Rectifier Bridge

Connect one ohmmeter lead to grounded heat sink. Connect second ohmmeter lead firmly against flat metal portion of terminal (not threaded stud). See Fig 1. Reverse leads to same heat sink and terminal. If readings are identical, replace rectifier bridge.

Fig 1: Bench Testing Rectifier Bridge All Models
Courtesy of GENERAL MOTORS CORP.
A good rectifier bridge will give one high and one low reading. Repeat test between grounded heat sink and 2 flat metal connectors. Test between insulated heat sink and each of its 3 flat metal connectors.
Check remaining bridge rectifiers in same manner. DO NOT replace rectifier bridge or diode trio unless one pair of readings are the same.