Conventional Instrument Cluster
Magnetic Gauges
Calibration Test (Using Rotunda Tester Model 021-00055)
The Rotunda Instrument Gauge System Tester Model 021-00055 or equivalent is used to diagnose problems in the oil pressure or coolant temperature gauge.
Test Set-Up
1. Disconnect connector from the appropriate sender and hook onto the matching terminals on the tester.
2. Turn the vehicle ignition switch lock cylinder to ON position.
Calibration Test (Without Tester)
Test the coolant temperature gauge with a 9.7-ohm resistor for high calibration, the oil pressure gauge with a short circuit for mid-scale check and a 74-ohm resistor for low calibration as follows:
Disconnect connector from sender. Turn the ignition switch lock cylinder to ON. For coolant temperature, connect a 9.7-ohm resistor between the connector and ground. For oil pressure gauge, short the connector directly to engine ground. The centerline of the pointer should fall within the band around the "H" mark for coolant temperature, and slightly above mid-scale for oil pressure.
Fuel Gauge Resistance
With the sending unit float arm in the empty stop position, resistance should be 15 ohms (below E). With the sending unit float arm in the full stop position, resistance should be 160 ohms (above F). The fuel gauge should read empty at 22.5 ohms and full at 145 ohms.
Fuel Gauge Sending Unit Test Procedure
With ohmmeter positive lead on the signal wire lead and the negative lead on ground, check the resistance values of the fuel sending unit at various positions of the float arm. An open circuit will indicate infinity on the ohmmeter and a short will indicate zero ohms.
Coolant Temperature Gauge
Connect a 74-ohm resistor between the gauge lead and ground. The center line of the pointer should fall within the band around the "C" mark.
^ If the coolant temperature gauge tests within calibration, replace engine coolant temperature sensor (ECT sensor)(12A648).
^ If the coolant temperature gauge still tests out of calibration, replace the gauge.
Coolant Temperature Warning
If the engine coolant temperature indicating system is still not indicating properly, check the engine coolant level to make certain that coolant level is still at full point. Also verify that water thermostat (8575) is operating and that fan belt tension is within specification.
Oil Pressure Gauge
With connector attached to the oil pressure switch and ignition switch lock cylinder on, the center line of the pointer should fall on or below the "L" mark.
^ If the oil pressure gauge tests within calibration, replace oil pressure switch.
^ If the oil pressure gauge still tests out of calibration, replace the gauge.
Charge Indicator Lamp
The integral alternator regulator (IAR) has a circuit in the voltage regulator that will indicate a high battery voltage condition. With the IAR system, three conditions can cause the charge indicator lamp to come on during vehicle operation:
1. No generator output - damaged generator, voltage regulator or wiring.
2. Overvoltage correlation - shorted generator rotor, regulator or wiring.
If the system is working normally, the following conditions will be present:
^ With ignition switch lock cylinder in OFF position, charge indicator (battery symbol) lamp is off.
^ With ignition switch lock cylinder in RUN (engine not running), charge indicator (battery symbol) lamp is on.
^ With ignition switch lock cylinder in RUN (engine running), charge indicator (battery symbol) lamp is off. Except as noted above.
If the charge indicator lamp does not come on with the ignition switch lock cylinder in the RUN position (engine not running), check the "I" circuit ( ignition switch lock cylinder to regulator "I" terminal) for an open circuit or burned out charge indicator lamp. Replace the lamp, if necessary.
3. If the charge indicator lamp does not come on, disconnect the wiring connector from the voltage regulator.
4. Connect a jumper wire from the wiring connector "I" terminal to the battery negative post cable clamp.
5. Turn the ignition switch lock cylinder to RUN (engine not running). If the indicator lamp does not light, check the indicator bulb for continuity and replace the bulb if it is burned out. If the bulb is still good, perform the voltage regulator "I" circuit tests.
Speedometer Accuracy Test, Ranger and Explorer
Speedometer accuracy can be checked by timing the vehicle on a measured mile and performing the following speedometer calibration test. The Ford Speedo Gear Supplement Charts show the proper speedometer gears to use for various rear axle and tire size OEM combinations.
Speedometer Calibration Test
1. Time the number of seconds it takes to travel one measured mile at an indicated 30 and 60 mph.
2. Calculate the vehicle speed using the following formula:
mph = 3600 divided by seconds
3. Acceptable ranges are the following:
Speedometer Cable and Housing Assembly, Ranger and Explorer
NOTE: An inaccurate speedometer reading should not be serviced by substituting speedometer gears to correct the concern, unless the odometer is also wrong.
1. Visually inspect the speedometer cable and housing assembly for kinks, cuts, burrs, abrasions or severe bends behind the instrument panel and routing to the transmission. If evidence of any such damage is observed, replace the speedometer cable and housing assembly.
2. Check the speedometer cable and housing assembly function as follows:
a. Remove the instrument cluster. Disconnect the speedometer cable and housing assembly at the speedometer.
b. Raise the rear wheels, start the engine and check the speedometer core rotation at the speedometer end.
c. If the cable does not rotate, disconnect the speedometer cable and housing assembly from the transmission end.
d. Twist cable; if it binds or will not rotate, replace the speedometer cable and housing assembly.
e. Examine the driven gear. A scored, nicked or gouged driven gear is usually indicative of a damaged drive gear. On those vehicles that have the drive gear integral with the transmission output shaft, the output shaft should be carefully inspected for imperfections and replaced if necessary. A driven gear with two or three adjoining teeth badly scored is indicative of improper assembly procedure. The gear should be inserted in the transmission while simultaneously turning the driveshaft. This will ensure initial engagement and prevent gear damage. Force should never be used. Whenever a drive gear is replaced, a new driven gear should also be installed regardless of its apparent condition.
f. Examine the cable tip for breaking or fraying. If damaged, replace.
Odometer Accuracy Test
To test the odometer accuracy, drive the vehicle over a measured distance of at least 10 consecutive miles (or 16 kilometers). Check measured distance against odometer measured distance. Acceptable odometer measured distance is 15.5-16.7 km (9.6-10.4 miles).