Full-field testing is not possible on some alternators with internal voltage regulators. To isolate and test the regulator on these types of alternators, the alternator must be removed from the truck and disassembled. Other internally regulated alternators have a hole through which the field circuit can be tested. The test will require the use of a short jumper with insulated clips and a stiff paper clip wire or a 1/32-inch drill bit.
1. With the engine off and all electrical accessories turned off, measure the voltage across the battery terminals. Make a note of the reading.
2. Start the engine and run it at the speed necessary to generate full alternator output.
3. Connect a short jumper to the alternator negative output terminal and to the shank of the straightened clip wire or drill bit.
4. Insert the wire or bit into the full-field access hole as far as it will go and make a note of the voltage reading. If a fault in the regulator or diode trio is causing the undercharged condition, the alternator output should climb to within 10 percent of its rated output.