Start-Stop Systems

A start-stop system shuts the engine off when the vehicle comes to a complete stop — like at a red light — and restarts it instantly when you release the brake pedal. It saves fuel by eliminating idle time. On a vehicle that spends 30 percent of its drive time sitting at lights, that is a significant fuel savings.

When you come to a full stop with the brake pedal pressed, the engine control module shuts down fuel and ignition. The engine stops. When you release the brake pedal to drive, a high-speed starter motor cranks the engine and it fires within half a second. Some systems use an enhanced conventional starter. Some use a belt-driven starter-generator that restarts the engine even faster and more quietly. The transition should be seamless — if the driver feels a lurch or delay, there is a problem.

A conventional flooded lead-acid battery cannot handle the constant deep cycling of a start-stop system. Every stop-and-restart is a discharge-and-recharge cycle. An Absorbent Glass Mat battery is designed for this. AGM batteries tolerate repeated partial discharge and recharge cycles without degrading the way a flooded battery would. If a start-stop vehicle gets a standard flooded battery during replacement, the system will either not function or the battery will fail prematurely. Always replace with the correct battery type — AGM for AGM, EFB for EFB. The battery management system may also require a registration or reset procedure after replacement so the module knows it has a new battery and adjusts its charging strategy.

The start-stop system has multiple conditions that prevent it from activating. If the engine is not at operating temperature — it stays running. If the battery state of charge is below a threshold — it stays running. If the climate control is demanding maximum heating or cooling — it stays running. If the hood is open — it stays running. If the driver seatbelt is not buckled on some vehicles — it stays running. These are all normal. Customers sometimes complain that the system does not work. Before diagnosing a fault, verify that all enabling conditions are met. Scan tool data shows the enable and disable conditions in real time.

The most common start-stop failure is a degraded AGM battery that can no longer hold sufficient charge for reliable restarts. The system disables itself to prevent a no-start condition. Test the battery with a tester capable of AGM testing — not all conductance testers are calibrated for AGM. A borderline AGM battery that passes a basic test may still not meet the higher threshold required for start-stop operation.