Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSFORD1990RANGER 4WD L4-140 2.3L SOHCREPAIR AND DIAGNOSISSTARTING AND CHARGINGCHARGING SYSTEMALTERNATORDIODE TRIOTESTING AND INSPECTION
1990 Ford Ranger 4WD L4-140 2.3L SOHC
Diode Trio: Testing and Inspection
1990 Ford Ranger 4WD L4-140 2.3L SOHCSECTION Testing and Inspection
TEST PROCEDURES
1. Disconnect electric choke, if equipped.
2. Disconnect voltage regulator wiring connector.
3. Connect a jumper wire between the ``A'' and ``F'' terminals of voltage regulator wiring connector, Fig. 9.
4. Connect voltmeter to battery clamps. Then start and idle engine.
5. Observe and note voltmeter reading.
6. Move voltmeter positive lead to the alternator ``S'' terminal and note voltage reading.
TEST RESULTS
1. If voltmeter reading is within 1/2 of battery voltage, diodes are satisfactory.
2. If voltmeter reading is approximately 1.5 volts, the alternator has a shorted negative diode or a grounded stator winding.
3. If voltmeter reading is approximately 1.5 volts less than battery voltage, the alternator has a shorted positive diode.
4. If voltage reading is approximately 1 to 1.5 volts less than 1/2 battery voltage, the alternator has an open positive diode.
5. If voltage reading is 1 to 1.5 volts above 1/2 battery voltage, the alternator has an open negative diode.
6. Reconnect electric choke into circuit after tests are completed, if equipped.
Fig. 9 Regulator Plug. Jumper Wire Connector:
1. Disconnect electric choke, if equipped.
2. Disconnect voltage regulator wiring connector.
3. Connect a jumper wire between the ``A'' and ``F'' terminals of voltage regulator wiring connector, Fig. 9.
4. Connect voltmeter to battery clamps. Then start and idle engine.
5. Observe and note voltmeter reading.
6. Move voltmeter positive lead to the alternator ``S'' terminal and note voltage reading.
TEST RESULTS
1. If voltmeter reading is within 1/2 of battery voltage, diodes are satisfactory.
2. If voltmeter reading is approximately 1.5 volts, the alternator has a shorted negative diode or a grounded stator winding.
3. If voltmeter reading is approximately 1.5 volts less than battery voltage, the alternator has a shorted positive diode.
4. If voltage reading is approximately 1 to 1.5 volts less than 1/2 battery voltage, the alternator has an open positive diode.
5. If voltage reading is 1 to 1.5 volts above 1/2 battery voltage, the alternator has an open negative diode.
6. Reconnect electric choke into circuit after tests are completed, if equipped.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.