Just replaced the alternator on my 2002 Honda Accord EX (v6) after seeing the battery light come on. AutoZone tested and confirmed it was bad and I installed the replacement myself.

Overall the car works fine now, but I've noticed the battery light flash on and off briefly a couple of times the past few days. I have a mobile radio hooked directly to the battery terminals and it has a built-in voltmeter which I've been using to monitor the voltage. When my AC is on I'm getting 14.5 volts consistently, but when I turn on AC off the voltage drops to 13.0 a few seconds later and mostly stays there, with a few brief jumps back up to >14 seemingly at random. When I turn the AC back on, the voltage jumps to 14.5 once again and remains there.

This is the opposite of what I would expect. I've noticed that I can sometimes get the battery light to flash by cutting off the AC. Why would the alternator voltage drop when I turn the AC system off?

Bonus question: why would this be happening after replacing the alternator with a brand new unit? I would expect these symptoms to disappear. I just had the battery replaced last month so it should be fine.

Thanks in advance for any advice or suggestions.

 

1. The reason the voltage drops when the A/C is turned off is due to Honda's dual mode charging system.

Honda's utilizes a dual mode charging system to increase fuel efficiency and reduce drag on the engine on start-up. The ECM/PCM determines charging based on info. from the ELD (Electrical Load Detector). During heavy load, the ECM/PCM can set charging voltage for high output (~14V - 15V). During light load and start-up, the ECM/PCM sets charging voltage for low output (~12 - 13V).

The ECM/PCM can control charge modes (high or low output).

2. The battery light on your dash is the charging system indicator. If that is flashing, there may be an issue with the alternator or connections. Don't be surprise if the Autozone alternator is still the issue.