Blocking AUTOSTOP Cause, Battery Monitoring

Voltage in battery too low 

The following causes block autostop:

• The voltage in the battery, measured by the battery monitoring module, is too low. Can be caused by extremely large connected power consumers such as ventilation fan, seat heaters, demister, radio, extra equipment (e.g. in the 12 V socket).

Checks:

• Check the battery's charge status according to the applicable procedures. If necessary, charge the battery.

Charge level of battery too low 

The following causes block autostop:

• The charge level of the battery, measured by the battery monitoring module, is too low. Can be caused by, with the engine off (and in autostop), large connected power consumers such as lighting, ventilation fan, radio, extra equipment (e.g. in the 12 V socket).

Checks:

• With the engine off (and in autostop), check for large connected power consumers such as lighting, blower fan, radio, extra equipment (e.g. in the 12 V socket).
• Check the battery's state of charge, see (Service Functions/Diagnostic Sequences/Vehicle's SoC), voltage and current history.

Battery capacity too low 

The following causes block autostop:

• The battery capacity, measured by the battery monitoring sensor, is less than 35 Ah. May be caused by a reduction in the battery capacity.

Checks:

• Check the battery's charge level according to applicable procedures.

HINT: In certain conditions, the battery monitoring module may undervalue the battery's capacity. Try resetting the battery monitoring module. See (Diagnostics/Components/Central Electronic Module (CEM)/Diagnostic Sequences/Resetting information about power supply), when replacing the battery. Then hand the vehicle over to the owner. On future drives the module can again calculate the capacity of the battery. If the vehicle comes back with the same fault, this indicates that the battery capacity is actually too low. 

Battery capacity too low during cold starts 

The following causes block autostop:

• The battery capacity during cold starts, measured by the battery monitoring sensor, is less than 6 V. May be caused by increased resistance in the circuit for the battery leads or by a reduction in the battery capacity.

Checks:

• Check the battery poles, wiring and connections to ground and the starter motor. Check for contact resistance and oxidation. Also check that the connections on the battery are correctly attached and are in good contact with the poles.
• Check the battery's charge level according to applicable procedures. Charge if necessary.
• In certain conditions, the battery monitoring module may undervalue the battery's capacity. Try resetting the battery monitoring module. See (Diagnostics/Components/Central Electronic Module (CEM)/Diagnostic Sequences/Resetting information about power supply), when replacing the battery. Start and stop the engine at least 10 times. To prevent the risk of overheating the starter motor, wait a while between each start. Each time the engine is started, the sensor calculates the battery capacity again during cold starting. If the same fault recurs, this indicates that the battery capacity is actually too low.

Voltage in secondary battery too low 

The following causes block autostop:

• The voltage in the secondary battery, measured by Central Electronic Module (CEM), is too low. May be caused by no power consumer being connected directly to the secondary battery's poles or in its circuit.

Checks:

• Check the secondary battery's pole voltage. See (Service Functions/Diagnostic Sequences/12 V current), overview. The voltage must be above approx. 12.5 V. If the voltage is too low, measure the battery's voltage with a multimeter on the secondary battery's poles. If the voltage is then correct, there is too much contact resistance in the circuit. Check the secondary battery circuit's wiring and its connections as well as the fuses in the circuit. Check for contact resistance and oxidation. If the voltage is the same for both readings, charge the battery. The secondary battery is best charged via the vehicle's charging system whilst driving. When using the battery charger to charge, the battery charger must only be connected directly to the secondary battery's poles. It is of the utmost importance that the battery charger is correctly connected as connecting to the wrong pin can damage the Engine Control Module (ECM) and that charging occurs with a low current so that the secondary battery is not overcharged. If the battery does not hold the correct pole voltage after charging, this means that there is something wrong with the battery.
• Then hand the vehicle over to the owner. On future drives using autostop, the Central Electronic Module (CEM) can resume measuring the battery's voltage. If the vehicle returns with the same fault this means that something is wrong with the secondary battery.

Average voltage in secondary battery too low 

The following causes block autostop:

• The average voltage in the secondary battery during autostart is too low. Can be caused by large power consumers connected during autostart, such as ventilation fan, seat heaters, demister, radio, extra equipment (e.g. in the 12 V socket).
• In the event of stalling the engine unintentionally, the secondary battery may become overloaded with too high current. This can happen when the battery relay closes (is deactivated) before the engine runs normally as the secondary battery is still connected. If, for example, the secondary battery's average voltage is 9 V, 15 operating cycles are required before the function is permitted again (average voltage is 10.5 V).

