Troubleshooting the Alternator

Charging System Diagnosis

AlternatorThe following alternator information has been created as a guide for charging system diagnosis and troubleshooting the alternator. Refer to the Original Equipment Manufacturer (OEM) service manual for specific information pertaining to charging system diagnostic procedures and safety precautions for your specific vehicle model.

Helpful Tips

1. What is the condition of the battery?

– A visual inspection and a performance test of the battery must always be performed before inspecting the charging system.

– The battery must be fully charged (12.6 volts) and the battery cables, terminals and case in good clean condition.

2. Does an amperage (amp) gauge or voltmeter indicate a charging system problem? 

Amp Gauge:

– Ignition ON engine not running

– The amp gauge should read zero.

– Ignition ON engine running

– The amp meter should display a current output. It will display a different level of charge depending on what electrical circuits are operating.

– Wires and connectors

– Corroded, broken, loose or frayed wires/connections could cause zero or inconsistent readings on the gauge.

Voltmeter:

– Ignition ON and engine not running

– Gauge readings should be between 12.0 and 12.6 volts for this condition.

– A reading below 12 volts could indicate insufficient charging, low battery, or corroded, broken, loose or frayed wires/connections.

– Ignition ON and engine running

– Gauge readings should be between 13.0 and 14.4 volts for this condition.

– A reading exceeding 14.4 volts could indicate a bad battery, failed regulator or poor wire connections.

– A reading below 13.0 volts could indicate a failed alternator or corroded, broken, loose or frayed wires/connections.

3. Are any fuses open (blown)?

– Check the fuses in all the truck fuse box locations.

– An open (blown) fuse indicates circuit problem(s), which may have an effect on the charging circuit.

– Check the OEM service manual for the location of each fuse box and proper circuit diagnostic procedures.

4. Is the fusible link(s) open?

– There may be several fusible links controlling battery electrical current to the vehicle electrical circuits.

– If a fusible link is open, then electrical current will be completely lost to all electrical circuits controlled by that fusible link. Check the OEM service manual for the location of each fusible link and proper circuit diagnostic procedures.

5. Is the alternator’s drive belt tension within specification?

Too Loose

– If the drive belt is too loose, then it will slip around the pulley causing the alternator to charge irregularly or not at all.

Too tight

– If the drive belt is too tight, internal bearing damage could possibly occur, causing premature alternator failure.

6. Is the alternator’s drive belt in good condition and the proper size?

Worn or too narrow

– If the alternator’s drive belt is worn or too narrow, then it will slip around the pulley causing the alternator to charge irregularly or not at all.

New drive belt

– A “new” alternator drive belt is a belt that has been used for less than 5 minutes.

– Once an alternator drive belt has been used for longer than 5 minutes, it is considered a “used” belt. It is important to check and adjust the belt’s tension to OEM specification after the initial 5 minutes of operation.

7. Has the vehicle been modified or additional equipment been installed?

Accessories

– Non-plant installed equipment can overload alternator performance and cause premature failure.

Improper accessory installation

– Improper accessory installation procedures can cause charging problems.

– Some of these problems may include poor ground points, loose connections or improper wiring.

8. Has any work been performed on the vehicle? 

Electrical ground points

– Check the ground circuits between the battery and engine, and also from the vehicle body to the frame for high resistance or missing ground straps.

– Many times when a vehicle has been repaired, the ground point(s) are disturbed or not re-secured properly.

– If your vehicle has multiple grounds, then each electrical circuit may be assigned to one or more ground points.

– If a circuit is activated while another electrical system is being operated, feedback from the controlled circuit due to a poor ground associated with the controlled circuit may be the root cause of the alternator malfunction and/or vehicle problem(s).