The following is an partial extraction from the Info 2000 Data CDROM.

Canister purge valve

Function:

The canister purge valve forms part of the emission control system. Petrol vapor from the petrol tank that is stored in the carbon absorption filter, is conveyed to the engine through this solenoid valve.
If the lambda control of the engine is functioning, the valve is powered by the control unit. The extra fuel in form of vapor that is conveyed through this system to the engine is compensated for by the lambda control.

Electrical control:

Generally the canister purge valve is closed normally. This means that the valve breaks the connection between the intake manifold and the active carbon filter, if no current is running through the valve. The activation of the canister purge valve occurs by means of the control unit connecting the valve to ground. During this time the voltage of the terminal from the control unit that connects the canister purge valve, amounts to 0 Volt. Frequently triggering takes place with pulses. The pulse ratio (duty-cycle) then depends on engine speed, engine temperature and engine load, among other things.

Specification:

Electrical diagnosis:

STATIC CHECK:

The canister purge valve is checked by carrying out the following steps:

• Resistance measurement. Turn ignition off. Remove the connector of the canister purge valve and measure the resistance across the two terminals of the coil. The resistance (depending on the type of valve) can vary from ± 15 ohm to ± 75 ohm.
• Checking the wiring. Turn ignition off. Remove the connector of the canister purge valve and the plug of the ECU. Measure the resistance between the terminal in the connector of the canister purge valve and the corresponding terminal in the plug of the ECU. The resistance should be less than 1 ohm. If not, check the wiring and connectors and repair them, if necessary.
• Voltage check. Turn ignition off. Remove the connector of the canister purge valve. Start the engine and measure the voltage on the positive terminal of the connector. Check if the supply voltage is present. If the supply voltage is not present, check the relay, and if it is present, check the fuse. Also check the wire connection between the relay and the canister purge valve.

DYNAMIC CHECK:

An oscilloscope is used to check the signal of a canister purge valve.

• Figure 1 shows the voltage image during triggering of the canister purge valve by the ECU. Connect one measuring pin of the oscilloscope to the signal wire of the canister purge valve, and the other to the ground of the battery. Start the car and check if the canister purge valve is triggered with pulses. Duty cycle triggering frequency differs per ECU. Triggering of the valve can also depend on engine temperature, engine speed and engine load, among other things.
• There are also systems whereby the valve is powered continually.

Figure 1: Example of an oscilloscope image of a ground-connected canister purge valve.
During the time that the ECU does not power the valve, the voltage amounts to 12 V.
During the time that the ECU powers the valve, the voltage amounts to 0 V.

Mechanical diagnosis:

• Visual inspection. Check the connectors and wiring for damage. Check the hose connection between the canister purge valve and the intake manifold, and between the canister purge valve and the carbon filter for damage. Check the canister purge valve for mechanical breakage. Repair these if necessary.
• Mechanical check. Remove the connector from the canister purge valve and check if the connection between the two connectors from the valve is closed. Connect 12 V on the canister purge valve and check if the connection is open.

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