Starting and charging circuit
Starter and alternator wiring diagram, explained
The starting and charging circuit is where the biggest currents in a car flow: the starter draws a hundred amps or more, the alternator puts it back. Here is how it is wired, terminal by terminal, and a live diagram you can crank and rev.
Open the editor · ▶ Run the live demo
Terminals you will see
| Terminal | Meaning |
|---|---|
| 30 | Permanent battery + |
| 15 | Switched + with the ignition on |
| 50 | Start signal to the starter solenoid |
| 31 | Ground |
| B+ | Main output of the alternator, or the main feed of the starter |
| D+ / EXC | Alternator excitation, wired through the charge warning lamp |
The starting circuit
- A thick cable runs from battery + straight to the starter’s B+ terminal; the starter draws about 100–200 A while cranking.
- The ignition switch (or a start button) sends a small current to the starter relay.
- The relay feeds terminal 50 / S of the starter solenoid, which engages the starter and closes its main contact.
- The engine block is grounded to the battery − with an equally thick cable — a bad ground is the most common reason a starter only clicks.
The charging circuit and the warning lamp
The alternator’s B+ output goes to the battery through a high-current fuse. With the ignition on and the engine stopped, current flows from terminal 15 through the charge warning lamp into D+, so the lamp lights and the alternator gets its excitation current.
When the engine runs, the alternator’s own voltage rises to that of the battery side of the lamp, there is no voltage left across the lamp and it goes out. If the lamp stays on with the engine running, the alternator is not charging.
Crank and charge in the simulator
- Open the “Starting and charging” demo.
- Turn on the ignition switch — the charge lamp lights.
- Hold the start button — the starter turns and draws about 100 A.
- Drag the engine speed up — the alternator charges and the lamp goes out.
- Click the battery cable and the alternator cable to compare the currents.
Questions
Why does the starter only click?
Usually the high current does not get through: a weak battery, a loose or corroded main cable, or a bad engine ground. The solenoid gets its small current and clicks, but the motor does not turn.
Does the starter cable need a fuse?
Often the cable from the battery to the starter is not fused, because no practical fuse survives cranking; it is kept short and well protected instead. Some cars use a high-current mega fuse.
Why does the battery light come on?
It lights when the alternator’s output is lower than the battery voltage: with the engine stopped (normal), or with a failed alternator, regulator or broken belt while driving.
What is terminal D+ on an alternator?
The excitation input. Current through the charge lamp into D+ starts the alternator; when it charges, the lamp goes out.
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