Article 1354 - Service Procedure for Instrument Cluster Gauges

TSB #89 - April 5, 1968
(All 1967-68 Vehicles)


CAUTION
The Ford Motor Company instrument cluster gauge system is a five volt system.
Applying twelve volts to any part of the system except the Instrument Voltage Regulator input (terminal marked IGN) or grounding any part of the system except as outlined under Test Procedures may result in burning out one or more of the components. When removing or replacing the Instrument Cluster, the Gauges, or the Senders the Battery Ground Cable must be disconnected.

FUEL GAUGE SYSTEM (lllus. S1260)
    The level of fuel in the tank governs the position of the Fuel Sender Float, which in turn controls the position of a contact on a variable resistor. When the fuel is low, the sender resistance is high and restricts the flow of electric current to the gauge. A high fuel level causes a low resistance and higher gauge current. The gauge pointer is controlled by a bimetal strip, which is heated by the current from the sender. When the current is low, there is little heating effect and the pointer moves a short distance. A higher current produces a proportionately higher heating effect and pointer movement. The Instrument Voltage Regulator provides a pulsing output with a peak at battery voltage but with an average value of five volts.

TEMPERATURE GAUGE SYSTEM
    The operation of the temperature system is the same as the fuel level system, except that the variable resistance is produced by a Thermistor located in the Temperature Sender. The resistance of the Thermistor is high when the temperature is low, and low when the temperature is high.

OIL PRESSURE GAUGE SYSTEM
    The operation of the Oil Pressure System is the same as the fuel level system except that a diaphragm controls the position of the contact on the variable resistor.

TEST PROCEDURES

    A. Wiring Harness
    Refer to the schematics in the Shop Manual for wire identification and color coding to facilitate proper testing procedure. A voltmeter or test light is recommended for finding a broken or open condition in the wiring harness system. Place the range selector of the voltmeter on the 20 volts D.C. scale. Attach the negative (minus) lead to a good ground and the positive lead to the sender wire. If the pointer moves in a pulsating manner or the test light flickers, there is continuity from the power source. If the meter pointer does not deflect or pulse, the open circuit exists between the test point and the power source. Repeating this procedure at the various connectors should define or isolate the faulty section of wire harness.

    B. Sender and Gauge
    Disconnect the sender lead wire of the sender and gauge being tested and connect to one lead of a Borroughs BT-11-7 Gauge tester or equivalent and the other lead to ground. With the ignition key on, adjust the dials on the tester for high, mid-scale and low and observe the gauge in each position. If a tester is not available, connect the Sender Lead to ground through a 10 OHM Resistor. This is equivalent to the HIGH or FULL scale setting of the tester.

    If the gauge does not indicate or operate during the test, the gauge or the wiring to it has an open circuit. If the gauge never worked or does not work after replacement, there may be an open circuit in the vehicle wiring system. (See test procedure A.)

    C. Instrument Voltage Regulator
    The Instrument Voltage Regulator may be tested in the vehicle as follows. Connect one lead of a test light or the positive lead of a voltmeter to the temperature sending unit terminal, oil pressure sending unit terminal, or fuel sender gauge terminal without disconnecting the vehicle lead. Connect the other lead to a suitable ground. With the ignition key on, a flashing light or pulsing voltmeter indicates that the Instrument Voltage Regulator is good. CAUTION: DO NOT GROUND OR SPARK EITHER TERMINAL OF THE INSTRUMENT VOLTAGE REGULATOR. This will burn out the dash Wiring Harness, the Instrument Voltage Regulator or both.

Image
Figure 15 - Schematic Diagram of Ford and Lincoln Mercury Instrument System
(CLICK TO ENLARGE)


DIAGNOSTIC PROCEDURE - All Gauges Pegged
POSSIBLE CAUSECORRECTION OR PROCEDURE
Loss of regulator case ground to vehicle.1. Use ohmmeter to check for proper ground (zero resistance).
2. Use ohmmeter to check for cracked ground circuit in printed circuit.
Faulty instrument voltage regulator.Use voltmeter or test light to check for pulsing instrument voltage regulator output (See test procedure C).


DIAGNOSTIC PROCEDURE - All Gauges Do Not Indicate With Ignition Switch ON
POSSIBLE CAUSECORRECTION OR PROCEDURE
Open circuit in radio noise suppression choke.Replace choke.
Faulty instrument voltage regulator.Use voltmeter or test light to check for pulsing instrument voltage regulator output. (See test procedure C).
Open circuit in battery feed to instrument voltage regulator.Use voltmeter or test light to locate open circuit in feed. (See test procedure A)
Open circuit in printed circuit.Use voltmeter or test light to locate open circuit in printed circuit.


DIAGNOSTIC PROCEDURE - Individual Fuel, Oil or Temperature Gauge Does Not Indicate or Operate
POSSIBLE CAUSECORRECTION OR PROCEDURE
Associated sender not grounded.Test sender ground with ohmmeter (zero resistance)
Open circuit in wiring harness or printed circuit.Use voltmeter or test light to locate open circuit. (See test procedure A).
Open circuit winding in gauge or sender.Test gauge and sending unit individually. (See test procedure B).


DIAGNOSTIC PROCEDURE - Individual Fuel, Oil or Temperature Gauge Pegged
POSSIBLE CAUSECORRECTION OR PROCEDURE
Harness or sender shorted to ground.1. By pass harness with jumper wire.
2. If still pegged, replace sender.
3. If operative, repair harness.


DIAGNOSTIC PROCEDURE - Erratic Operation of All Gauges
POSSIBLE CAUSECORRECTION OR PROCEDURE
Dirty contacts in instrument voltage regulator.Replace instrument voltage regulator.


DIAGNOSTIC PROCEDURE - All Gauges Read Low
POSSIBLE CAUSECORRECTION OR PROCEDURE
Radio noise choke connected to output terminal.Connect radio noise choke to input (IGN) terminal.
Instrument voltage regulator out of calibration.Check calibration as described in Sender and Gauge. (See test procedure B).

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