This Ford charging system uses a voltage regulator bolted to the rear of the generator housing. The regulator is turned on by a small current from the dash light circuit when the ignition switch is turned on. It monitors two voltages to determine if the system is working properly, and turns on the warning light to indicate a problem. Troubleshooting will be easier if you have a basic understanding of electrical theory and can read a digital volt-ohmmeter. In some instances, a less expensive test light will be recommended because it will complete an open circuit causing current to flow and voltage drops to occur. The result of these voltage drops is a dim test light. If you do not understand electrical theory but would still like to diagnose this system, read the "Safety" section first, then follow the instructions for "Basic Diagnosis". A digital voltmeter is preferred, but a test light should provide sufficient information.
WARNING: Some systems have two heavy output wires in a single connector on the side of the generator. DO NOT unplug this connector for any reason. Disregard any of the following steps that call for plug removal.
Figure 1. 1994 Tempo AC generator.
There are a few different designs that use this voltage regulator bolted to the rear housing of the generator. This one is on a '94 Tempo.
Field current comes into the voltage regulator on the yellow wire. This circuit goes directly back to the battery positive, usually through a 15 amp fuse or fuse link wire, and is used to sense system voltage. At first it looks like the regulator comes before the field winding, but actually, the circuit branches off internally and goes to the first brush for the field. There is a corresponding external test point. The second brush, which also has a test point, goes to the voltage regulator. Field current continues through the regulator, to ground, then back to the battery negative.
To prevent the field circuit from draining the battery, the voltage regulator remains off until told to turn on by current flow through the green / red wire. That circuit comes from the warning light on the dash. Current flow through the bulb creates a voltage drop of approximately 10 volts. That leaves around 2.0 volts at the generator. If the dash light turns on when the ignition switch is turned on, that circuit is working. When the light does not turn on, a break in the green / red wire will result in the regulator not turning on to allow field current flow. Common places to find that break include corroded pins in the voltage regulator connector and corroded or lose pins in a two-wire connector often found a short distance from the generator. A burned out bulb in the instrument cluster will not cause a failure of the regulator to turn on. There is a resistor across the bulb that will pass enough current to get things started. The charging system will work fine with a burned out warning light bulb. The only clue to a problem is you won't see the light turn on when the ignition switch is turned on but before the engine is running.
Once the engine is running and voltage is being induced in the stator winding, the alternating current is rectified by the diodes and sent to the vehicle's electrical systems and to the battery to recharge it after starting the engine. Some generators have a fourth pair of diodes connected to a centertap of the stator winding to capture some of the wasted energy. This can increase the current capacity slightly and reduces ripple.
Ford has a more important use for that tap in the stator winding. That circuit leaves on the white / black wire and goes into the voltage regulator on the "S" (stator) terminal to tell it the system is working. Knowing that, the regulator applies battery voltage on the green / red wire. With battery voltage on both sides of the dash light, the net difference is zero, so the bulb turns off. The voltage on the white / black wire will be very close to half of system voltage but measuring it has little value other than to verify the charging system is working or not working, . . . something you already know. This same stator circuit was used by Ford for a long time on older cars with external mechanical and electronic voltage regulators, but where those systems turned on the warning light when there was no generator output, this regulator will also turn on the light during an undercharge and an overcharge condition.
Figure 2. Voltage regulator.
Here you can see the four screws that hold the voltage regulator to the rear housing. In addition, notice the other two screws. The brushes are attached by them and will come out with the regulator when it is removed. These screws are two important test points. The one on the left is labeled "Ground here to test" and is designated the "F" terminal for "field. The screw on the right typically has a protective plastic cap. It is designated the "A" terminal which is used to identify a power source, in this case battery voltage. The "A" and "F" can be found between the two screws.
Testing the Input Circuit
When the charging system is inoperative, a real fast test is to ground the test point, but for this story, we'll take things in order beginning at the yellow wire. Battery voltage must be present all the time. If it is missing, suspect a blown 15 amp fuse or a burned open fuse link wire. To double-check for a corroded or loose connection in the plug, you might also want to verify that voltage is present at the right screw on the voltage regulator. That is where battery voltage is applied to the field brush. From this brush, current flows through the field winding, then out the other brush at the "Ground here to test" screw. This is where the current enters the voltage regulator, then goes to ground, but only after it is turned on. The "battery" warning light being on would normally prove the turn-on circuit is working, but there is always the remote chance the green / red wire is grounded. Measure the voltage on that wire in the regulator's connector. If you find close to 2.0 volts, the wire is not grounded and it is okay. If you do find 0 volts at the regulator, unplug the connector. If the warning light is still on, that green / red wire is grounded and must be repaired. If the light goes out, you should find full battery voltage on the wire. That proves the regulator is shorted internally and must be replaced.
To bypass the voltage regulator, use a jumper wire to ground the "F" test point screw. If the system voltage goes up, there is no need to look further. The voltage regulator is open and bypassing it allowed current to flow through the field winding. The fact that the output circuit started working proves all of the wiring is okay. When that test fails to produce generator output, measure the voltage on the two test screws. Full battery voltage on the right "A" terminal and 0 volts on the left "F" one indicates the brushes are worn and the field circuit is open. A continuity test with an ohm meter will verify the open circuit.
If you find between 4 and 11 volts on the test point screw, the input circuit is working, and the problem is in the output circuit. A load test will further identify the problem within the generator. A shorted or open diode will result in one third of the generator's maximum current rating being all that it will deliver. A more common problem is no output current and no increase above battery voltage during the full-field test. No voltage will be on the white / black stator wire so the regulator will turn on the battery warning light. Suspect an open stator winding. There are two different styles of stator winding. Ford uses a "wye" winding which has a common connection for the three individual windings. That is where the white / black stator wire comes from that goes to the regulator. A brass connector is crimped around the copper wires to make the connection. When road salt and water sprays into that connection, it is only a matter of time before corrosion causes that connection to fail. The fastest repair is to replace the generator, but should you choose to attempt a repair, you'll need to completely disassemble it, then clean the wires until they are shiny and solder them. Coat the connection with silicone gasket sealer or another sealer to prevent a repeat problem.
Voltage Regulator / Brush Assembly Replacement
Figure 3. Location of the Test Terminals.
If you have enough room to access the four mounting screws, the regulator can be removed without removing the generator from the engine. To prevent damaging the new brushes, they must be held in the retracted position until the assembly is installed over the slip rings on the end of the rotor, (field winding). That is the purpose of the small hole midway between the "A" and "F" screws. Use a toothpick or stretched out paper clip to hold the brushes retracted. The wire or toothpick can be removed once the new assembly is bolted in place.