Vaanth
Well-Known Member
Introduction
Variable resistance sending units and gauges of the King-Seeley Constant Voltage type were used by Chrysler from 1960 through 1993 on domestic rear wheel drive vehicles.The thermal type gauges used in the subject vehicles are Chrysler’s adaptation of the King Seeley thermal gauge concept. The gauge functions from an applied, effectively constant voltage to the connected gauge and sending unit. The voltage derived from the instrument cluster voltage limiter, or regulator, is of a pulsating form due to the thermally activated contact points open/close action. The voltage supplied by the voltage limiter is dropped from the electrical system 12 volts to 5 volts. Although pulsating, for the thermal gauge response, it was considered a regulated constant source. The effective 5 volts is applied across the series connected gauge and variable resistance sending unit.
The voltage limiter is a self contained module that plugs into the instrument cluster circuit board, or fastens to the rear of the cluster and connects via wire jumpers, or is integrated into the fuel gauge or temperature gauge. The three terminals of the voltage limiter are as follows:
- +12 volt input (denoted “I”)
- ~+5 volt output (denoted “A”)
- System ground
The voltage limiter consists of a heating coil on a bimetallic arm actuating one half of a set of contact points. The points open and close as the arm heats (closed) and cools (open).
The following image shows a limiter internally. The limiter shown is the external version, but the version combined in a gauge is the same principle.
Although not a common occurrence, it is possible for the points in the limiter to stick closed. When this occurs, the full constant 12 volts is output to the gauges and will ultimately burn them out if left operating long enough. The limiter was not intended to be serviced internally, but with careful attention, the points might be freed up. However, often, the point surfaces are burnt and will likely stick again. Cleaning with an ignition point file might work, but the components are delicate.
The gauges that use the aforementioned principle are the fuel gauge, the temperature gauge, and the oil pressure gauge (if equipped). The gauges operate with a similar heating coil and arm driving the needle movement instead of the open/close points.
The fuel gauge, temperature gauge, and oil pressure gauge, all receive the ~5 volt output from the voltage limiter. All three gauges read the same resistance ranges. Actually, other than the faces, and physical layout for position and mounting, the gauge internal part can be interchanged and still read the same. The gauge voltage input is denoted “A” and the gauge sending unit input is denoted ”S”.
The voltage limiter is not adjustable, but the gauges can be with mechanical adjustments to the movement. The adjustment varies depending on the gauge and is covered in the factory service manuals. The oil pressure and temperature sending units are not adjustable, but the fuel sending unit has some indirect variance with arm bending. The actual resistance of the sending unit is not adjustable though.
For more information about the voltage limiter and gauges, in general, refer to the Master Technician's Service Conference (MTSC) training book at Thermal-Electric Gauges.
Gauge and Sending Unit Testing
The following sections provide basic testing of the gauges and sending units.The gauge is tested, in the vehicle, under power by applying varied resistance to the sending unit input, “S”. This can be performed in the vehicle by powering the cluster and applying a varying resistance in place of the sending unit. Note that a bad or poor connection in the circuit from the sending unit terminal to the gauge in the cluster will affect the results.
The sending unit is tested, in the vehicle, under power by checking the unit resistance at various points in operation.
Cardinal Check Point Values
To check cardinal points of a gauge, use the following resistance values:Chrysler Specification:
- Low/Cold/Empty = 72 Ω
- Middle = 22 Ω
- High/Hot/Full = 9 Ω
Miller Special Tool Specification (used by Chrysler for testing):
- Low/Cold/Empty = 75 Ω +/-5% -> 71.2 - 75.8 Ω
- Middle = 22 Ω +/-5% -> 20.9 - 23.1 Ω
- High/Hot/Full = 10 Ω +/-5% -> 9.5 - 10.5 Ω
Gauge position tolerance at the cardinal points are listed in the service manual for different model gauges, but in general, observe the following:
- With the 72 Ω resistance, observe "Cold/C, Low, or Empty" plus or minus 3/32" on the gauge.
- With the 22 Ω resistance, observe the gauge to slowly advance to the halfway position.
- With the 9 Ω resistance, observe "Hot/H, High, or Full" plus or minus 3/32" on the gauge.
