How to Service an Onan 6500 Generator

by Gus Stephens

The Cummings Onan 6500 Model HGJAE is a gasoline powered open frame generator designed for commercial applications. At standard output, the 6500 produces 5 kilowatts of 120/240 volts AC power. Its maximum output is 5.5 kilowatts. The rated amperage at 120 volts is 4.6 and the 240 volt rated amperage is 20.8. The generator is driven by a 13 horsepower, 389 cc 4-stroke engine with a continuous run time of nine hours. Cummings specifies a routine service regimen for proper generator maintenance.

Checking the Oil

Check the engine oil every day of operation.

Place the generator on a level surface. Shut off the generator engine.

Lift the maintenance access cover and remove the oil fill cap/dipstick at the bottom front of the engine.

Wipe down the attached dipstick. Screw the oil cap/dipstick back in, remove again and read the oil level on the dipstick. Add the manufacturer's recommended viscosity of engine oil, if necessary, to bring the level up the "Full" mark on the dipstick.

Engine Oil and Filter Service

Change the engine oil and filter after the first 20 hours of operation and after every 150 hours of operation thereafter.

Start the generator engine. After allowing the engine to warm up, shut it off.

Put a pan under the oil drain hose and oil filter. These components are located underneath the generator, just behind the front edge.

Remove the oil fill cap. Open the oil drain valve located just below and to the immediate right of the oil fill cap. Allow the oil to drain completely into the pan and then shut the oil drain valve. Spin off the oil filter.

Apply a film of fresh engine oil to the new oil filter gasket, screw it on the oil filter adapter and hand tighten. Refill the engine crankcase with 2 quarts of oil.

12-volt Starting Battery Service

Check the 12-volt starting battery and cables every month.

Clean the battery case and terminals and make sure they are kept dry.

Remove the battery cables when necessary using a proper battery terminal puller. Remove the negative cable first and reconnect it last to prevent arcing.

Air Filter Element Service

Replace the air filter element after every 150 hours of operation.

Unlatch the three spring clips on the air filter cover located on top of the engine. Rotate the cover away from the hooks and remove it from the engine compartment. Remove the air filter element from the cover.

Clean the filter cover sealing surfaces. Place a new filter element into the filter cover. Reassemble the filter cover and secure it with the three spring clips.

Spark Plug Service

Replace spark plugs after every 450 hours of operation.

Remove the spark plug cable connector from the spark plug at the top of both cylinder heads. Remove the spark plug using a spark plug socket and socket wrench.

Hand-thread a new spark plug into the hole until it seats. Tighten the spark plugs with a torque wrench and spark plug socket to 10 foot-pounds.

Spark Arrestor Service

Clean the spark arrestor after every 50 hours of operation.

Locate the square-headed cleanout plug on the bottom of the top drum muffler; it is accessible from beneath the machine. Remove the cleanout plug with a 7/16-inch eight point socket wrench.

Start the generator engine and place it under electrical load. Run the engine near full power. Allow the engine to run for five minutes to clear soot from the muffler.

Stop the engine. Wait until the muffler cools and reinstall the cleanout plug.


  • check Follow the manufacturer's recommendation for proper oil viscosity according to expected ambient temperatures.

Items you will need

About the Author

Gus Stephens has written about aviation, automotive and home technology for 15 years. His articles have appeared in major print outlets such as "Popular Mechanics" and "Invention & Technology." Along the way, Gus earned a Bachelor of Arts in communications. If it flies, drives or just sits on your desk and blinks, he's probably fixed it.