GM Auto Engine Stop/Start Systems

The Engine Stop/Start system, which automatically turns off the engine when the vehicle comes to a stop in appropriate driving conditions, can reduce fuel consumption and carbon dioxide (CO2) emissions by up to 5% in mixed driving conditions. In an urban environment and in heavy traffic with frequent stops, the savings may increase to as much as 10%. These numbers show why the Stop/Start system (RPO KL9) has grown to become standard equipment on many GM models.


First introduced in the 2014 model year, the following models offer a Stop/Start system: 2014-2018 Malibu; 2015-2018 Impala; 2016-2018 Encore, Envision, CT6, Cruze; 2017 Verano; 2017-2018 LaCrosse, Regal, ATS, CTS, XT5, Trax, Acadia (VIN N); and 2018 Equinox and Terrain.


The Stop/Start system will automatically turn off the engine, referred to as an Auto Stop (Fig. 1), when the brake pedal is applied and the vehicle is at a complete stop, if various operating conditions are met. Once the brake pedal is released or the accelerator pedal is depressed, the engine will restart. It takes about 0.3 seconds for the engine to start from the time the brake pedal transition initiates the restart.


Fig. 1


Three Systems


There are three types of 12V Stop/Start systems that have been used in GM vehicles:

  • The Two-Battery system that includes an auxiliary 12-volt battery.
  • The DC to DC system that features a power supply transformer.
  • The Ultra Capacitor system that has capacitors that provide a voltage boost during engine restarts.


All Stop/Start systems operate fundamentally in the same way. The Engine Control Module (ECM) monitors a number of inputs — such as the engine coolant temperature, vehicle speed, hood ajar switch, brake booster vacuum, battery current, brake pedal position, accelerator pedal position, and engine speed — to determine when the engine should be turned off. The ECM also controls the auxiliary coolant pump motor for cabin heating and, depending on the vehicle, the auxiliary transmission fluid pump motor or auxiliary transmission fluid accumulator solenoid valve that maintains proper transmission fluid pressures while the engine is off.


There are a number of other components that work together to determine Stop/Start system activity. The Transmission Control Module (TCM) monitors the gear selection. The vehicle must be in Drive to turn off the engine. The Body Control Module (BCM) monitors data on the state of the battery based on the battery sensor module along with the Heating, Ventilation and Air Conditioning (HVAC) system settings.


The Stop/Start system also uses an enhanced starter motor to better manage the greater number of engine starts. It has a high performance electric motor and a stronger pinion engagement mechanism than a conventional starter. It also has independent control of the pinion and motor.


Two-Battery System


The Two-Battery system includes the primary 12V battery as well as an auxiliary 12V battery. (Fig. 2) Both batteries are Absorbent Glass Mat (AGM) batteries. AGM batteries are used in all GM models with a Stop/Start system. The auxiliary battery is used to power vehicle loads, except the starter, during stop/start cranking events. The Dual Battery Control Module switches primary and auxiliary batteries in and out at appropriate times to support vehicle loads and battery charging. The Two-Battery Stop/Start system is automatically activated each time the ignition switch is turned on.


Fig. 2


DC to DC System


In a DC to DC system, the power supply transformer (Fig. 3), or DC to DC converter, helps power the vehicle loads temporarily when the engine cranks. The power supply transformer monitors battery voltage and provides a boost of voltage when the engine restarts to ensure proper operation of the radio, instrument cluster and other components.


Fig. 3


The climate controls on some vehicles feature an ECO (economy) switch that can change the air conditioning mode to Off, Comfort, or Economy. These settings change the parameters for the activation of the Stop/Start system. In Comfort mode, the focus is on passenger comfort. While in Economy mode, the focus is on fuel economy.


Currently, on Cadillac models with the DC to DC system, the Stop/Start function can be disabled with the Stop/Start button on the instrument panel.


Ultra Capacitor System


The Ultra Capacitor system (Fig. 4) also provides a burst of power to help restart the engine along with providing additional power to the vehicle’s electrical system. The high voltage drop caused by current drawn during an engine restart is compensated by the voltage provided by the two double-layer capacitors, which store energy in an electric field. The capacitors are switched in series to the standard battery and are controlled by the Stop/Start Capacitor Control Module (SSCCM). The SSCCM controls the charging and discharging of the capacitors and communicates with the ECM.


Fig. 4


When the engine starts, the module charges the capacitors to a voltage in the range of 3.2– 4.5 V (depending on the vehicle engine). In normal operating mode, the module connects the battery to chassis ground. During an engine restart, the module opens the switch to ground and closes the switch to the capacitors to provide a boost in voltage to the vehicle’s electrical system.


The Ultra Capacitor system features a Stop/Start disable switch located on the instrument panel or center console. (Fig. 5) The Stop/Start system is enabled at every ignition cycle.


Fig. 5


TIP: Follow all Service Information diagnostic and repair procedures to discharge the Ultra Capacitor system when requested.


Enabling Criteria


The following vehicle conditions must be met for an Auto Stop event to take place.


  • Initial minimum vehicle speed during the drive cycle must reach 12 MPH (19 km/h). Subsequent minimum speed may vary from 1-6 MPH (2-10 km/h), depending on the vehicle.
  • In a vehicle with a manual transmission, the transmission must be in Neutral and the clutch pedal fully released.
  • Ambient and engine coolant temperature correlation must meet specified values.
  • Ambient and transmission fluid temperature correlation must meet specified values.
  • Hood switch status is closed.
  • Brake pedal is depressed beyond a specific value (approximately 27%).
  • Accelerator pedal is in the learned home position.
  • Brake booster vacuum is greater than 45 kPa (7 PSI).
  • Transmission gear selector is in the Drive position.
  • Vehicle speed is less than 3 MPH (5 km/h).
  • Engine speed is below 1500 RPM.
  • Engine coolant temperature is less than 248°F (120°C).
  • No A/C compressor request from the HVAC system (A/C or Defrost modes).
  • Battery voltage greater than 12 V.
  • Battery state of charge in the ECM is greater than 75% (changes with state of health).


The following conditions will enable an engine restart.


  • Transmission gear selector is moved from the Drive position.
  • In a vehicle with a manual transmission, the driver depresses the clutch pedal.
  • Driver removes pressure from the brake or depresses the accelerator pedal while the vehicle is in the Drive.
  • Brake booster vacuum is less than 40 kPa (6 PSI).
  • A/C compressor request from HVAC (A/C or Defrost modes).
  • Battery voltage less than 11 V.
  • Battery state of charge is less than 73% (changes with state of health).
  • Hood switch status changes to open.
  • Auto Stop time exceeds 2 minutes.


– Thanks to Dan Jaszkowski and Jack Woodward


Service Know-How
Enhanced Starter Motor Operation in Engine Stop/Start Systems

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