Cummins/Onan B43 / B48 — Flywheel Alternator Output Test
Testing & diagnosis
For the Cummins/Onan B43 / B48, from the Cummins/Onan service manual.
Applies to: Cummins/Onan B48
Before you start
- Reversing positive and negative battery connections or allowing engine to run without being connected to the alternator will result in engine electrical system damage.
Procedure
- With engine running, check battery terminal voltage (regulator output) using a DC voltmeter; should be 13.6 to 14.7 VDC.
- If voltage is greater than specified replace regulator-rectifier; if less, proceed to step 2.
- Examine all wires for loose, corroded, broken connections, short circuits; check fuses; repair as needed.
- Disconnect plug from regulator-rectifier and test AC voltage at plug with engine running near 3600 rpm.
- If AC voltage reads more or less than specified, proceed to step 4; if AC voltage is as specified but DC voltage low, replace regulator-rectifier.
- Use Rxl scale on ohmmeter to detect opens in stator (unit not running); refer to Table 1 for resistance specifications.
- If resistance is not as specified, replace stator; if windings are grounded the stator must be replaced.
Tools required
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- Battery voltage 12 VDC
- Regulator output 13.6 to 14.7 VDC
- 15A system stator AC voltage 40 VAC minimum, 60 VAC maximum
- 20A system stator AC voltage 31 VAC minimum, 51 VAC maximum
- 15A stator resistance 0.1 to 0.2 ohms
- 20A stator resistance 0.3 to 0.5 ohms
Full service-parts list: Cummins/Onan B48 →
Source: Cummins/Onan service manual. Steps and values are as documented; always cross-check against the official manual and any later revisions for your exact serial number before servicing.