Starting & Charging Components
Starting and charging is a big share of the ASE A6 test, and it's where voltage-drop diagnosis earns its keep. A weak crank or a battery that keeps dying is usually resistance in a cable or connection, not a dead starter or alternator. This reference covers the battery, starter and solenoid, alternator and regulator, and the circuits that tie them together — with the failure each shows and how to test the path under load instead of replacing parts.
How the two systems connect
The battery supplies the starter to crank the engine; once running, the alternator recharges the battery and carries the electrical load, with a regulator holding output in range. Every one of these depends on low-resistance connections, which is why A6 frames so many starting/charging questions as “voltage is present but the system underperforms.” The answer is almost always to measure voltage drop under load.
Component reference
| Component | Job | Common failure | Symptom |
|---|---|---|---|
| Battery | Store/supply energy | Low state of charge; lost capacity | Slow crank, repeat dead battery |
| Starter motor | Crank the engine | Worn brushes, high draw, open | No-crank or slow crank |
| Starter solenoid | Engage drive, switch current | Burned contacts | Click, intermittent crank |
| Alternator | Recharge + power vehicle | Worn brushes, bad diode | Low/no charge; AC ripple if a diode fails |
| Voltage regulator | Control output | Over/under-charging | Boiling battery or chronic undercharge |
| Charging circuit cables | Carry charge to battery | High-resistance connection | Battery undercharged despite good alternator output |
State of charge vs. capacity — the test’s favorite distinction
A battery can be fully charged and still bad, or good but discharged. State of charge (a voltage/specific-gravity reading) tells you how full it is; a load or conductance test tells you whether it can still deliver current. A6 wants you to know that a 12.6 V resting reading doesn’t prove a healthy battery — you have to load it.
State of charge
- Open-circuit voltage or specific gravity
- Tells you how full the battery is right now
- 12.6 V = fully charged, but may still fail under load
- A surface charge can fool this reading
Capacity (load/conductance test)
- Battery loaded to simulate cranking
- Tells you whether it can deliver current
- A fully charged battery can still fail here
- What ASE wants before condemning or clearing the battery
The call: A 12.6 V resting voltage does not prove a healthy battery. Run a load or conductance test — state of charge and capacity are two different things.
The parts techs mix up
- Starter vs. its circuit. A slow crank with battery voltage at the post is often voltage drop in the starter feed or ground, not a worn starter. Drop-test both sides under crank before replacing.
- Alternator vs. charging cable. If the alternator outputs 14+ V at its B+ terminal but the battery sees only 13 V under load, the cable/connection is the fault — the alternator is fine.
- No-charge vs. AC ripple. A failed rectifier diode lets AC ripple through; the battery light may be off but charging is poor and ripple shows on a scope. Don’t stop at “voltage looks okay.”
| Symptom | Real cause | Exam trap / correct call |
|---|---|---|
| Slow crank, battery tests good at rest | High resistance in starter feed or ground cable | State of charge looks fine — voltage-drop test the circuit under crank before condemning the starter |
| Battery keeps going low, alternator reads 14 V at B+ | High-resistance charging cable between alternator and battery | Not a bad alternator — measure at the battery under load, not at the alternator terminal |
| Charging voltage looks okay, battery drains over days | Failed rectifier diode letting AC ripple through | DC voltage 'looks acceptable' — check AC ripple on the charging line with the engine running |
| Battery boiling, short life | Overcharging from a failed voltage regulator | Don't condemn the battery until you've checked regulator output under load |
| Slow crank after sitting overnight, cranks fine once started | Low state of charge from a parasitic draw or marginal alternator | Run a load test and measure key-off current draw — surface charge can make battery voltage look normal |
ASE traps
- A 12.6 V resting battery can still fail a load test — state of charge isn’t capacity.
- An alternator that reads good output at its own terminal can still leave the battery undercharged if the cable has resistance — measure at the battery under load.
- A failed diode causes AC ripple and poor charging even when DC voltage “looks” acceptable.
Study the full test in the A6 Electrical guide, drill it with the A6 practice test, and see electrical & HVAC study for the broader track.