1950-1/2 to 1956 Willys Utility Vehicles

 Some of this information comes from Willys service manuals and some from discussions on the WillysTech mailing list.

 Questions or comments please E-mail me. Click here for contact information.

Description of Operation

Diagnostic Information and Procedures

Specifications

Wiring Diagrams

Component Locator

12 volt Conversion Information

Common Problems
 
 

The Fuel, Oil and Temperature gauges are of the bi-metallic type construction. They are supplied with regulated power from the Instrument Voltage Regulator to improve accuracy. The fuel gauge is connected by a single wire to a float-and-slide-rheostat sending unit in the fuel tank. The temperature gauge is connected by a single wire to a sealed bulb unit mounted in the water jacket of the cylinder head. The oil pressure gauge is connected by a single wire to a diaphragm-type unit (75 ohm rheostat) mounted in an oil gallery of the engine block. The fuel, temperature and oil pressure gauges all appear to be the same mechanism with different faces. Each gauge has 2 terminals. The internal mechanism consists of a coil of "heater wire" wrapped around a bimetallic strip. Each terminal on the gauge is connected to one end of the heater wire and there is no connection to ground. The more current that flows through the heater wire, the more the bimetallic strip bends and the more the indicator needle moves to the right. Therefore the lower the resistance of the sending unit to ground, the more the gauge will deflect to the right. Due to their construction the gauges are not polarity sensitive and do not need to be grounded. Each gauge has two internal adjustments.

The Fuel, Oil and Temperature gauge sending units provide a variable resistance ground path for each gauge. The less the resistance of the sending unit, the more current flows through the gauge and the more the needle deflects to the right. This makes the sending units connection to ground very important.

The amp meter is a moving vane type meter and is different than the other 3 gauges. It is electro-magnetic and wired in a series circuit.

The Instrument Voltage Regulator maintains a constant amount of power to the gauges to improve their accuracy. This power is regulated by varying the duty cycle of the output, not by actually changing the voltage. The Instrument Voltage Regulator is mounted to the Fuel/Oil gauge housing, making the ground of this housing very important. Input voltage in excess of 8 volts will not affect gauge readings but will result in premature wear of the Instrument Voltage Regulator contacts. Input voltage of less than 5 volts will make the output too low and cause low gauge readings. The Instrument Voltage Regulator is a small rectangular box and it has two screw terminals. The case is ground. One terminal is labeled IGN, which is connected to ignition power. The other terminal is the regulated output terminal. It goes to each of the fuel, temperature and oil pressure gauges to supply them with regulated power. The internal mechanism of the Instrument Voltage Regulator consists of an on/off contact controlled by a coil of "heater wire" wrapped around a bimetallic strip. When the ignition switch is turned on power flows into the Instrument Voltage Regulator and through the closed contact then out to the 3 gauges. Power also goes through the heater wire to ground. As the heater gets hot, the bimetallic strip bends and opens the contact. With the contact open the power flow is interrupted to the gauges and the heater wire, so the bimetallic strip cools until the contact closes again. The on/off duty cycle determines the amount of power supplied to the gauges by the Instrument Voltage Regulator. There is an internal adjustment screw on the mechanism.

Keep in mind that even when operating properly the gauges are not extremely accurate. If a gauge reads too high, there is too much power flowing through it. If a gauge reads too low, there is not enough power flowing through it. The most likely problems are bad connections and miss-routed wiring. There may be more than one problem causing the symptom. Before replacing any part, check it against the specifications charts to be sure it is defective.

Suggested tools- Standard Automotive Test light, Digital multimeter, Jumper wires.
 
 

Fuel, oil and temperature gauges ALL read too high or too low

If all three gauges read too high or too low, the Instrument Voltage Regulator may not be functioning properly. Go to Testing the Instrument Voltage Regulator
 
 

Only One Gauge Reads Incorrectly (except Amp Meter)

1. Disconnect the lead wire at the sending unit. With the ignition switch on, the gauge should have a below zero reading. If it doesn't, the lead wire may have an unwanted ground or the gauge may be wired incorrectly. Substitute the lead wire and check the gauge for proper wiring with the wiring diagrams.
2. Connect a test light from the battery ground to the disconnected lead wire. With the ignition switch on, the test light should flash at about 1-2 times per second and the gauge should read about 1/3. (Actual reading may vary depending on the bulb in the test light.) If this tests correctly, the sending unit may be bad.
3. Test the Instrument Voltage Regulator and bench test the gauge.

