Constant Voltage Regulator Type (CVR)
NOTE: 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.
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.
OPERATIONAL TEST
1. Disconnect wiring harness connector at sending unit. Connect a test lamp or voltmeter between terminal in wiring harness connector and ground.
2. With ignition switch in ON position, light should pulse or meter reading should fluctuate.
3. If lamp lights but does not pulse, or if meter reading remains steady, check ground to IVR. If ground is satisfactory, IVR is defective.
4. If lamp fails to light, or if meter reads 0 volts, check for open circuit across IVR terminals, indicator gauge terminals, or open circuit in wiring harness and printed circuit between components.
5. Connect test lamp or voltmeter ground lead to ground terminal in sending unit wiring harness connector. Light should pulse or meter reading should fluctuate as in step 2. If not, locate open in ground circuit. Do not apply battery voltage to system or ground output terminals of IVR, as damage to system components or wiring circuits may result.
SENDING UNIT TESTS
Fuel Tank Gauge
1. Disconnect wiring harness connector at sending unit and connect ohmmeter between ground terminal and resistor terminal on sending unit.
2. Meter should read 1-88 ohms on 1984-87 Cherokee and Wagoneer and 1986-87 Commanche, 10-63 ohms on 1984-87 Grand Wagoneer and Truck, or 10-63 ohms on all other models. If reading shows no continuity (infinite reading), check ground connection to tank gauge.
3. If ground is satisfactory, but reading is not within specification, tank unit is defective.
4. If reading is within specification, remove fuel tank gauge from vehicle and connect ohmmeter between resistor terminal and ground terminal (metal housing on single terminal units).
5. Observe meter while slowly moving float rod between empty and full stops. Meter should read at high end of specifications listed in step 2 at empty stop and at low end of specifications listed in step 2 at full stop. Change in readings should be smooth, without hesitation or jumping.
6. If tank unit fails to operate as outlined, unit is defective. Before installing fuel tank gauge, connect wiring harness connector to gauge and move float rod from empty to full position with ignition key in ON position. If dash gauge reading is incorrect, check IVR and dash gauge. If system tests prove satisfactory, but system still does not operate correctly, check that tank gauge rod is not bent or binding and that float is not damaged, loose or filled with fuel.
Oil & Temperature Sending Units
1. Test dash gauge and IVR as outlined above.
2. If system is satisfactory, start engine and allow it to reach operating temperature.
3. If no reading is indicated on the gauge, check the sending unit-to-gauge wire by removing the wire from the sending unit and momentarily ground this wire to a clean, unpainted portion of the engine.
4. If the gauge still does not indicate, the wire is defective. Repair or replace the wire.
5. If grounding the new or repaired wire causes the dash gauge to indicate, the sending unit is faulty.