Cummins/Onan GV12 Generator Set

Tools: feeler gauge

1. Bar engine to No. 1 VS mark; both valves closed.
2. Loosen locknut and back off adjusting screw.
3. Insert feeler gauge between rocker lever and valve stem or crosshead.
4. Turn screw down until lever just touches gauge.
5. Tighten locknut to 70/80 foot-pounds.
6. Final adjustment at engine operating temperature.
7. Repeat for each cylinder per firing order.

Tools: wire gauge

1. Loosen crosshead adjusting screw locknut and back off one turn.
2. Hold crosshead in contact with valve stem B with light finger pressure at 1/4 inch.
3. Turn down adjusting screw until it touches valve stem C.
4. With new crossheads, advance screw additional 20 degrees.
5. With worn crossheads, advance up to 30 degrees.
6. Tighten locknut to 25/30 foot-pounds.
7. Check clearance between crosshead and valve spring retainer; minimum 0.020 inch.

Tools: timing light

1. Crank engine until rotor arm centered in timing window and timing marks indexed for No. 1 cylinder.
2. Rotate Mag-Tronic drive until rotor arm centered in timing window.
3. Position Mag-Tronic to drive assembly, engaging drive coupling; secure capscrews loosely.
4. Plug wiring harness into Mag-Tronic.
5. Attach timing light to No. 1 cylinder ignition transformer.
6. Start engine; rotate Mag-Tronic to obtain correct timing.
7. Secure Mag-Tronic in desired position.

⚠ Never use a by-pass filter in place of a full-flow filter.

1. Remove drain plug; drain oil.
2. Remove clamping ring capscrew; lift off cover.
3. Unscrew hold-down; lift out element(s); discard.
4. Clean housing and hold-down in solvent.
5. Inspect spring and seal; replace if damaged.
6. Install new element(s) in housing.
7. Replace cover O-ring; install cover and clamping ring.
8. Add oil; loosen vent plug; start engine; close vent when oil reaches vent.

1. Remove drain plug from filter case; drain oil.
2. Loosen center bolt or remove capscrews; remove filter case.
3. Withdraw filter element, inspect, discard.
4. Remove seal ring from filter head; discard.
5. Clean filter case thoroughly.
6. Install new element over pilot and/or valve assembly.
7. Position new seal ring; assemble; tighten center bolt to 30/35 foot-pounds.
8. Check oil level; run engine and check for leaks.

⚠ Do not pry belts on with a tool; do not roll tight belts over pulleys.

Tools: ST-968 Belt Tension Gauge

1. Tighten belts until 90 to 110 pounds on ST-968 Belt Tension Gauge.
2. If gauge unavailable, use finger pressure to deflect belt per Table 3-3.
3. Replace pairs of belts in complete sets.
4. Pulley misalignment must not exceed 1/15 inch per foot between pulley centers.
5. Do not allow belts to bottom in pulley grooves or protrude over 3/32 inch above groove.
6. Re-tension new belts after running one hour or more.

1. Check all pipes and fittings for leaks; tighten as necessary.
2. Remove cover plate to drain pump.
3. Lift out rubber impeller; check for cracks, breaks, damage.
4. Clean out sediment; install new impeller if necessary.
5. Install new cover plate gasket; use 0.015 inch gasket for proper clearance.
6. Install cover on pump.

⚠ Do not concentrate air pressure in one spot.

Tools: compressed air

1. Wipe dirt from cover and upper portion.
2. Loosen clamps; remove cover.
3. Unscrew wing bolt; remove element carefully.
4. Remove dust cap and clean.
5. Tap element side or bottom ring with palm or soft mallet.
6. Blow out element from clean-air side with compressed air, maximum 100 psi.
7. Wash element with non-sudsing detergent and warm water 120-140°F; dry with 40 psi air.
8. Inspect element for holes; replace if damaged.

⚠ After engine runs, add oil to compensate for filter and oil cooler absorption.

Tools: priming pump

1. Fill crankcase to L mark on dipstick.
2. Remove pipe plug from oil filter head (GV12) or oil pump (GNH).
3. Connect priming pump line to oil plug boss.
4. Prime until 30 psi minimum pressure.
5. Crank engine with ignition OFF at least 15 seconds, maintaining 15 psi minimum.
6. Remove external oil supply and replace plug.
7. Fill crankcase to H mark with specified oil.
8. Check oil level after running; add oil as needed.

⚠ Do not crank continuously more than 30 seconds; wait 1 to 2 minutes before recranking.

1. Push Reset button if overspeed stop equipped.
2. Set throttle for idle speed.
3. Disengage driven unit or open main disconnect.
4. Open gas supply shut-off valve.
5. Turn on ignition switch; press starter or stop-run-start to start.
6. Hold start until engine starts and oil pressure comes up to normal.

1. Run at 800 to 1000 rpm for 4 to 5 minutes or until water reaches 140°F.
2. Operate at partial load at approximately 75% governed rpm until water reaches 160°F to 165°F.
3. Apply load gradually until oil temperature reaches 140°F.

⚠ Remove transformer cover with care; transformer assembly is spring loaded.

Tools: 13/16 inch deep-well rubber insert spark-plug socket

1. Remove ignition harness wire from transformer terminal.
2. Remove screws securing transformer cover, retainer, spring, ground wire; lift off cover.
3. Lift transformer, spring, retainer, cover from spark plug adapter tube.
4. Use 13/16 inch deep-well rubber insert socket; remove spark plug and gasket.
5. Check spark plug gap; should be 0.018 inch.
6. Clean adapter seating area; position new gasket; tighten spark plug to 28 to 30 foot-pounds.
7. Insert transformer spring and retainer; secure cover and ground wire.
8. Connect ignition harness lead to transformer terminal.