Common signs of a failing alternator
Alternators are part of the charging system and they recharge the truck's battery. A failing alternator will cause dim lights, malfunctioning electrical equipment or even a dead truck.
- Low batteries.
- Corroded or frayed cables or loose connections.
- Misaligned or loose mounting, or belt tension and belt tensioner failure.
Alternators are at the heart of a truck’s charging system, which also includes the battery, cables, tensioner and belt. With so many moving parts, an electrical problem with the truck may indicate a failing alternator, but it could be more complicated.
What do alternators do?
Alternators are designed to monitor the voltage of the battery and maintain the battery’s charge depending on the power demands of the application.
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“An alternator is not designed to charge a dead battery, keep charging a bad battery, or keep up with added accessory loads,” says Mark Weremy, technical product manager for industrial rotating electric power at Terrepower, maker of Wilson alternators and starters.
What are common problems with heavy-duty alternators?
Weremy says some common causes of alternator failure are:
- Low batteries: A low battery condition can cause alternators to run hot and leads to straining and premature bush wear, leading to a reduction of the alternator’s life span.
- Corroded or frayed cables or loose connections: These issues can cause resistance and a failure in the electrical system, leading to shorts and improper current to the truck’s components.
- Misaligned or loose mounting, or belt tension and belt tensioner failure: These can cause bearing failure and well as failure or damage to other components on the vehicle.
“A loose or poorly mounted alternator can lead to belt misalignment, vibration and inconsistent electrical output, all of which reduce performance and can cause premature failure,” says Don Jordan, DENSO product manager. “Proper mounting ensures stable operation, correct belt tension and reliable charging — critical for heavy-duty truck systems.”
What are signs of an alternator problem?
Catching signs of a problem in the electrical system early can prevent downtime and expensive repairs.
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“Operators should keep an eye out for symptoms, such as frequent jump starts, low batteries, erratic or abnormally high voltage readings or unusual noises, specifically whining or grinding sounds,” says Brian Thompson, North America fleet director at PHINIA. “While these symptoms may not definitively point to alternator failure, they may be signs of underlying electrical system issues that warrant further investigation.”
Also look for dim lights, battery warning lights and malfunctioning accessories.
What are routine maintenance practices for alternators?
The entire charging system should be inspected regularly to look for signs of corrosion, contamination nor damage, Thompson says. The system should also undergo regular voltage drop testing to ensure proper electrical performance and technicians should check to make sure they’re using the proper torque specifications to optimize the alternator’s lifespan.
“It’s essential to verify that the drive belt and tensioner are functioning correctly,” Thompson says. “Regular application of corrosion protection on terminals and batteries can also prevent premature alternator failure.”
Jordan says listening for unusual noises also can help catch bearing issues early.
Finally, operators should pay attention to how accessories and power take-offs are working. They don’t necessarily impact the performance of the alternator, says Jeremiah Packett, application engineer for PHINIA, but they can have an impact on the overall electrical system.
“Power take-offs, specifically, can affect performance depending on the engine speed and alternator pulley ratio,” Packett says.
