A vehicle that is equipped with a stop/start system may soon be coming to your bays or might already be there on some European and Asian vehicles. These systems can improve fuel economy by 3-5 percent, depending on the driving habits of the customer. For your shop, it is essential to understand what is different on these vehicles, and how those differences might change your diagnostic approach.

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). This type of battery requires different testing methods and shop chargers. Some systems use a second battery to handle non-starter loads, so the customer doesn’t notice any disruption with HVAC or audio.

2. Battery Management
Imagine if a stop/start car shut down, and when the driver lifted his foot from the brake pedal, the engine did not start. To prevent this from happening, stop/start systems have three or more ways 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 windings, 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.

With this information, the ECM can predict if a stop/start cycle is possible. It is also a very good indicator of the health of the battery. Many systems will turn on a battery life indicator if it detects a low battery.

3. Transmission
When the engine stops, the transmission pump behind the torque converter stops turning and line pressures drop. Stop/start vehicles use an electric pump and possibly an accumulator to keep the transmission fluid under pressure. If the pump fails, the transmission will slam into gear when the engine is started.

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

5. 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 foot from the brake pedal to the gas pedal, the engine has started.

6. Engine Oil
New oil standards from the OEMs, API and ILSAC have new tests to help measure if the engine oil protects components like the camshaft, crankshaft and the bearings during stop/start operation. Most of these tests measure how the oil sticks to the surfaces. In addition, engine manufacturers are using electric pumps, accumulators and valving to keep the oil flowing and prevent too much oil from draining into the pan. Turbocharged engines that have a stop/start system are equipped with a pump that circulates oil to prevent it from coking up in the housing and lines.

7. Starter
High-end stop/start systems use a large generator/motor combo between the engine and transmission to turn the engine and generate power. But, the majority of systems are using a starter that looks a lot like a conventional starter.

These starters operate in 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 is lower. This allows the starter to turn slower, which reduces wear on the brushes and bearings. In addition, the armature and bushes are upgraded to extend the longevity of the starter.

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. The system will look for cylinders that are on a downward stroke. 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.

10. Smarter Vehicles With More Sensors and Shared Information
A stop/start system is not just a single module. It is a shared strategy among many modules using information ranging from 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.