Rectifier Bridge

Connect an ohmmeter to grounded heat sink, with second lead held firmly against flat metal connector (not threaded stud). See Fig 1 . Then reverse leads to same heat sink and connection. If both readings are the same, replace rectifier bridge.
A good rectifier bridge will give one high and one low reading. Repeat this same test between grounded heat sink and other 2 flat metal connectors. Then repeat same tests, this time between insulated heat sink and each of the 3 flat metal connectors. This makes a total of 6 tests with 2 readings each. Check other bridge rectifiers in same manner.
NOTE: Do not replace rectifier bridge or diode trio unless at least one pair of readings is the same. Also, never use 110 volt test lamp for this test.

Fig 1: Bench Testing Rectifier Bridge And Identification Of End Frame Components
Courtesy of GENERAL MOTORS COMPANY