Symptoms
- New alternator installed but car still won't start
- May crank slowly or not at all
- Battery light may still be on
- Belt may look correctly installed
Likely causes
- CommonAlternator connector or wiring not fully reconnectedIt's easy to leave a plug loose after alternator work.
- CommonOriginal problem was the battery, not the alternatorIf the battery was drained flat before the alternator work, it may still need charging or replacement.
- PossibleSerpentine belt not properly routed after the jobA misrouted belt can fail to spin the new alternator correctly.
- PossibleBattery never recharged after sitting dead during the repairA battery run completely flat sometimes needs a slow charge to recover, not just a new alternator.
- Less commonBlown fuse related to the charging systemSome charging systems are fused and a blown fuse can remain even after alternator replacement.
What to do
- Check that the battery has a full charge; if it sat dead during the repair, give it a slow charge with a battery charger before assuming there's still a fault.
- Visually confirm the serpentine belt is correctly routed according to the routing diagram (often on a sticker under the hood).
- Check that all electrical connectors on the new alternator are firmly plugged in and locked.
- Try jump-starting the car to see if it starts and runs, which would point to the battery needing more charge or replacement rather than the alternator.
- Check the charging system fuse if the car starts but the battery light stays on.
- Have the new alternator's output tested to confirm it's actually charging correctly.
- If you or a shop did the work, double-check torque on the alternator mounting and belt tensioner.
- Have the shop that installed the alternator recheck their work if the car still won't start, since a loose connection is a common oversight.
Tools and parts
- Multimeter
- Battery charger
- Jumper cables
Group-size car battery (if needed)Search: “car battery group size for [year make model]”
When to call a pro
If a full battery charge and connection check don't get the car running, have the installer or another shop verify the alternator installation and output.
Safety
Use caution
Keep hands and tools clear of the serpentine belt and pulleys, and be cautious of sparks near the battery.