Cummins/Onan 3A1.7 Engine Engine

• Voltage is unstable, high or low
  Fix: Push to reset breaker. Does generator AC output voltage build up? If no, flash exciter field per TEST [D]. Does generator output voltage build up?
• Residual voltage across terminals 2 and 3 of VR21 is less than 5 to 10 VAC
  Fix: Disconnect stator leads from leads 1 and 2 from AC across the leads. Is residual voltage 4 and 5,5 VDC or more now? Check reactor CMR21 and transformer T21 per TESTS (B) and (E). For shorts per TEST [MI.
• NO AC OUTPUT VOLTAGE AT RATED ENGINE RPM
  Fix: Check lead continuity between AC terminals on A1 and terminals 2 and 3 on VR21 per TEST [Ml. Replace defective field breaker. Remove one lead from breaker and check continuity with ohmmeter. Is breaker open? Is control panel field breaker CB21 on?
• Engine starts but stops after running several seconds.
  Cause: A fault condition indicated by fault breaker CB12 on control panel or DC control breaker trips when the generator set is started.
  Fix: Check for a possible fault condition. If none, check fault monitors S1 through S5 and fault breaker CB12 for improper wiring. Check all B+ wiring for shorts to ground.
• Engine does not crank.
  Cause: Battery dead.
  Fix: Check battery per Checkout [A] and recharge or replace. Check battery charger operation per jumper battery cable B+ connection to 81 starter solenoid terminal. Does engine crank? If no, check battery cables for clean tight connections and motor-if bad, repair or replace. (ref. Checkout [B]). Check B1 starter solenoid With S11 in Start position, is battery voltage present between K11 terminal S and ground? If not, check B+_/wiring between K11 and B1 solenoid terminal. Replace if bad. If K11 does not energize, test per Checkout [D] and replace if bad. Is battery voltage present at B1 solenoid terminal with S11 in Start position. Check breakers CB11, CB13, or fuse F1. Check S11 and replace if bad. Check B+/wiring to S11 start terminal and K11 -S terminal.
• Engine cranks but does not start.
  Cause: Heater circuit operating improperly or incorrect fuel.
  Fix: Check heater solenoid K13 per Checkout [HI. Check diodes CR2 and CR3, wiring, heaters, and resistors START. Check fuel supply system: fuel tank level, shutoff valves, fuel lines and connections, fuel filters, transfer pump and injection blue-white, and fuel flow steady from fuel return line. Check fault reset breaker. Check B+ circuit through K11-I to Engine monitor board terminal All-1, All-CB12 Line, All-SW B+, to K1. Does K1 fuel solenoid energize when S l l is in Start position? Incorrect fuel? See Operator’s Manual recommendations.

Compiled from service & troubleshooting documentation across every generator set that uses the Cummins/Onan 3A1.7.

Pooled from every document that names this exact engine — confirm the interval and part number against your unit's serial number.