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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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In this article I’ll show you what all the dials and indicators mean in the instrument cluster of Volvo’s EMS (Electronic Multiplex System) v2.0, the symbols, and what they mean. This dash is not just used on Volvo’s B7RLE/B7/9TL buses, it is also used on their range of coaches too, there may be differences, but the specifics are here. Their FH & FM range of trucks have the same indicators and dials, but the cluster layout is different.

A. Oil pressure gauge, engine

This gauge indicates the current engine oil pressure. The needle must not enter the red zone when the vehicle is being driven! This can sometimes be unavoidable on very hot days, but if it does, lower your revs as much as possible to lower the pressure and thus the temperature. The engine can be seriously damaged if oil pressure is lost due to an oil leak for example. This gauge gives a reading when the ignition key is in the drive position.

The LED in the lower left-hand corner lights when the engine control unit sends a low oil pressure signal, no matter what the gauge reads. The STOP exclamation indicator will light if the oil level becomes too low due to a leak, if it does, stop the bus and have it checked. If the bus is driven with a leak and the oil runs out, serious engine seizure and damage will result.

B. Temperature gauge, coolant

This gauge shows the engine coolant temperature. The same procedure above as for the engine oil should be followed.

C. Turbo pressure (boost) gauge

This needle shows how much boost the turbo is giving. Generally it will be higher under heavy acceleration and on hills, but you should not stress it too much otherwise the engine may go into limp home mode, shutting the turbocharger off to protect it from overheating.

D. Display (Infopanel)

This is where everything is displayed. Trip measurement, fuel economy, time/date. It also displays “Check diagnostics for xxx ECU” messages, along with the CHECK light (see J.). Failure icons and messages are also displayed, along with the STOP exclamation triangle (See I.). Info messages are also displayed, with the INFO indicator (See K.)

E. Tachometer

The tachometer displays the current engine speed in revolutions per minute (RPM) from 500 revs to 3,000 revolutions. Engine tickover (idle) speed is around 650rpm, and optimal gear upshift/downshift revs are between 1,000 and 2,000 revs. As always, observe the safety colur band in the rev ranges, and do not go over the yellow range maximum, otherwise engine damage will result.

F. Speedometer

The speedometer (speedo for short) displays the current total vehicle speed in km/h, some UK vehicles have it in mp/h, don’t get the two confused!

G. Fuel gauge

This gauge shows how much fuel you have left in the tank. This relates to single diesel tank fitted buses and trucks. For biodiesel and adblue vehicles, and Volvo vehicles fitted with dual diesel tanks there may be another separate dial. For adblue and biodiesel these are not normally Volvo fitted, but fitted by the body manufacturer or retrofit company to customer order (for buses) or the fleet company (for trucks). For information consult your vehicle documentation or builder.

H. Brake pressure gauges

These dials show the air pressure in the brake cylinders of the system. The pressure must not be allowed to get too low, otherwise brake failure will result. If it gets too low or high a Check Diagnostics warning is displayed in the Infopanel display, and a failure of the system will normally result in the STOP exclamation indicator coming on, in which case the vehicle MUST NOT be driven any further. Damage to the vehicle, other road users, passengers and yourself will result.

I. STOP exclamation indicator

This indicator will illuminate if a CRITICAL failure or other critical event happens, such as engine oil, coolant and/or fuel getting too low, an ECU or CAN/J1939 communications bus failure on the multiplex controller area network electrical system, brake failure, engine failure. If this indicator illuminates, STOP THE BUS IMMEDIATELY and have the cause rectified by an engineer. They, like myself, have the tools, knowledge and software to determine the cause by communicating with the vehicle electrical computer systems to find fault codes that tell us exactly where the fault is.

You MUST get the vehicle towed if this light comes on, for utmost safety. If you’re carrying passengers, make sure they are your first priority. Shut off the engine and use the emergency cutoff switch in the cab if you are worried about fuel or electrical fires or problems, it cuts both off safely. You’d be putting passengers, yourself and road users in extreme danger by driving a possibly seriously faulty vehicle that weighs over 18 – 30 tons by thinking the indicator is coming on for nothing, this is rarely the case. Volvo multiplex systems and control units are very very accurate. I can’t stress it enough.

J. CHECK warning indicator

This indicator comes on when there is a warning condition in one or more electronic control units (ECU’s) in the bus. They are not extreme failures, just friendly warnings that a service is needed. It will illuminate along with the following messages displayed in the Infopanel display:

1. “Check Diagnostics for Brake ECU at next stop”

This is the most common one you’ll see, and it means the brake wear sensors have detected the pads or discs are nearing their wear limit. It will also trigger when slight drops in pressure or leaks occur on the brake air system. Drops in pressure can be determined by ear. Listen to the discharge pattern when the system lets off steam regularly. The system discharges in two stages, two hisses about 2 seconds apart, every 5 or so minutes. If they are too long, too regular, the pressure may need to be looked at. Determine this by reading the pressure gauge in the dash as earlier in the article.

2. “Check Diagnostics for Engine ECU at next stop”

This message appears when a warning is triggered from the engine’s ECU. Situations include engine temperature sensor failure (look for the oil temp needle being cold when it shouldn’t), engine coolant sensor failure, tachograph sensor failure (for detecting RPM’s) and various others.

3. “Check Diagnostics for Light Control Unit at next stop”

The Light Control Unit controls all vehicle illumination and signalling equipment. Headlights, tail lights, indicators and brake lights. On buses it also controls gangway lighting, as they are wired into the main lighting system along with the destination board, normally wired in with the running lights (sidelights). The Light ECU monitors resistance across all lighting circuits, and any failing bulbs or LED brake/indicator clusters (used on Volvo buses with Wright Eclipse Urban/Gemini bodies) cause high resistance, so the ECU knows a light circuit has failed.

I am unaware if the Light ECU can trigger the STOP indicator.

K. INFO indicator (Can illuminate in conjunction with CHECK indicator)

This light comes on when the Infodisplay is set to Info messages in the display, such as fuel consumption etc, and a CHECK warning is displayed. Switch the display over using the control stalk on the steering column to view any CHECK messages and the INFO light will extinguish.

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