Next Generation Start/Stop Systems -

Next Generation Start/Stop Systems

Although stop/start idle ­systems have been used on ­hybrid vehicles for many years, a growing number of non-­hybrid late-model import models are also being equipped with this fuel-saving technology.


Although stop/start idle ­systems have been used on ­hybrid vehicles for many years, a growing number of non-­hybrid late-model import models are also being equipped with this fuel-saving technology. In 2015, almost every major automaker offered one or more models with stop/start idle control. Many experts predict that within the next two years, over half of all vehicles sold in the U.S. will be factory-equipped with stop/start systems.

What does this mean to you? It means a growing service ­opportunity to diagnose and repair these systems as they come out of warranty in the years ahead. The 2015 model year marked the tipping point where stop/start technology really took off. You’ll find it on everything from entry-level cars such as the Kia Rio (with Eco package) to the Acura TLX and various Mazda models, to top-of-the-line luxury and performance cars such as Audi, BMW, Jaguar Mercedes-Benz, Porsche and VW. And as each new model year comes along, stop/start technology will become almost universal.

Stop/start technology has been around for nearly a decade on full hybrid vehicles such as the Toyota Prius and Honda Insight. It’s also used on many “mild hybrids” such as the Honda ­Accord, Civic and CRZ hybrids, Hyundai Sonata hybrid, various Lexus hybrids, Nissan Altima ­hybrid, Toyota Camry and Highlander ­hybrids, and VW Touareg hybrid. On these applications, the high-voltage hybrid battery is used for the stop/start function. It may also provide additional power assist when accelerating, depending on how the system is configured.

What’s different about the next generation stop/start systems is that the idle shut-off feature is being incorporated into non-hybrid cars, trucks and SUVs. In other words, most of these applications will not use a special high-voltage nickel-metal-hydride or lithium ion battery, but instead will get their cranking power from an AGM (absorbent glass mat) 12-volt lead-acid battery.

Some of these new applications (BMW and VW, for example) actually use two separate batteries: a 12-volt AGM battery for cranking the engine, and a second conventional 12-volt wet-cell, lead-acid battery to power the onboard electronics. On the BMW and VW applications, the batteries are mounted in the trunk (one on each side). What’s more, each battery may be charged independently of the other depending on electrical load and demand – which will complicate charging diagnosis if either battery is not being maintained at full charge, or there is a key-off power drain that’s running down one or both batteries.

Source: Mercedes-Benz
Source: Mercedes-Benz


There is no federal rule that ­requires stop/start idle control on any new vehicle. It’s just one of the many steps that automakers are having to take to achieve the Corporate Average Fuel Economy goal of 52.5 mpg by 2025 (which is required by law). In Europe, where fuel prices are considerably higher, about 60% of late-model vehicles have stop/start idle control systems. Although we are currently enjoying a temporary drop in gasoline prices, the push for higher fuel economy numbers is more about global climate change and reducing CO2 emissions into the ­environment.

Stop/start systems (also called “idle stop” or “idle stop & go”) use inputs from the steering wheel position sensor, vehicle speed sensor, throttle position sensor and brake pedal position sensor to determine when the vehicle has come to a halt and will likely ­remain stopped for a period of time. The powertrain control module (PCM) will then shut off the engine by killing fuel and ignition. When the driver lifts his/her foot off the brake pedal (or depresses the clutch pedal if the vehicle has a manual transmission), the PCM starts the engine so when the driver pushes down on the accelerator pedal the engine will ­respond and drive the car as if nothing has happened.

Most of these systems will restart the engine in less than half a second, ­allowing a more-or-less seamless stop/start function. On the 2015 Acura TLX, active engine mounts are used to offset and dampen the engine starting vibrations.

So how much fuel does idle stop/start actually save? It depends on how the vehicle is driven. In an urban setting with heavy stop-and-go traffic, a stop/start system can improve overall fuel economy as much as 10 to 12%. Most automakers estimate stop/start idle control saves the average motorist about 6% on his/her fuel bill. For vehicles that are driven mostly on the open road and spend very little time stopped, the fuel savings are ­minimal.

On many applications, the stop/start system can be temporarily disabled by pressing a button. On some vehicles, the stop/start function will remain off until the driver pushes the button to turn it back on.

On all stop/start systems, an indicator light on the instrument panel tells the driver when the ­engine has stopped. Consequently, you may have to educate some customers as to what the indicator light means and how the system operates.


