Ford Alternator Repairby Dan Ferrell
Before you start troubleshooting your Ford alternator, perform a visual inspection. Make sure there is no apparent damage to the case, the drive belt is tight and the electrical connections are in good condition and free of rust. Then disassemble the alternator, lay each part in order on a workbench and clean them with electrical contact cleaner. Again, start by examining each part for wear and damage. Now, you are ready for the next tests.
Start with the rotor and check for continuity between the two slip rings mounted on the center shaft using an ohmmeter (see the Resources box at the bottom for more information). Your meter should register continuity between the two rings; if not, replace the rotor. Then check for continuity between each rotor slip ring and the rotor shaft. Your meter should register no continuity; if not, replace the rotor. If the rotor checked OK on these tests, polish the two slip rings using 400-grit sandpaper.
Inspect the three stator electrical terminals (see the Resources box at the bottom for more information). Using your ohmmeter, check for continuity between the center terminal and the one on the left. Then check for continuity between the center and right terminal. Your meter should register continuity in each test; if not, replace the stator. Now check for continuity between each terminal and the outer stator ring. Your meter should register no continuity; if not, replace the stator.
Some Ford alternators come with separate diodes mounted in the rear case. Check the resistance of each diode terminal and the case; then reverse the ohmmeter leads and repeat the test. You should get a high and a low reading for this test; if the two ohmmeter readings are too close to each other on any one of the diodes, replace that diode.
Diode Trio Test
On alternators with a diode trio, check the resistance between the side terminal and each one of the three terminals seating next to each other. Repeat the test reversing the ohmmeter leads. One of the readings should be higher than the other; otherwise, replace the diode trio.
Rectifier Bridge Test
Inspect the rectifier bridge, if your alternator is equipped with one (see the Resources box at the bottom for more information). The fins on one side of the rectifier are insulated, while the ones on other side are not. Check the resistance of the rectifier by probing one side of the rectifier and each one of the three clips in the middle of the bridge. Repeat the same test reversing the ohmmeter probes. One of your readings should be higher than the other; if not, replace the rectifier bridge. Now, repeat the same test, this time between the other side of the bridge and the three clips. One of your readings should be higher than the other; if not, replace the rectifier bridge.