3-second answer: Test first: with the engine running, battery voltage should read 13.5-14.7V. Below ~13V = not charging. Check the big wire + ground connections before condemning the alternator (corroded terminals fake dead alternators constantly). The swap: unbolt, unplug, reverse — ~20 minutes, one belt.
The symptoms
- Battery dies overnight or while driving
- Lights dim at idle, brighten with revs
- Whine in the radio that rises with RPM (diode noise — usually the alternator, sometimes a bad ground)
- Battery light on (the classic: the light on at idle that goes out with revs = worn brushes)
Test before you buy
- Battery voltage key-off: ~12.6V = healthy battery. Below ~12.2 = battery (or drain), not necessarily the alternator.
- Engine running: 13.5-14.7V = charging fine. <13V = charging problem.
- Load test: lights + blower on, still 13V+ = good.
- Check the voltage at the battery vs the alternator output post — a bad connection shows the drop.
The swap
- Disconnect battery, unplug the harness + the output wire (watch the big nut — 8mm or 10mm, anti-seize on reinstall).
- Unbolt the two through-bolts + the adjusting bracket.
- Reverse, re-tension the belt (1/2 in deflection mid-span), reconnect battery.
- A 130A upgrade is a common mod (the video covers it) — worth it if you run lights/winch/audio.
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- FTE search: alternator · charging