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GM Stop/Start Vehicles

It is essential to understand what is different on these vehicles and how it might change your diagnostic approach.

Since 2013, GM has offered stop/start technology on a growing number of engines and platforms. For your shop, it is essential to understand what is different on these vehicles and how it might change your diagnostic approach.

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1. LARGER OR MULTIPLE BATTERIES

When the engine stops so does the alternator. The battery must run the HVAC, infotainment and other systems with little or no disruption. Most stop/start systems use a deep cycle aggregated glass mat battery (AGM). In some GM applications, there is a second or accessory battery.

2. TRANSMISSION

When the engine stops, the transmission pump behind the torque converter stops turning and line pressures drop. Most GM stop/start vehicles use an electric pump and maybe an accumulator to keep the transmission fluid under pressure. If the pump fails or valves fail, the transmission will slam into gear when the engine is started and the person pulls away from a light.

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3. HVAC

Managing the temperature inside the cabin is critical. Roasting a driver for better fuel economy is not a good idea. When the engine shuts down, the HVAC system will monitor outside and cabin temperatures to determine if the engine needs to be started to power the A/C compressor or if the auxiliary coolant pump needs to be turned on to pump hot coolant to the heater core.

4. BRAKE PEDAL SENSOR

Don’t think of the brake pedal position sensor as just a switch. Stop/start vehicles measure brake pedal travel and force, so by the time the driver has moved his/her foot from the brake pedal to the gas pedal, the engine has started. For most GM vehicles with stop/start systems, the sensor needs to be calibrated if replaced. 

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5. OIL

GM Dexos Gen 2-approved engine oils protect components like the camshaft, crankshaft and the bearings during stop/start operation. Dexos tests measure how the oil sticks to the surfaces. For Ecotec turbocharged engines that have a stop/start system, a coolant pump circulates coolant to prevent it from coking up in the housing and lines.

6. ELECTRICAL SYSTEM

Imagine if a stop/start car shut down and when the driver lifted his/her foot from the brake pedal the engine did not start. To prevent this from happening, stop/start systems have three ways or more to measure the condition of the battery. The first measurement is voltage coming from the battery. Second, the system is measuring current and loads at the battery with coil winding like a current clamp on the positive battery cable. Third, most systems measure the temperature of the battery directly or through data PIDs for underhood temperatures.

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7. STARTER

The majority of systems use a starter that looks a lot like a conventional starter.

It’s not only appearance; these starters operate the same way but with some critical internal differences. The first thing you will notice is that the gear on the starter is larger, and the gear reduction system. Also, the armature and bushes are upgraded to improve the longevity of the starter.

The starter’s pinion gear will engage the flex plate when the engine drops below 50 rpm during stop/start operation. Most of these starters will have an extra connection that will engage the pinion gear. 

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8. THE ENGINE

With improved engine position sensors and gasoline direct injection, the ECM is able to do a neat trick. Since the system knows the position of the piston and valves, it can use the fuel injector and spark plug in the cylinder to “nudge” the engine over to make life easier for the starter. When the engine needs to start, a small amount of fuel will be injected and ignited to get the engine moving.

9. COOLING SYSTEM

Some stop/start vehicles are using an auxiliary water pump to move the coolant in the block, head and heater core so the engine will not be heat soaked from the coolant not moving. If ambient temperatures are too hot, the stop/start system will not be active.

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10. SMARTER VEHICLES WITH MORE SENSORS AND SHARED INFORMATION

A GM stop/start system is not a module. It is a shared strategy among many modules using information like cabin humidity to crankshaft position to determine what happens at the next stoplight. For the technician, it means that serial data communication bus and electrical diagnostics will be much more important in the future.

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