For a stop/start system to function normally, the ­engine management system has to look at how the vehicle is being driven. It monitors engine speed, temperature and load, as well as vehicle speed, and the positions of the brake and accelerator pedals, steering wheel and transmission gear selector. The PCM may also consider accessory electrical loads on the engine (headlights, wipers, state of battery charge, etc.) and A/C cooling requirements. Using all of these inputs, the control module will then ­decide whether or not to shut the engine off when the vehicle stops.

With most stop/start systems, the engine will shut off after the vehicle has been motionless for a few seconds if the transmission is in drive and the driver is holding his/her foot on the brake pedal. Some systems may even anticipate a stop by shutting off the engine when the driver lifts his/her foot off the accelerator pedal when the vehicle is decelerating.

On most systems, the stop/start function may not occur if the cranking battery voltage is low (less than 75%), if there is a high A/C cooling load due to high ambient temperatures, or if there are ­usually high electrical loads on the charging system (lights, ­defrosters, heater, etc., all on at the same time).

On Mazda’s “i-stop” system, the starter motor is used to crank the engine. The PCM also knows the position of the crankshaft and the engine’s firing order, so it also injects some fuel into a cylinder that is past top dead center on its power stroke and fires the spark plug to give the crank an extra kick. This makes the restart much easier and faster, ­reducing the load on the starter and the time it takes to restart the engine to 0.35 seconds.

No more wasting fuel waiting for red lights to turn green. Automatic idle stop/start systems can ­improve fuel economy by 6% to 12%.


The new next generation non-­hybrid stop/start ­systems also require some additional changes to the vehicle itself. In addition to a stronger AGM battery with more reserve ­capacity than a standard battery, a beefed-up starter motor is also required to handle the increased number of startup cycles. Others use a two-way alternator that serves as both a generator and a starter motor to crank the engine.

The main and rod bearings on some engine applications may also have a scuff-resistant, friction-­reducing coating to reduce the risk of metal-to-metal contact from frequent restarts. When the engine shuts off, oil pressure drops to zero. Most late-model cars use relatively thin oil (0W-20, 0W-40, 5W-20, etc.) so the oil may drain out of the bearings fairly quickly depending on oil temperature, bearing clearances and how long the vehicle sits without running. The coating on the bearings provides an extra layer of protection until oil pressure can restore ­normal oil pressure and flow ­following a restart.

On some applications, auxiliary electric pumps may be used to keep coolant circulating to maintain heat during cold weather or to supply hydraulic pressure to the automatic transmission. On a full hybrid vehicle, such as a Toyota Prius, an electric A/C compressor is used to maintain cooling when the engine is not running. We will likely see more of these electric A/C compressors being used on some next generation stop/start non-hybrid applications for the same purpose.

Another change you’ll find on some applications is an auxiliary module that maintains constant voltage to key electronics when the engine is in cranking mode. On a 2013 Kia Rio, for example, there is a DC-to-DC converter behind the glove box. The DC-to-DC converter senses system voltage and maintains steady voltage to the main power relay so system voltage doesn’t drop when the engine is cranking. If this module goes bad, it may cause the radio to stop working when the ­engine restarts due to low system voltage.


Stop/start systems may fail in a number of ways and for a variety of reasons. The system may not shut the engine off when it should. The stop/start system may kill the engine, but fail to restart it when it should. Or, the system voltage may drop noticeably when cranking the engine.

The underlying problem may be electronic (PCM or other control module fault), electrical (low battery voltage, weak battery or poor cable connections), mechanical (bad starter motor or alternator, damaged flywheel teeth, etc.) or sensor related (bad brake pedal, accelerator pedal, throttle position or other sensor). The onboard diagnostic system should detect any major system or sensor faults and set an appropriate code, but as we all know, many faults never set a code.

If a stop/start system is not functioning normally, start with the ­basics. Check the charge and condition of the battery, check the battery cables (clean and tighten as needed to ensure a minimal voltage drop across the connections), check charging output, and use your scan tool to check the function of key sensors that provide input for the stop/start system.

Your scan tool sensor data should tell you if the brake pedal and accelerator pedal sensors are ­responding when the pedals move. Ditto for the steering position sensor. Is the vehicle speed sensor providing an accurate signal? Does the transmission gear selector show the proper gear when it’s moved from one position to another?

