This SEAT Arosa 1.0 was brought into my garage because the red ignition warning light had been glowing. It wasn’t actually glowing when they brought it to me, but the owner was wise enough to know that problems don’t normally fix themselves.

I used a simple volt meter to check the output at the battery. At first it looked to be OK, putting out 14.2V on tick over. When the headlamps were switched on, the voltage quickly dropped to below 12V at idle. Even when revving the engine slightly, the voltage would not go above 12.4 V.

I checked through the continuity of the circuit, which was all good, and decided that the alternator should be changed. After replacing the alternator and the auxiliary drive belt, I re-checked the output to find that with the headlamps on, a steady 14V could still be read off.

Charging problems on modern vehicles are not always straight forward. Not only can the alternator fail, but on some motors the smart charging system control unit may be at fault. No longer can you just replace the alternator because the charging light is on, the system needs to be checked first to ensure that the problem isn’t in another part of the circuit.

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