Control-O-Matic Onan Generators

These units were the staple of the industry for many years. Their low cost and durable operation, made them a good choice for many installations. They had a few features that could be persnickety, but if reasonably maintained they could last almost a life time.

Model Number Guide Chart Page
Note: Opens in new window.

Check the engine to make sure it has been filled with oil and fuel. Fill cooling system and prime water pump. If engine fails to start at first attempt, inhibitor oil used at the factory or during the winterize,  may have fouled the spark plugs -- remove, clean in a suitable solvent, dry thoroughly and reinstall. Heavy exhaust smoke when the engine is first started is normal, and is caused by the inhibitor oil.

NOTE: The Control-O-Matic will run the bilge blower for about
            5 minutes before cranking the engine. The blower cycle
            may be eliminated, see Control-O-Matic section.

Crankcase Oil:
Use a good quality detergent oil that meets the API (American Petroleum Institute) service designations MS, MS/DG. Oil should be labeled as having passed the MS sequence tests (also known as the ASTM G - IV sequence tests) and the MIL-L-2104B tests. Recommended SAE oil numbers for expected ambient temperatures are as follows:
Above 30oF      SAE 30w
OoF to 30oF     SAE 10w
Below OoF       SAE 5w-20

Do not use service DS oil. Do not mix brands or grades. Refer to Maintenance Section for recommended oil changes and complete lubricating oil recommendations.
Recommended Fuel: Use clean, fresh, regular grade automotive gasoline. Do not use highly leaded premium types. Never fill the tank when the engine is running. Leave some fuel expansion space. Open fuel line valve (when used).

The following shows the sequence of operation. This shows operation with manual control at the plant and automatic control, either at the plant or remotely located.


1. Operator pushes START button or plant is started by
remote control.

2. Start solenoid energizes.

3. Battery current flows to
a) series field
b) STOP relay
c) electric choke

4. Ignition coil fires spark plugs when breaker points open.
5. Engine starts
6. Operator releases START button
7. Start disconnect relay energizes
8. Engine continues running.


1. Operator pushes STOP button
2. Stop relay opens ignition circuit
3. Engine stops
4. Operator releases STOP button.

An emergency stop is caused by low oil pressure or high water temperature.

1. Low oil pressure switch closes. emergency relay energizes, opening ignition circuit.

2. High water temperature switch opens ignition circuit.
3. Engine stops.

NOTE: If shutdown is caused by high water temperature,
            plant will crank for about 90 seconds. If it does
            not start, it will stop cranking.

Allow plant to warm up before connecting a heavy load and keep the load within nameplate rating. Continuous generator overloading may cause high operating temperatures that can damage the windings

The battery charge rate is controlled by a charge regulator. This regulator is pre-set to allow the proper charge rate at operating speed. Do not attempt to change this setting.

A high-water-temperature switch in the cooling system stops the plant if the engine overheats.
The engine has a low-oil-pressure switch which will stop the plant through an emergency relay in the control if oil pressure drops below a safe operating pressure. After an emergency stop, investigate and correct the cause.

CONTROL-O-MATIC Operation (optional)
A 3 position toggle switch controls plant operation. The operator may choose either of two running procedures.

1. Move switch to RUN position. The bilge blower control will go through its cycle. The engine will then crank and start.

2. Move switch to AUTO position. The engine will start when the load demand circuit senses a need for current. This occurs after the bilge blower starts and finishes its cycle. The plant will stop when there is no load on the AC line

3. Move switch to STOP position. The plant stops. If stop circuit fails, close fuel valve.

The built-in emergency relay shuts down the plant for these conditions:

1. Engine fails to start. The engine will crank for approximately 90 seconds before the emergency relay shuts it, down.

2. Low oil pressure

3. High water temperature. The engine will stop, start cranking again for about 90 seconds; then stop, and  not attempt to restart.

To reset the emergency relay, remove the cause of the emergency. then push the red RESET button on the Control-O-Matic front panel.

The bilge blower may be connected to the Control-O-Matic in two ways. Fig. 15 shows the bilge blower connected to tile Control-O-Matic. The blower operates from the electric plant cranking batteries. The B+ and BB terminals are labeled in the Control-O-Matic Box. With this circuit, the running current of the blower must not exceed 5 amperes. The user may add switches as shown to operate bilge blower without operating the electric plant.

An auxiliary relay is used so that the bilge blower can operate from a separate battery or power source. A larger blower may be used, as long as the relay has a 12 volt DC coil and the relay contacts can safely carry the current needed by the blower.

The bilge blower circuit may he disabled so the plant can be started without delay. This requires changing one wire connection on the thermal delay relay, mounted on the inside bottom of the Control-O-Matic box. Fig. 17 shows this relay actual size. Take care with the following step, as the terminals posts can easily be broken. Lift the clip from the rear terminal post of terminal 1 and place it on the rear post of terminal 3. With the front-mounted toggle switch in the "AUTO" position, the plant will respond immediately when the automatic demand control senses a load. The bilge blower will run. The Control-O-Matic should not need attention, but if it does not function properly, see the Trouble-Shooting section for repair procedures.

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