Diagnostic Tips for Auto Stop/Start Operation

The automatic engine Stop/Start system (RPO KL9), which turns off the engine under required conditions when the brake is applied and the vehicle is at a complete stop, is becoming a standard feature on many GM models.

The Stop/Start system is used to improve fuel efficiency in stop-and-go driving. Turning off the engine in appropriate conditions at a traffic light helps reduce tail pipe emissions and saves fuel that would otherwise be used idling the engine. The engine instantly restarts when the brake pedal is released or the accelerator pedal is pressed. It only takes the system approximately 0.3 seconds to start the engine.

To support the increased number of engine starts, the starter motor is upgraded with a high performance electric motor and a stronger pinion engagement mechanism with reduced noise levels. In addition, an intelligent battery sensor connected to the battery continually monitors the battery charge and state of health. The Engine Control Module (ECM) uses this information from the intelligent battery sensor to determine if the battery charge and health is sufficient for Stop/Start operation.
There are many operating conditions to be aware of that must be met in order for the Stop/Start system to turn off the engine. If a vehicle does not engage in Auto Stop mode or automatically restarts during an Auto Stop event, it does not necessarily mean the vehicle is not operating properly.
A system malfunction may be the cause for the loss of Stop/ Start operation, and a Diagnostic Trouble Code (DTC) may be set. Many of the conditions that allow, or shorten, an Auto Stop event are not visible or controlled by the driver. In general, a skipped Auto Stop should be considered normal. Auto Stop mode is based on many factors and it’s important to first verify that all conditions are met to enable the mode.
Operating Criteria
To ensure the needs of the driver and the vehicle are not compromised during Stop/Start operation, the engine may not turn off under certain circumstances.

Criteria for enabling an Auto Stop include:
• Initial minimum vehicle speed during the drive cycle must be 12 mph (19 km/h) or greater. Subsequent Auto Stop minimum speed may vary from 1-6 mph (2-10 km/h), depending on the vehicle
• Ambient and engine coolant temperature correlation meets specified values
• Ambient and transmission fluid temperature correlation meets specified values
• Hood switch status is closed
• Brake booster vacuum is greater than 7 psi (45 kPa)
• Transmission gear selector is in the Drive position
• Vehicle speed is less than 3 mph (5 km/h)
• Engine speed is below 1,500 rpm
• Engine coolant temperature is less than 248°F (120°C)
• Ambient temperature is greater than 14°F (−10°C)
• No A/C compressor request from HVAC (A/C or Defrost modes)
• Battery voltage is greater than 12 V
• Battery state of charge is greater than 75 percent (changes with state of health).
The engine will automatically restart if:
• The driver removes pressure from the brake or depresses the accelerator pedal while the vehicle is in the forward Drive gear
• Brake booster vacuum is less than 6 psi (40 kPa)
• A/C compressor request from HVAC (A/C or Defrost modes)
• Battery voltage is less than 11 V
• Battery state of charge is less than 73 percent (changes with state of health)
• Hood switch status changes to open
• Auto Stop time exceeds two minutes.

On some models, the Stop/Start system can be disabled by pressing a button on the instrument panel or center console. When the system is disabled, the Auto Stop symbol on the instrument cluster will change from green to white.