Checks:

• Check for large power consumers connected during autostart, such as blower fan, seat heaters, demister, radio, extra equipment (e.g. in the 12 V socket).
• Charge the secondary battery. The secondary battery is best charged via the vehicle's charging system while driving. When using the battery charger to charge, the battery charger must only be connected directly to the secondary battery's poles. It is of the utmost importance that the battery charger is correctly connected as connecting to the wrong pin can damage the Engine Control Module (ECM) and that charging occurs with a low current so that the secondary battery is not overcharged.

HINT: If the power is cut to the Central Electronic Module (CEM), the stored average value is reset and the Central Electronic Module (CEM) starts the calculation with a predefined value (13.0 V). This enable the automatic start/stop of the engine function again. Test drive the vehicle. Whilst driving, the Central Electronic Module (CEM) stores the average voltage again. If the vehicle comes back with the same fault, this indicates that the secondary battery's capacity is too low. 

Battery temperature too low or too high 

The following causes block autostop:

• The battery's temperature is outside the interval 0 - +55 °C. May be due to either a high outdoor temperature and in combination with driving or an outdoor temperature below 0 °C.

In the event of temperature changes, the battery's temperature changes more slowly than the outdoor temperature. If the outdoor temperature has been below 0 °C for a long period of time and then rises above 0 °C, the battery's temperature takes longer to reach 0 °C. If the outdoor temperature is above 0 °C the battery temperature may be too low and the function is blocked. Also applies at high temperatures, where the battery's sluggishness does not follow the outdoor temperature.

HINT: Read off the battery's temperature via Service Functions/Diagnostic Sequences/12 V current, overview. 

Reliability in calculating the battery status 

The following causes block autostop:

• The battery monitoring sensor's calculation of the battery status is temporarily unreliable and the function is therefore blocked. May be due to:

• The battery's negative pole is used during charging. The battery monitoring sensor then does not register the current that the battery is being charged with. When the ignition is switched on, the battery monitoring sensor registers that the voltage across the battery poles has changed since the last registration. The battery monitoring sensor can interpret this as that its calculation of the battery's state of charge(SoC) currently does not correspond and sends messages about this to the Central Electronic Module (CEM). On these occasions, the sensor's calculation of the battery's status is not reliable and the function is therefore blocked.
• The standby current consumption is more than 250 mA for 7 days. This means that the sensor never has the opportunity to measure the battery's standby voltage and thus does not calculate the battery's charge status.

Checks:

• Read off the status of the sensor's calculation via Service Functions/Diagnostic Sequences/Vehicle's SoC, voltage and current history. If the calculation is currently reliable, check the standby current. Otherwise, continue as follows. In order for the battery monitoring module to calculate the battery's correct state of charge, the vehicle must be in sleep mode for 4 hours  . Sometimes, up to 6 hours in sleep mode may be required. This is on the condition that the ignition key has been removed, doors and hatches closed and the car is locked.

  Then read off the sensor's calculation again. If the calculation is reliable, disconnect the battery monitoring sensor's connector, wait a few seconds and then reconnect the sensor.

  NOTE:

  Before disconnecting the battery monitoring sensor, switch off the ignition, remove the key and wait 10 minutes. This is to minimize the vehicle's current consumption and allow the battery's pole voltage to resume best possible standby voltage. 

  Disconnecting the sensor means that the battery monitoring sensor assumes a SoC of 80% and the automatic start/stop of the engine function is permitted. This is on the condition that the battery is fault free and charged.

• If the standby current is greater than normal, check the causes of the high standby current consumption according to fault tracing.

HINT: Read off status standby consumption via CEM / Parameters. 

The vehicle is set to transport mode 

The following causes block autostop:

• To ensure that the battery is always charged in transport mode, the automatic start/stop of the engine function is blocked.

Checks:

• Set the vehicle to normal mode.

DTCs for the battery monitoring sensor 

The following causes block autostop:

• If DTCs for the battery monitoring sensor are stored and the fault is permanent, the function is blocked.

Checks:

• Fault trace according to the description for DTCs.

DTCs for alternator 

The following causes block autostop:

• If DTCs for the alternator or for high or low voltage are stored and the fault is permanent, the function is blocked.

Checks:

• Fault trace according to the description for DTCs.