Recently bought a new (to me) 2010 Fit with ~160k miles. Was loving every minute of driving it until the electrical issues started, now I can't safely drive it until I get to the bottom of the problem. Seems like the issue is a failing alternator but tests with a voltmeter were inconclusive. Wondering if anyone can help me get to the bottom of the problem

The problems showed up suddenly- during my commute home yesterday, the battery warning light came on for the first time, then the radio backlight began to flash, then power steering died. I pulled over and shut off the engine then couldn't get the car to start again. Had to get it towed home. This all happened within the span of about 15 minutes

Borrowed a voltmeter today and tested the battery: the reading was 12V. Jump started the car then tested again with the engine running: 13.6- 14V reading, indicating the alternator was sending power to the electrical system. Drove the car around the block and the battery warning light flashed on and off every 15 seconds or so. Parked the car, left the engine running, and tested voltage again: 14V. Turned the car off and tested the battery voltage one more time: 12V

So, the car is behaving like its alternator is failing, but tests with a voltmeter indicate that both battery and alternator are producing the voltages they should

For context: as far as I can tell from Carfax reports, the alternator has never been replaced; the serpentine belt was replaced in Feb 2023, about 20K miles ago; the current battery is from Nov 2020, a little over 4 years old.

This means the alternator and battery are probably nearing the end of their useful lives and it would probably be smart to preemptively replace them. However, again, tests with a voltmeter show that both of them are producing the voltages they should

What should my next troubleshooting steps be? Could the root cause be something different- electrical wires, parasitic drain, or something else? Or should I just go ahead and replace the alternator and/or battery and hope it solves the problem?

 

Make sure connections to the battery are good, tight and without corrosion.
Make sure grounds in the engine bay are not corroded.
Load test the battery: it could have 12 V, but no more cranking amps (this test simulate trying to start the car).

That would be, for me, the first steps to perform.

 

Sure enough I cleaned the battery terminals with baking soda slurry and a wire brush then reconnected and now everything is working fine

Could just be a coincidence though- maybe an intermittent ground fault is the real culprit, so I'll take your suggestion and check the grounds in the engine bay for good measure

 

The GE's alternator still has a simple pull-to-ground warning light output. That is, the alternator is triggering the warning lamp, in spite of the signal going through the ECM and F-CAN to reach the gauge cluster. The diagnostic pages in the Honda service manual are questionably written, but the gist seems to be:


Engine Electrical / Charging System / Indicator Circuit Troubleshooting

- Check that the Lamp wire between the alternator and ECM is in good condition. Unplug the alternator connector and ECM connector B. *Both* connectors must be unplugged to prevent damage and get valid test results. Check for continuity to ground (should not exist), check resistance from alternator connector to ECM connector (should be less than 1 ohm, ideally use an incandescent test light here).
- Check warning lamp circuit function. Plug the ECM and alternator connections back in. Turn the ignition on, engine off, warning light should illuminate. Unplug the alternator connector, check that the warning light goes out. Connect a jumper wire from the Lamp terminal of the alternator connector to ground and check that the warning light illuminates.
- If that all checks out, replace the alternator.

Though honestly the battery not charging is a dead ringer for a failing alternator. Alternators have wear points, very similar to DC motors - two electrically conductive block "brushes" that lightly press (by spring tension) against a pair of contact "slip ring"s on the armature/rotor/spinning part. Together, they carry electrical current from the electronics on the body of the alternator to the electromagnet coils of the spinning armature. The alternator's electronics (regulator) vary the current through the coils to adjust the magnetic field strength of the armature, which varies the power output of the alternator to match the car's needs. When the brushes wear down far enough, they stop making consistent electrical contact with the slip rings, the magnetic field ends up weaker than it should be. Weak field means weak electrical output that can't keep up with the car's electrical demands, which drains the battery. The sequential failure of electrical accessories is also very typical as the battery voltage drops. The radio is usually the first to start acting up, engine ignition system is usually last. This is all very typical Honda charging system stuff, dating back at least to the 80's.

The Honda service manual for the '09-12 Fit has overhaul instructions for the alternator if you want to take it on yourself. You will need an air impact driver to remove the pulley nut. I would expect to replace the brushes (2) at a minimum. Looks like they're soldered to the regulator assembly, so that could mean replacing the whole regulator if you're not comfortable soldering. Replacing the front and rear bearings would be wise as preventative measures. Replacement slip rings are often available if yours are worn - again, soldering required. The advantage of doing it yourself over buying a rebuilt alternator is knowing all the wear parts were replaced and the brand (quality) of the parts used - you get to do your own quality-assurance.