Diagnostic Trouble Code (DTC) Detecting Criteria (H6DO): Overview

COMPONENT DESCRIPTION

Atmospheric pressure sensor is built in ECM.

Scheme 2054

Scheme 2054: ENABLE CONDITION

ECM OPERATION AT DTC SETTING

Memorize the freeze frame data. (For test mode $02)
Memorize the diagnosis value and trouble standard value. (For test mode $06)

Memorize the freeze frame data. (For test mode $02)
Memorize the diagnostic value and trouble standard value. (For test mode $06)

Pressure Control Solenoid Valve

Pressure control solenoid valve maintains the fuel tank pressure equal to the atmospheric air pressure. Normally, the solenoid is set to OFF, and the valve mechanically opens and closes in accordance with the difference between the tank pressure and atmospheric air pressure, and the tank pressure and canister pressure.

During the diagnosis, the valve is forcibly opened by setting the solenoid to ON.

Scheme 2055

Valve Operation and Air Flow

As in the X parts below, there are the area with atmospheric air pressure above the diaphragm and the area with tank pressure below the diaphragm. Also, as in the Y parts below, there are the area with tank pressure above the diaphragm and the area with canister pressure below the diaphragm. In the table below the air flow from each port in accordance with pressure difference is shown with the atmospheric air pressure port A, tank pressure port B and canister pressure port C.

Scheme 2056

Scheme 2057

Scheme 2058

Scheme 2059

When A < B (Solenoid OFF)
When B < C (Solenoid OFF)
When solenoid is ON
Drain valve

Drain valve controls the ambient air to be introduced to the canister. Normally the solenoid is set to OFF and the valve is open. The valve is closed when the solenoid is set to ON only at the time of diagnosis.

Scheme 2060

Tank Pressure Switching Solenoid

Fuel tank sensor control valve is connected to fuel tank pressure sensor and opened to atmosphere. A normal condition, fuel tank sensor control valve is OFF, and circuit of fuel pressure sensor is open to atmosphere. If fuel tank sensor control valve is ON, then circuit is closed.

Scheme 2061

Purpose of this solenoid

Fuel tank pressure sensor detects the difference between the atmospheric air pressure and the tank pressure, and the ECM monitors the pressure difference.

Even if the tank pressure is constant, the atmospheric air pressure varies depending on the driving height and the pressure signal transmitted to ECM will change.

Scheme 2062

Especially, in the small leakage [0.5 mm (0.02 inch)], minute change in the tank pressure has to be detected. This diagnosis period is long (approx. 29 seconds). And if the driving height changes during the diagnosis, he atmospheric air pressure changes. In this case, it becomes difficult to precisely detect the tank pressure variation, causing erroneous diagnosis. Therefore, using the atmospheric pressure switching solenoid, atmospheric air is sealed between the fuel tank pressure sensor and atmospheric pressure switching solenoid, maintaining the air pressure constant and enabling the detection of minute variation of tank pressure.

Scheme 2063

Note. ECM also has the atmospheric air pressure sensor, and always monitors atmospheric air. However, as the monitoring range is large, that is, 53.3 to 107 kPa (400 to 800 mmHg, 15.7 to 31.6 inHg), it is not suitable for detection of minute pressure variation. In the case of small leakage diagnosis, the tank pressure variation is very small, that is, 0.13 to 0.26 kPa (1 to 2 mmHg, 0.04 to 0.08 inHg), and the fuel tank pressure sensor is equipped.

Memorize the freeze frame data. (For test mode $02)
Memorize the diagnostic value and trouble standard value. (For test mode $06)