Cummins/Onan — Final Check-Out Procedure
Testing & diagnosis · starting system
From a Cummins/Onan service manual covering: 3.0 AJ, 4.0 BFA, 5.0 BGA, 6.5 NH, T-012. Check it applies to your exact model before use.
Applies to: Cummins/Onan 4.0 BFA
Procedure
- Remove spark plug wires so set will not start when cranked.
- Connect voltmeter to battery terminals, battery side of start solenoid, and set frame.
- Crank engine and measure voltage at battery.
- Then measure voltage from start solenoid to set frame; do not allow more than five seconds between readings.
- Subtract voltage at set from measured battery voltage during cranking; difference should not exceed 0.6 volt.
- If difference exceeds 0.6 volt, check all connections; a separate ground cable may be needed.
- Alternate test: remove spark plug wires, crank engine and measure cranking speed.
- Connect separate ground cable from battery to set frame, crank engine and again measure speed.
- If cranking speed increased more than ten percent, check connections and consider permanent ground cable.
Tools required
Tool links are Amazon searches (affiliate links); manufacturer special tools may only be available from a dealer.
Specs & values
- Difference should not exceed 0.6 volt
- Cranking speed increase threshold: 10 percent
Full service-parts list: Cummins/Onan 4.0 BFA →
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.