Instrument Panel, Gauges and Warning Indicators: Testing and Inspection

Gauge failures are often caused by defective wiring or grounds. The first step in locating trouble should be a thorough inspection of all wiring, terminals and printed circuits. If wiring is secured by clamps, check to see whether the insulation has been severed, thereby grounding the wire. In the case of a fuel gauge installation, rust may cause failure by corrosion at the ground connection of the tank unit.

CONSTANT VOLTAGE REGULATOR TYPE (CVR)
The Constant Voltage Regulator (CVR) type indicator is a bimetal-resistance type system consisting of an Instrument Voltage Regulator (IVR), an indicator gauge, and a variable resistance sending unit. Current to the system is applied to the gauge terminals by the IVR, which maintains an average pulsating value of 5 volts.
The indicator gauge consists of a pointer which is attached to a wire-wound bimetal strip. Current passing through the coil heats the bimetal strip, causing the pointer to move. As more current passes through the coil, heat increases, moving the pointer farther.
The circuit is completed through a sending unit which contains a variable resistor. When resistance is high, less current is allowed to pass through the gauge, and the pointer moves very little. As resistance decreases due to changing conditions in system being monitored, more current passes through gauge coil, causing pointer to move farther.

Voltage Limiter Test

1. Connect one lead of a voltmeter to temperature sending unit and other lead to a good ground. Do not disconnect sending unit lead from sending unit.
2. Turn ignition switch to On position and observe voltmeter.
3. A fluctuating voltmeter indicates that voltage limiter is operating.

VARIABLE VOLTAGE TYPE
The variable voltage type dash gauge consists of two magnetic coils to which battery voltage is applied. The coils act on the gauge pointer and pull in opposite directions. One coil is grounded directly to the chassis, while the other coil is grounded through a variable resistor within the sending unit. Resistance through the sending unit determines current flow through its coil, and therefore pointer position.
When resistance is high in the sending unit, less current is allowed to flow through its coil, causing the gauge pointer to move toward the directly grounded coil. When resistance in the sending unit decreases, more current is allowed to pass through its coil, increasing the magnetic field. The gauge pointer is then attracted toward the coil which is grounded through the sending unit.

Dash Gauge Test

1. Disconnect electrical connector from sending unit.
2. Turn ignition to On position.
3. Ground sending unit lead and observe gauge. If gauge does not move to high side of scale, the gauge or wiring is defective. If gauge responds when grounded, replace sending unit.