General Description General Description Engine speeds are shown in revolutions-per-minute (rpm) by the electronic tachometer (Fig. 1). The tachometer is used as a guide for shifting, and to keep the engine speed in the proper rpm range. The electronic tachometer consists of a gauge, magnetic sensor (Fig. 2), and the electrical wires that complete the circuit between the two components. The electronic tachometer receives signals from the magnetic sensor, which extends through a threaded bore in the flywheel housing (Fig. 3). When the vehicle is moving, the teeth of the turning flywheel generate electrical pulses in the sensor s magnetic field. These pulses are transmitted to the tachometer head, which converts the signals to an rpm reading. 1 2 1. Flywheel Teeth 2. Flywheel Housing 3. Magnetic Sensor 10 15 20 3 5 25 f540184 RPM X 100 30 Fig. 3 f600173 Fig. 1 1 2 3 1. Locknut 2. Sensor 3. Weather Pack Connector f540144 Fig. 2, Magnetic Sensor Assembly Heavy-Duty Trucks Service Manual, Supplement 0, July 1992 050/1
Tachometer Head Replacement Replacement 1. Apply the parking brakes. 2. Turn off the ignition. 3. Remove the screws that attach the tachometer/ speedometer panel to the dash. Pull the panel out to gain access to the back of the tachometer head. See Fig. 1. 4. Unplug the electrical wiring connectors from the tachometer head. 5. Remove the nuts that attach the mounting bracket to the tachometer head, and remove the bracket. Remove the tachometer head by pulling it out from the front of the panel. 6. Calibrate the new tachometer head to the vehicle s engine. For instructions, refer to Subject 120. 7. Place the new tachometer head in the dash panel. Attach the mounting bracket to the tachometer head, then tighten the nuts. 8. Connect the electrical wiring connectors to the tachometer head. 9. Position the panel, then install the screws that attach the tachometer/speedometer panel to the dash. f540076 Fig. 1 Heavy-Duty Trucks Service Manual, Supplement 0, July 1992 100/1
Magnetic Sensor Replacement and Adjustment Replacement and Adjustment 1. Disconnect the Weather Pack connector. See Fig. 1. 2. Loosen the magnetic sensor locknut. Remove the sensor from the flywheel housing. 3. Screw the new magnetic sensor into the flywheel housing until it bottoms out on the gear teeth. Back off the sensor one-half turn. See Fig. 2. IMPORTANT: The tachometer reading will be wrong if the correct adjustment is not maintained. 4. Hold the magnetic sensor in place to keep it from turning. Tighten the locknut 18 lbf ft (25 N m). 5. Connect the Weather Pack connector. 6. Test drive the vehicle and check that the tachometer is operating. If there is no tachometer reading, the magnetic sensor was probably not adjusted correctly; readjust the sensor. 1 2 3 Fig. 2 1. Flywheel Teeth 2. Flywheel Housing 3. Magnetic Sensor f540184 1 2 3 1. Locknut 2. Sensor 3. Weather Pack Connector f540144 Fig. 1, Magnetic Sensor Assembly Heavy-Duty Trucks Service Manual, Supplement 0, July 1992 110/1
Calibrating the Tachometer Head Calibrating the Tachometer Head LIGHTBULB If a new tachometer head is installed, or if a new engine is installed, you must also recalibrate the tachometer head. The Stewart-Warner tachometer is calibrated by setting eight DIP switches at the back of the tachometer head. See Fig. 1. The DIP switches are set according to the engine model used in the vehicle. Determine the tachometer calibration setting as follows: 1. Adjust the magnetic sensor. For instructions, refer to Subject 110. 2. Obtain the calibration code from the table listing codes in Specifications, 400. 3. Remove the tachometer head from the panel. For instructions, refer to Subject 100. 4. Remove the plastic cover from the DIP switch panel at the back of the tachometer head. Calibrate the tachometer by moving DOWN each switch that corresponds to the 0 (zero) positions of the calibration code. See Fig. 2. Move UP each switch that corresponds to the 1 (one) positions of the calibration code. See Fig. 2. 5. Install the plastic cover on the DIP switch panel. 6. Install the tachometer head in the panel. For instructions, refer to Subject 100. DIP SWITCHES DIP SWITCHES Fig. 1 0 1 1 1 0 1 1 1 CALIBRATION CODE Fig. 2 f540187 f540186 Heavy-Duty Trucks Service Manual, Supplement 0, July 1992 120/1