Testing the Instrument Voltage Regulator

Clean the connections at the Instrument Voltage Regulator and check for correct wire connections using the wiring diagrams. Check the voltage supply to the Instrument Voltage Regulator ignition terminal. It should be battery voltage when the key is on. Connect a jumper wire between the Instrument Voltage Regulator case and the battery ground terminal to help isolate ground connection problems. Connect a test light to the output terminal of the Instrument Voltage Regulator and with the key on it should be flashing at about 1-2 times per second, otherwise replace the Instrument Voltage Regulator. If a problem still exists go to One Gauge Reads Incorrectly and test each gauge individually.
 
 

Bench testing a Gauge

Remove all wires from the gauge to be tested. Make sure the gauge terminals are insulated from the housing. To check a Fuel, Oil or Temperature gauge, connect a D cell battery to the gauge terminals. The gauge should read about 1/4. If it does not, the gauge is bad. To check an amp meter, measure the resistance between the two terminals. It should be very close to 0 ohms. The gauge pointer should be centered.
 
 

Amp Meter With the engine off and the headlights on, the amp meter should read toward the discharge side of the scale. When the engine is started, the needle should move to the charge side, then slowly return to center as the battery is recharged. If the amp meter reads in reverse, the wire leads are connected backwards. If the gauge does not operate properly it is most likely wired incorrectly. Check the wiring diagrams for proper connections.
 
 

 Resistance

All resistance checks are done with the component out of the circuit
 
 

Voltage

All voltage checks are done with the components installed and the key on
 
 

Gauge Wiring Diagram drawn from actual 1951 Wagon, 4x4, F4 wiring harness.
 
 

Instrument Voltage Regulator - Screwed to the Fuel/Oil gauge housing. View backside of instrument panel

Temperature Sending Unit - L6, left rear of cylinder head - F4, right rear of cylinder head

Oil Pressure Sending Unit - L6, Right rear of engine block - F4, Left rear of engine block

Fuel Tank Sending Unit - Inside the fuel tank screwed to the top flange
 
 

The Fuel, Oil and Temperature gauges will operate normally on 12 volts. The higher voltage can shorten the life of the Instrument Voltage Regulator. The voltage can be reduced several ways. Probably the simplest method is to use a resistor in series with the power lead of the Instrument Voltage Regulator. A current limiting15-ohm power resistor reduces the voltage to about 6 volts. Another method is to use a voltage regulator. Converters are available from electronic stores that are used to power small electronic devices from the cigarette lighter of a car. Usually they come with plug adapters to fit the power plug of small radios and tape players. Select a converter that has a 6-volt output. Cut the cigarette plug off the converter. Remove the power feed wire from the instrument voltage regulator. Connect the converter wire that was hooked to the center contact of the cigarette plug to the removed power feed wire from the instrument voltage regulator. Connect the other wire that went to the cigarette plug to ground. Connect the 6-volt converter output to the ignition terminal of the Instrument Voltage Regulator. Be sure the wires are connected with the correct polarity.

The amp gauge measures current and is not affected by voltage.
 
 

E-mail me the solution to your gauge problem and when I see a common problem I will add it to the list. Click here for contact information.

Various problems

The most common problems with the gauges are bad connections and modified wiring. On older vehicles the connections and wires have corroded to the point where the resistance is too high. Previous wire harness repairs may have changed the wiring, rendering gauges inoperative. Check all wires for proper routing. Check all connections and grounds with an ohmmeter. Use a jumper wire to connect the case of the Instrument Voltage Regulator to the battery ground post to help isolate poor ground connections.

Fuel gauge doesn't go all the way to full

1. The fuel tank doesn't have a good ground. Run a wire from a sending unit mounting screw to a clean connection on the frame.

2. Float needs adjustment. The float hits the inside top of the fuel tank before the sending unit resistance is less than 10 ohms. Bend the float lever and check with an ohmmeter.

Fuel, Temperature and Oil Gauges all read too high

1. The Instrument Voltage regulator has a bad ground. Clean and tighten the mount screw of the regulator and clean the mounting surfaces of the Fuel/Oil gauge housing.