Glossary

D+ signal

Also D+, D+ terminal

What is D+ signal?

A signal from the alternator indicating that it is running and charging. It is used to switch split-charge relays, DC-DC chargers or loads depending on the engine.

What matters about D+ signal in practice?

Not every modern alternator provides a classic D+ — in regulated systems it is missing or behaves differently. Devices that use it therefore usually offer voltage-controlled activation as an alternative.

The signal is intended for control purposes and cannot carry a load. Anyone connecting larger loads to it needs a relay in between.

Where does the D+ signal come from?

Conventional alternators have a D+ terminal that carries voltage as soon as the alternator charges. This is where the charge warning lamp used to be connected. When the engine is off, D+ is dead. That makes it a simple sign for "engine running and charging".

What is it used for?

  • Split charge relays connect starter and service battery only while the engine runs.
  • DC-DC chargers only start once the alternator charges, so they do not drain the starter battery.
  • Cool boxes or other loads run only while driving.

What if there is no D+?

On vehicles with a smart alternator or CAN-bus controlled electrics, D+ is often missing or not accessible. Many DC-DC chargers then detect engine running from the voltage and switch on as soon as it rises above a threshold. Because smart alternators lower the voltage at times while driving, the device must be expressly designed for this. More about charging from the engine is on the page boat alternator.

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