MOTOR TRUCK SERVICE MANUAL
Electrical current supplied by the battery travels
through the ignition switch and enters the gauge at point
between the two alternate paths result from changes in
A (see Fig. 26). From point A two possible paths exist to
the sender's resistance. The magnetic fields around the
complete the circuit to ground. The current can flow
coils in each respective case position the pointer at the
through the series connection of Coil 1, Coil 2, and the
appropriate scale reading.
calibrating resistor to ground; or it can travel through Coil
3 to the sender. Since current will travel through the path
The calibrating resistor connects the ground terminal
of least resistance, the flow path is determined by the
with the tie-in terminal, Fig. 27.
resistance of the sender as follows.
The ground terminal is tin coated for identification.
When the oil is cold, the sender's resistance is high.
This terminal grounds the gauge to the housing or cover.
The current will, therefore, flow through Coil 1, Coil 2 and
The housing is grounded to the chassis through the
the calibrating resistor to ground, which offers less
resistance than the sender path. A small amount of the
current, however, follows the path through Coil 3 and the
The sender terminal is the first terminal clockwise
sender. (When current flows through a coil, a magnetic
from the ground terminal (when viewed from the back
field is developed around the coil.) Since Coils 1 and 3
side). The ignition terminal is directly opposite from the
are oppositely-wound, their resulting magnetic fields
oppose each other. The remaining field around Coil 2
has an attraction for the magnet. Since the magnet is on
To determine faulty component parts, proceed as
the same spindle as the indicating pointer, the spindle
rotates toward the colder scale reading corresponding to
the oil temperature.
1. Disconnect sender wire at sender unit (point A), Fig.
When the oil is hot, the sender's resistance is low.
2. Connect one lead of Fastestor to sender wire and
Thus, the majority of the current will flow through Coil 3
the other lead to a good ground.
and the sender. The remaining current will travel the
3. Set Fastestor left-hand knob at "120" and right-hand
alternate path and create weak magnetic fields around
knob at "0. " Turn ignition switch "on. " Gauge
Coils 1 and 2. The magnetic field around Coil 3
should read approximately 170°F.
dominates the attraction for the magnet. This field
4. Turn Fastestor left-hand knob to "30" and right-hand
rotates the pointer to the higher temperature scale
knob to "3." Gauge should read approximately
300°F. Turn ignition switch "off." Reconnect sender
reading corresponding to the oil temperature.
wire to sender unit (point A).
Fig. 27. Oil Temperature Gauge Circuit