Troubleshooting Troubleshooting The following is a troubleshooting process requiring the use of a voltmeter and an ohmmeter. Make the checks in the order listed below. Incorrect tachometer head readings may occur whenever a new tachometer head or engine is installed. In each case, calibrate the tachometer head using the procedure in this section. Inaccurate readings will also occur if the magnetic sensor is not adjusted correctly. Before troubleshooting the system, make sure the magnetic sensor is correctly adjusted. For instructions, see Subject 110. Also, make sure the batteries are fully charged. MAGNETIC SENSOR VOLTAGE CHECK NOTE: To receive an accurate tachometer reading, a minimum of one volt AC output is needed from the magnetic sensor. 1. Disconnect the electrical connector from the sensor. Make sure the sensor terminals are free of corrosion and moisture. 2. Connect an AC voltmeter to the terminals of the magnetic sensor. 3. Run the engine at idle speed, and check the voltage output from the sensor. If the voltage is less than one volt, refer to the "Troubleshooting Chart" for possible causes and corrections. 4. Turn off the engine and disconnect the voltmeter. MAGNETIC SENSOR RESISTANCE CHECK USING AN OHMMETER Connect the ohmmeter leads to the terminals of the magnetic sensor. The resistance should measure 876 ohms. Readings between 788 and 963 ohms are acceptable. If the resistance is acceptable, then inaccurate tachometer readings are caused by either the wiring to the tachometer head or by the tachometer head itself. Go to "Wiring Check: Magnetic Sensor to Tachometer Head." If the resistance is outside the acceptable range, replace the sensor with a new one. For instructions, see Subject 110. WIRING CHECK: MAGNETIC SENSOR TO TACHOMETER HEAD 1. Remove the screws that attach the tachometer/ speedometer panel to the dash. Pull the panel out to gain access to the back of the tachometer head. 2. Unplug the electrical wiring connector from the tachometer head. 3. Check for an open or short circuit by disconnecting the magnetic sensor leads, circuit numbers 162 and 163. Connect the ohmmeter to the leads of circuit numbers 162 and 163. The resistance should measure 876 ohms. A reading between 788 and 963 ohms is acceptable. If the above reading is not obtained, and the magnetic sensor is operative, the problem is in the wiring between the magnetic sensor and the tachometer head. Repair or replace the damaged wiring. If the reading is between 788 and 963 ohms, connect the magnetic sensor leads to the tachometer head. Go to "Battery Voltage Check Using a DC Voltmeter." BATTERY VOLTAGE CHECK USING A DC VOLTMETER 1. With the ignition switch off, attach the probes of the DC voltmeter across the positive and negative terminals of the tachometer head. NOTE: Attach the negative (black) probe to the yellow wire terminal and the positive (red) probe to the red wire terminal. 2. Turn on the ignition switch. The voltmeter should read 9 to 16 volts DC. If the voltage is between 9 and 16 volts DC, but the tachometer meter readings are inaccurate, then the tachometer head, not the wiring is causing inaccurate readings. Turn off the ignition switch, and disconnect the voltmeter probes. Replace the tachometer head. For instructions, see Subject 100. If the voltage is not between 9 and 16 volts DC, check the wiring between the ignition switch and the tachometer head. Turn off the ignition switch, Heavy-Duty Trucks Service Manual, Supplement 0, July 1992 300/1
54.07 Troubleshooting Stewart-Warner Electronic Tachometer and disconnect the voltmeter probes. Repair or replace the damaged wiring. 3. Place the panel in the dash; install and tighten the screws. Troubleshooting Chart Problem Less Than One Volt Reading from the Magnetic Sensor Possible Cause Magnetic sensor gap is not properly adjusted. The magnetic sensor is damaged. Adjust the magnetic sensor. Replace the magnetic sensor. Remedy Problem Tachometer Head Will Not Register Possible Cause Magnetic sensor gap is not properly adjusted. The magnetic sensor is damaged. The tachometer head is damaged internally. Loose or corroded terminals, broken contacts, broken wires, shorting, or frayed wires are causing an incomplete circuit. Adjust the magnetic sensor. Replace the magnetic sensor. Remedy Replace only after checking all other possible causes. Replace the damaged component(s), as necessary. Problem Inaccurate Tachometer Readings Possible Cause Magnetic sensor gap is not properly adjusted. Shorts are occurring in the magnetic sensor. The tachometer head is incorrectly calibrated. Erratic readings are resulting from loose electrical connections. Adjust the magnetic sensor. Remedy Check the resistance of the magnetic sensor. Replace the sensor, if necessary. Recalibrate the tachometer head. Check all the wiring in the tachometer system. Tighten all loose connections. 300/2 Heavy-Duty Trucks Service Manual, Supplement 0, July 1992
Specifications Calibration codes by engine for Stewart Warner electronic tachometers. Engine Manufacturer Engine Model Calibration Code Detroit All 01110111 Caterpillar 3306 10010111 3406 and 3176 01110001 Cummins NTC Big Cam III & IV L10 01100011 Table 1 Heavy-Duty Trucks Service Manual, Supplement 0, July 1992 400/1