pw5599

2005 Mazda 3, 2.3 litre auto.
My battery light has been on quite a bit in the past week and a voltage measurement on the battery when the engine is off and when running confirms a no charge condition when the light is on and the odd time it's off, it is charging. It does go off at times while driving but is mainly on. Battery checks good, cables, connections are good so I've picked up an alternator and will install that in the next day or so. Before I install it I'm going to take resistance measurements between all terminals which may or may not help someone diagnose a faulty alternator but any info is good to have. I'm hoping it's the alternator because if it's not, I'm looking at the pcm or worse. Anyone else have any ideas, experiences, common issues with the charging system?

djs2571

issue#1 - look for corrosion... or possibly the fuse / fusable link connecting to the alternator.

pw5599

Thanks. I did check cables, connectors, battery and all were good but cleaned the battery posts/connectors anyway. I'll check the voltage on both sides of the fusable link to make sure they're the same.
After all that it looks like the next step is installing the new alt.

pw5599

The alternator was the problem. A resistance measurement both ways between pin 2 (furthest from the output bolt) to the case was 480 ohms as compared to 9500 ohms on the good one. According to the diagram that points to a leaky/shorted transistor, probably a switcher or regulator circuit.