Toyota Celica, Corolla, ECHO & MR2 1999-05

General Information


This section describes the operating principles of sending units, warning lights and gauges. Sensors which provide information to the Electronic Control Module (ECM) are covered in Section 4 of this manual.

Instrument panels contain a number of indicating devices (gauges and warning lights). These devices are composed of two separate components. One is the sending unit, mounted on the engine or other remote part of the vehicle, and the other is the actual gauge or light in the instrument panel.

Several types of sending units exist, however most can be characterized as being either a pressure type or a resistance type. Pressure type sending units convert liquid pressure into an electrical signal which is sent to the gauge. Resistance type sending units are most often used to measure temperature and use variable resistance to control the current flow back to the indicating device. Both types of sending units are connected in series by a wire to the battery (through the ignition switch). When the ignition is turned ON , current flows from the battery through the indicating device and on to the sending unit.

The ECM contains a built-in, self-diagnosis system which detects troubles within the engine signal network. Once a malfunction is detected, the Malfunction Indicator Lamp (MIL), located on the instrument panel, will light.

By analyzing various signals, the ECM detects system malfunctions related to the operating sensors. The ECM stores the failure code associated with the detected failure until the diagnosis system is cleared.

The MIL on the instrument panel informs the driver that a malfunction has been detected. The light will go out automatically once the malfunction has been cleared.