Outdoor Power Equipment, Small Engine, Lawn Mower Repairs, Troubleshooting and Safety

Float-type switch with sensor module. The switch wire(s) pass through the side of the crankcase from the inside, above the oil level. The switch itself is mounted inside the crankcase, at the base of the oil reservoir. It is necessary to remove the crankcase cover to service the switch.

If there are two wires coming out of the engine (see inset), one goes to ground. Otherwise, the switch is grounded internally (when oil level is low).

Wiring diagram for float-type switch with sensor module.

The yellow wire from the oil level switch is connected to the sensor (oil alert). The black wire of the sensor is connected to the primary winding of the ignition coil and to the engine run/stop switch. The sensor passes a small part of the voltage from the coil to the oil level switch. If the switch closes due to low oil, the ignition primary is grounded to the engine block through the sensor's casing and mounting screw.

Some engine applications may be equipped with a low oil pressure switch. The switch is a normally closed type, but is held open by engine oil pressure during cranking and running. Should engine oil pressure drop below approximately 10 psi for any reason, the switch contacts will close. With the switch contacts closed, the following will occur:
1. A low oil pressure indicator will turn on and glow as power is available for its operation.
2. Switch contacts closure will connect the primary ignition circuit to ground, ignition will terminate, and the engine will shut down.

Low oil indicator light used with some pressure switch systems.

There is a low pressure shutdown module located inside the engine shroud near the indicator light which senses when the pressure switch closes, grounds out the ignition coil, and powers the lamp while the engines coasts to a stop.

Generac low oil shutdown delay module

There is a low pressure shutdown module located inside the engine shroud near the indicator light which senses when the pressure switch closes, grounds out the ignition coil, and powers the lamp while the engines coasts to a stop.

Low oil pressure shutdown wiring diagram.

In this diagram the low oil pressure switch (LOP) and the engine on / off switch are connected to ground by a black wire. Commonly the LOP is grounded internally and the engine switch is grounded separately.

The low oil shutdown module (sensor) is commonly grounded through its case and mounting screw, rather than by wire as is shown here. The LOS is connected to the primary winding of the ignition coil and to the engine switch. When oil pressure is low the pressure switch closes, making a path to ground. The module senses this condition and grounds the ignition coil to the engine block, stopping the engine. Typically, the module waits about 10 seconds before shutting down the engine. This allows time for pressure to build when the engine starts up.

Diagnostic flow chart for pressure-type switch with delay module.

CHECK OIL PRESSURE SWITCH

If the engine cranks and starts, then shuts down almost immediately ( within around 10 seconds ), the cause may be one or more of the following:

• 1. Low engine oil level.
• 2. Low oil pressure.
• 3. A defective oil pressure switch.

LOW OIL SWITCH

Testing the Low Oil Pressure Switch

• 1. Check engine crankcase oil level. If necessary, add the recommended oil to the dipstick FULL mark. Do not overfill above the full mark. Also, check if the oil has been thinned due to gasoline leaking past the carb float and into the crankcase. If so, it may flow as thin as water. Disconnect wire(s) from the oil pressure switch terminals. Remove the switch and install an oil pressure gauge in its place.
• 2. Start the engine while observing the oil pressure reading on gauge. Normal oil pressure is approximately 35-40 psi with engine running. If oil pressure is below about 10 psi, shut engine down immediately. A problem exists in the engine lubrication system or the oil is too thin. The oil pressure switch is rated at 10 psi for single cylinder engines.
• 3. Remove the oil pressure gauge and reinstall the oil pressure switch. Do NOT reconnect wires to the switch terminals. Connect the meter test leads across the switch terminals. With engine shut down, the meter should read CONTINUITY. Start the engine. After several seconds, the meter should read INFINITY.
• 4. If the LOP switch tests good in Step 3 and oil pressure is good in Step 2, but the unit still shuts down with a LOP fault, check all wiring connections between the START-STOP-RUN switch and the LOP pressure module and the LOP sender for a short to ground. Any ground on this wire will cause the ignition coil to receive a ground also inhibiting spark from occurring. If a short to ground is found replace the respective wire.

RESULTS:

• 1. If LOP switch, oil pressure and wiring all test good, replace the oil pressure module (sensor).
• 2. If the LOP switch failed, replace the switch.
• 3. If no pressure was measured, an internal failure of the oil pump may have occurred.

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