Oil Sending Unit and Gauge Simplified Circuit Diagram and Test Points
For testing the oil sending unit and gauge operation, the following points and resistance values of the sender internal variable resistor, as measured from the external post to ground at "0-8 psi", "~40 psi", and "80 psi", are as follows:
- 0-8 psi: The needle moves to L mark on the oil gauge, sending unit resistance = 72 Ω 72 Ω ( 69 - 75 Ω )
- 40 psi: The needle moves to middle on the fuel gauge, sending unit resistance = 22 Ω ( 21 - 23 Ω )
- 80 psi: The needle moves to H, or 80 if marked, mark on the fuel gauge, sending unit resistance = 9 Ω ( 8 - 10 Ω )
Temperature Sending Unit and Gauge Simplified Circuit Diagram and Test Points
For testing the temperature sending unit and gauge operation, the following points and resistance values of the sender internal temperature dependent resistor, as measured from the external post to ground at ambient, "cold", mid range (ie. normal), and "hot", are as follows:
- Ambient temperature at about 77 degrees Fahrenheit (25 degrees Celsius), before running engine, gauge needle at lowest point, sending unit resistance = ~360 Ω
- Engine at ~120 degrees Fahrenheit (49 degrees Celsius): The needle moves to Cold mark on the temperature gauge, sending unit resistance = 72 Ω
- Engine at ~190 degrees Fahrenheit (88 degrees Celsius: The needle moves to middle on the temperature gauge, sending unit resistance = 22 Ω (approximate range of ~18 to ~26 Ω)
- Engine at ~230 degrees Fahrenheit (110 degrees Celsius): The needle moves to Hot mark on the temperature gauge, sending unit resistance = 9 Ω
Fuel Sending Unit and Gauge Simplified Circuit Diagram and Test Points
For testing the fuel sending unit and gauge operation, the following points and resistance values of the variable resistor as measured from the external post connected end of the wire wound resistor through the wiper to ground at "empty", "half", and "full", are as follows:
- Tank empty: The needle moves to E mark on the fuel gauge, sending unit resistance = 72 Ω ( 69 - 75 Ω )
- Tank half full: The needle moves to middle on the fuel gauge, sending unit resistance = 22 Ω ( 21 - 23 Ω )
- Tank full: The needle moves to Hot mark on the fuel gauge, sending unit resistance = 9 Ω ( 8 - 10 Ω )
Bench Testing a Gauge
The gauge can be bench tested with a 5 volt, 2 amp, DC power source and select resistance values or the Miller C-3826 gauge tester.To test a gauge, set up a test circuit and use varying resistance to check the gauge movement and reading.
Set up a circuit as shown in the following diagram:
To check cardinal points of the gauge, do one of the following:
Using resistors
Point resistances can be made using fixed resistors or a combination thereof, variable resistors, or decade resistance boxes.
To check points of the gauge, use the following resistance values:
- Low/Cold/Empty = 72 - 75 Ω
- Middle = 21 - 23 Ω
- High/Hot/Full = 9 - 10 Ω
- With the 72 - 75 Ω resistance, observe "Low/Cold/Empty" plus or minus 3/32" on the gauge.
- With the 21 - 23 Ω resistance, observe the gauge to slowly advance to ½ position.
- With the 9 - 10 Ω resistance, observe "High/Hot/Full " plus or minus 3/32" on the gauge.
The Miller C-3826 tester can be used to test these points. If using the Miller tester, substitute it in place of the resistor on the diagram.
Gauge position tolerance at the test points is listed in the service manual, but in general, observe the following:
- With the gauge tester on "L", observe "Cold or C" plus or minus 3/32" on the gauge.
- With the gauge tester on "M", observe the gauge to slowly advance to the halfway position.
- With the gauge tester on "H,", observe "Hot or H" plus or minus 3/32" on the gauge.
Note about Measuring Low Resistance
When measuring low resistances such as continuity or low resistance values of the sending unit, account for the resistance of the test leads connected to the measuring device. This can be done by shorting the leads together, noting the resistance, and subtracting it from the measurement readings, or use the meter's relative setting (if equipped) similarly with the lead shorting, or use a meter that measures conductance and convert accordingly, or use a four wire ohms measurement setup with the appropriate device.Although it is about measuring ballast resistors, for more about measuring low resistance, see the following document:
The foregoing content is excepted from Chrysler Thermal Gauges, Sending Units, and the Voltage Limiter, which is under construction.