If you find one or more fault codes (which could be powertrain, body, suspension or other), follow the vehicle manufacturer’s diagnostic procedures to isolate each fault. This means accessing the service information data on the OEM website or via any of the aftermarket data services. Also, check for any TSBs that may relate to the problem.

At some point down the road, we may also see computer reflashes as a “fix” for certain stop/start issues. This will require a scan tool that is capable of doing reflashes and gaining access to the OEM ­updated calibration.


The next generation stop/start systems on non-­hybrid vehicles are 12 volts, so there are no special precautions to follow other than to make sure the ­engine is off if it has a pushbutton start and/or smart keyfob.

With full and mild hybrid vehicles, however, you do have to watch out for the high-voltage battery. Hybrid voltages may range from 120 up to 330 volts, depending on the application. This kind of voltage can be deadly, so avoid any contact with the orange high-voltage hybrid wiring and battery until the battery has been isolated. Follow the vehicle manufacturer’s procedures for isolating the ­battery. Insulated gloves capable of withstanding up to 1,000 volts are required if working on a live system, and insulated hand tools are recommended.

For non-hybrid 12-volt applications with stop/start, you can use the same hand tools, diagnostic tools, battery and charging system test equipment that you use on other vehicles.

Since most of the next generation stop/start ­applications come factory-equipped with an AGM battery, you should replace same with same if the battery has failed. Substituting a less expensive conventional wet-cell lead-acid battery is not ­recommended.

AGM batteries typically provide greater cranking power thanks to their denser design. They also have a much longer service life than ordinary wet-cell lead-acid batteries because they have no liquid electrolyte in their cells. The electrolyte is held in spongy mats between the lead cell plates. This not only makes such batteries spill-proof, but also less vulnerable to outgassing and loss of electrolyte over time.

Another difference is that AGM batteries have a slightly different charging rate and voltage. AGM batteries recharge about 15% faster than a conventional battery (which is important for maintaining battery charge for reliable starting). A fully charged AGM battery will typically read 12.8 to 13 volts or higher (versus 12.65 volts for a conventional ­battery). A reading of 12.5 to 12.8 volts indicates a 75% charge. A reading of less than 12.5 volts means the battery is low and needs to be recharged and/or load tested.

A low-voltage reading may mean the charging system is not producing enough current to maintain battery charge, or there is a key-off current drain on the battery that’s causing it to run down. This may require testing charging output and/or checking for an unusual key-off power drain.

Battery condition can be determined by load testing or by using a capacitance tester on the battery. For accurate results, a load test requires a battery to be 75% charged. Charge doesn’t matter if you’re using a capacitance tester on an ordinary battery, but with an AGM battery it does. The battery should be 75% charged because an AGM battery has less internal resistance than an ordinary battery.

AGM batteries can be damaged by overcharging. To reduce the risk of this happening, use a smart charger that has a different setting for AGM batteries.

If a starter motor has failed, make sure the replacement is a quality reman or new unit, not a cheap rebuilt that won’t stand up to repeated start cycles. A stop/start system can really work a starter motor to death, so it’s important to use a starter that’s robust enough to handle the job.

You May Also Like

Fuel Pumps and Cranking

Diagnosing the problem comes down to understanding what causes a loss of fuel pressure.

Here is a diagnostic puzzle: An owner comes to your shop complaining that it takes longer than expected to start his car’s engine in the morning. There are no codes for the crankshaft and camshaft sensors. And, the oil pressure is within specifications. If you are lucky, you might have a code for low fuel pressure on the high side of the direct injection system. What could be the source of the problem?

Wheel Bearing Service Tips

When faced with any noise complaint, take the time to test-drive the car.

Navigating the Intricate Landscape of Coatings and ADAS

With refinish and ADAS, the theme time and time again comes back to: check the OEM repair recommendations.

Proper Paint Prep: Cleaning and Abrading

Nothing is faster or more profitable than doing the job one time.

How Battery Service Continues To Change

Your attention to a battery’s condition will tell you if it needs to be replaced – and with what.

Other Posts

Catalytic Converter Replacement Rules

The conditions under which the catalytic converter was installed is of great importance to the part manufacturer.

ADAS False Activation Evolution

The logic behind most ADAS warnings or corrections is to examine the plausibility of the situation.

Audi Active Suspension Service

The key to servicing these systems is proper information and understanding precisely how these systems operate.

Diagnosing Misfires

What if there are no codes and a misfire is intermittent? This is where it gets complicated.