Starting The Engine

Start sequence 

HINT: The start procedure and the components involved differ between the diesel engines and gasoline engines/flexifuel engines, and between engines with different number of cylinders. 

Courtesy of VOLVO CARS CORPORATION

The starter motor (6/25) is supplied with power via the starter motor relay (2/35), the starter motor relay is controlled by the Engine Control Module (ECM) (4/46).

In a system with a diesel engine, the glow plugs (20/22-26) activate in ignition position II. This is carried out by the Engine Control Module (ECM) sending a digital signal to the glow plug control module (4/109). Ignition is activated even at normal startup as below. See: Function - Glow Plug Control.

The start process is as follows: 

1. The start button is pushed (ignition position III).
2. Signal from the start button (3/1) reaches the Central Electronic Module (CEM) through a directly connected cable from the button.
3. The central electronic module (CEM) uses the button signal together with the signal from the clutch pedal switch (3/271) (manual gear) or brake light switch (3/9) (automatic gear) and generates a start request that is sent to the engine control module (ECM) via CAN communication.
4. The Engine Control Module (ECM) uses the received start request, and the signal from the directly connected cable from the start button, and activates the starter motor solenoid by grounding and powering the starter motor relay coil. Activation only occurs, however, if all the starting conditions are met, see: Conditions for starting below.
5. The relay closes the circuit between the starter motor solenoid and the fuse in the relay/fuse box in the engine compartment, activating the starter motor.

Conditions for starting 

Activation of the starter motor is not permitted or is interrupted if:

• the engine is running (the engine speed (RPM) above a certain value)
• the immobilizer function does not allow a start. For further information also see Design and Function, Immobilizer
• the clutch pedal is not depressed (manual gear)
• the gear selector is not in "P" or "N" (automatic transmission)
• the brake pedal is not depressed (automatic gearbox).

If the engine does not turn or if the engine speed is extremely low when the start relay is activated, the Engine Control Module (ECM) interrupts start relay activation.

Auto start 

If the start button is released before the engine started, the starter motor will continue to run, provided the following conditions are met. The starter motor runs until the engine is started or until a certain time is reached. How long the starter motor is allowed to run is dependent on engine temperature; longer time is allowed at low engine temperature.

The time for start is monitored by the Engine Control Module (ECM) and DTC is stored if exceeded.

Autostart is not permitted or is cancelled if a fault is detected on one of the signals that supply the Engine Control Module (ECM) with information about:

• the gear selector's position (automatic transmissions). The Engine Control Module (ECM) receives a signal indicating the position of the gear selector from the Transmission Control Module (TCM) (4/28) via both the CAN and from a directly connected cable between the Engine Control Module (ECM) and Transmission Control Module (TCM). Also see Design and Function, Transmission Control Module (TCM).
• the clutch pedal position (manual gearbox). Signal about the clutch pedal position is received by the Engine Control Module (ECM) from the directly connected clutch pedal connector (3/271), and the clutch pedal position sensor (7/123), connected to the central electronic module (CEM) (4/56), via the CAN.
• brake pedal position. Signal about the brake pedal position is received by the Engine control module (ECM) from the directly connected brake light switch (3/9), and via the CAN from the brake pedal switch (3/284), which is connected to the central electronic control module (CEM).

Normal start, also called semi start, when the button is held in during the entire start procedure, also works if one of the signals for the gear selector, clutch pedal or brake pedal is deemed faulty. In the event of a double fault, i.e. when more than one signal is deemed to be faulty, semi-start is also prevented for safety reasons.