Auto repair oscilloscope measuring automobile carbon canister solenoid valve signal waveform

The purpose of the fuel evaporative pollution control system is to prevent volatile hydrocarbons (HC) from evaporating and polluting the atmosphere. The fuel stored in the carburetor or fuel tank will evaporate during use. About 50% of automotive hydrocarbon emissions come from a faulty evaporative system. For this reason, today's fuel tanks are sealed and have a vent pipe to collect vapors in a carbon canister.

This canister is usually installed in the engine compartment. The canister contains activated carbon or activated carbon particles. The collected fuel is released from the canister into the intake manifold through a solenoid valve controlled by the control module. Most evaporative control systems reduce fuel vapor emissions by absorbing evaporative fumes into the canister when the car is idling on the road, stationary, or parked in strong sunlight. Once the engine reaches its normal operating temperature, the stored hydrocarbons are released into the intake manifold and become part of the combustible air/fuel mixture. This allows the evaporated hydrocarbons to be burned in the engine instead of being released into the atmosphere.

The solenoid valve is controlled by the ECM. When the engine is turned off or idling, there is no vacuum signal in the cutoff diaphragm chamber, so the evaporated fumes in the carbon canister are prevented from being released into the intake manifold. When the engine is running above idle speed, there is a relatively high vacuum in the signal line. This causes the vacuum cutoff valve to lift off its seat, allowing fresh air to enter the bottom of the carbon canister through the center pipe. The air then spreads out at the bottom of the carbon canister, rises through the carbon canister outlet, and pushes the hydrocarbon vapors out through the cutoff valve and into the intake manifold.

Next, let's take a look at how to use an oscilloscope to measure the working signal of the carbon canister solenoid valve:

The carbon canister solenoid valve has two connections, one is the 12V power line, and the other is the ground line for on-off. Connect a BNC to banana head cable to the oscilloscope's channel 1, connect a red needle to the red banana head of the BNC to banana head cable, and connect a black alligator clip to the black banana head. Insert the red test needle into the wire of the carbon canister solenoid valve, and connect the black alligator clip to the negative ground. Then adjust the channel attenuation ratio of channel 1 to 1X, the vertical range to 2V/div, the time base to 50ms, and you can turn on the low-pass filter to 30KHz to reduce interference signals.

Some oscilloscopes have built-in auto repair package software, which can complete the relevant settings of the oscilloscope with one click, which is faster and saves debugging time.

After the connection is completed, start the engine and keep it at 2500 rpm for 2-3 minutes until the engine is fully warmed up and the fuel feedback system enters a closed loop. Turn off all auxiliary electrical equipment, put the car in the parking gear or neutral position, jack up the drive wheel and observe the carbon canister solenoid valve signal waveform on the oscilloscope.

Under certain conditions, the engine control module (ECM) controls the switching of the negative circuit ground to activate the solenoid valve. This switching action can be seen on the sample waveform. This waveform is only seen when the engine reaches normal operating temperature, the throttle is open, and the engine speed rises to cruising speed. When the car reaches a certain speed, the engine computer controls the opening of the solenoid valve through a duty cycle signal. When decelerating, this signal stops and the valve closes.