Rectifier Short Or Ground Test

1. Using an ohmmeter set to X1 range, connect one ohmmeter probe to alternator rectifier "B+" terminal. Connect other ohmmeter probe to STA terminal. See Fig 1. Reverse probes and repeat test.
2. Ohmmeter should indicate approximately 6.5 ohms resistance in one direction and infinity when probes are reversed. Same reading in both directions indicates bad positive diode or shorted radio suppression capacitor.
3. Perform same test using STA terminal and rear alternator housing. Same reading in both directions indicates bad negative diode, grounded stator winding, grounded stator terminal, or shorted radio suppression capacitor.
4. If, in preceding tests, no needle movement occurs in one direction, or high resistance (considerably more than 6.5 ohms) is indicated in other direction, rectifier is defective.

Fig 1: Alternator Plug Terminal ID

Courtesy of FORD MOTOR CO.