.
If wiring is OK, replace gauge.
OIL PRESSURE WARNING LAMP
A second or back-up oil pressure warning system is also
used. This system uses a second oil pressure warning
switch mounted on the engine.
When engine oil pressure is in its normal operating
range (high) the pressure switch is held in its off position
and no current is sent to warning light in the instrument
When engine oil pressure is below its normal operating
range (low) the pressure switch will close to its "on"
position and deliver current to warning light in instrument
CTS-4140Y - Page 12