The following functions are handled by the chassis control unit (CIM) on Volvo’s B7R coach, B7RLE low entry city bus, and B9TL double decker bus. Any faults usually result in a CHECK lamp illumination or a warning icon illumination, accompanied with a “Check diagnostics for Chassis Control Unit at next stop” message in the driver infodisplay.

Brake lights

The control unit controls the body’s brake lamp with the help of a retarder signal and a brake lamp signal from the bus Vehicle Electronic Control Unit (VECU).

Idle

The chassis control unit’s idling function signals to the body when the accelerator pedal is in idling position.

Kneeling

The kneeling function works together with the electronically controlled suspension (ECS) and activates a lamp on the instrument panel when the bus kneels. The function also activates a CHECK warning when the ECS indicates that pressure is too low.

Buzzer

The buzzer function controls the external buzzer in the dashboard which is connected to the chassis control unit (CIM). The buzzer is activated on request from the body and from the internal CIM functions. The buzzer sounds when CHECK messages or other info is displayed in the instrument panel.

Check of battery voltage level

The function senses when the bus’s batteries (2x12v batteries, each with an alternator) voltage is less than 22 Volts and then shuts down all voltage users that have low priority. When battery voltage hits less than 16 Volts an internal signal is generated for low battery level, and with voltages exceeding 36 Volts an internal signal is generated for high battery level. These internal signals are used for opening and closing certain inputs and outputs. The signal for low battery level also closes the normal signal transmission on the J1587/J1708 link. Low battery voltage will result in erratic engine starting and other electrical failures.

Retarder control

In normal operation the retarder (engine brake, known as JAK, JaKob’s engine brake) will only accept a number of preset braking torques. In those cases when the vehicle control unit (VECU) wants the retarder to brake with a torque which is not preset, this limiting function must be disconnected. This is achieved by activating an earth signal on the retarder control unit’s inputs.

Brake pressure

The brake pressure function reads off the air pressure on the front and rear brake tanks respectively, and controls the corresponding gauge on the instrument panel via data link J1587/J1708. The function also activates an alarm on the instrument panel when insufficient pressure is shown.

Vehicle speed

The function gives three output signals when the vehicle’s speed exceeds 5 km/h, 20 km/h and 95 km/h. The function also controls a frequency-modified signal with constant pulse width which represents the vehicle’s speed. (This signal is known as the C3 signal.)

Starter motor

The chassis control unit checks that the requirements for starting are fulfilled and then activates the starter motor’s relay.

Dimmer

This function reproduces the rheostat level which is given from the instrument panel to be used in the body instrument’s lamps.

Parking brake

The parking brake function gives a signal indicating the parking brake’s status to the body system when the parking brake is engaged. The body system uses the status signal to activate an alarm if the door to the driver’s cab is open when the parking brake is disengaged. The chassis control unit (CIM) also activates the parking brake lamp on the driver’s instrument panel.

Fire alarm

If a fire is discovered in the engine bay or in the bus body, a fire alarm is activated on the instrument panel. The fire alarm also activates the instrument cluster buzzer.

Main switch

The vehicle’s main electrical circuit cannot be permitted to close down in an uncontrolled way, i.e immediately. After the ignition is switched off certain functions must be provided with power for a certain period of time. The gearbox takes ten seconds to close down, which is the longest closing-down time. For this reason, the chassis control unit’s holding circuit function releases the holding circuit ten seconds after the ignition has been switched off and leaves the switching off of the main circuit to the main circuit’s switch.

Steering wheel adjustment

This function checks that the parking brake is applied and that the ignition is on before it permits adjustment of the steering wheel and instrument panel.

Fuel level

The fuel level function receives signals from the fuel level sensors and controls the fuel level gauges on the driver’s instrument panel. The function also sends a warning to the instrument panel when insufficient fuel level is recorded.
The calculation of the fuel level is determined by how the jumpers are set, (so that the function can be adapted to different fuel tanks). In those cases in which there are two fuel tanks, the jumpers decide the relationship between the sizes of the tanks. If there is only one tank, the jumpers give the setting the shape of the tank. (The jumpers can only be set to 15 V, earth or not be connected at all).

Fuel cut-off

This function controls the fuel cut-off’s solenoid valve and retains its position until the valve needs closing. A pulse for closing the valve is sent when the emergency switch is activated or when the main switch is switched off. The valve is opened when the emergency switch is switched off and the main switch is active or if the main switch is activated while the emergency switch is switched off. If both are activated at the same time, the emergency circuit breaker overrides.

Bogie

The bogie function checks the air pressure in the bogie axle’s air suspension bellows and activates a lamp on the driver’s instrument panel when the air pressure is released. This funtion only applies to tri-axle B7R coaches with a double rear axle (bogie). Most B7RLE and B9TL doble deckers have one front and one rear axle. The inner-most axle of a bogie can be raised or lowered using the air suspension.

Alternator

The chassis control unit (CIM) senses the status of the alternators, illuminating icons on the instrument panel if any of the alternators are not charging. This warning is shown before the engine is started in order to ensure that the function works. If the alternator lights do not go out once the engine is running, check the alternator(s) drivebelt, or the regulators on the alternators for any faults. If the battery voltage in the infodisplay is less than 22v but the batteries are fine, there is a fault in one or more alternator regulators.

Engine start/stop

The chassis control unit (CIM) handles the start signal to the vehicle control unit (VECU) and checks that established requirements for starting are fulfilled.

Temperature

The temperature function reads from an inside sensor and an outside sensor and sends the signals to the instrument panel where the temperatures are shown on the infodisplay.

Gearbox

In those cases where the vehicle is equipped with MGS or EGS, the gear position function signals to the instrument panel if the gearbox is in high gear. The function also informs the instrument panel if the EGS gives a warning signal.

Open door

The open door function senses whether a door is open and, if so, lights up a symbol on the instrument panel, sounding the instrument cluster buzzer all the time the door is open. Once the open door is closed, the buzzer turns off. The open-door function is used with a conventional electrical system in the body. It monitors doors such as the emergency exits, cab door (not all bodies support this) and the bonnet. On some vehicles the passenger entry door is linked to this so the bus cannot be moved if it is open or has a faulty potentiometer.

On the B7RLE and B9TL, a faulty potentiometer or potentiometer track switch on the passenger door usually results in the STOPPING sign not extinguishing once the doors open. It should go out when the doors open, not when they close.

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