I think my alternator has failed. What should I do?

If you are experiencing power problems, you can follow these steps to help diagnose the underlying issue. Most power problems our customers experience stem from an underlying problem with their battery or batteries.

1. Check your battery or batteries. With the engine and ignition off, check the voltage across the front battery red and black terminals with a volt meter. The reading should be at least 12.3 volts DC.

If there are dual batteries, disconnect the positive of the front battery, then measure front and rear batteries voltage with a VOM, one at a time. If the voltage is below 12.3 volts DC, then you need to replace. If there are dual batteries, replace both even if only one batter is below 12.3 volts DC.

Important note: Do not charge the battery with a charger before checking it. Check the battery as-is.

2. Ensure proper connection. Make sure there is a heavy gauge wire (about 3/8" thick) running from the red post on the alternator to the red post on the battery.

3. Ensure proper ground. Make sure there is a heavy gauge ground wire (again, about 3/8" thick) running from the alternator mount to the negative side of the front battery or to the vehicle chassis if the battery is also solidly grounded to the chassis.

Important tips for ensuring a proper ground:

  • Do not ground the alternator to the engine. This is not sufficient.
  • Ensure there is no paint on the alternator mounting bracket. Thick paint on the mounting bracket can cause an insufficient connection to ground the alternator.

4. Align belts. Proper belt alignment is critical for alternator function.

First, verify that the belt that runs the compressor and alternator is seated on the correct grooves. The compressor will usually have eight grooves, and the alternator will have six grooves. Align the belts correctly using a belt alignment laser, or use the operations manual to do it yourself. Typically (but not always) the belt should ride on the rear-most six grooves of the eight-groove pulley.

5. Use the alternator checklist. Download the Alternator Troubleshooting Checklist and run all the tests as described. This checklist is designed to correct most alternator problems.

If you follow these steps and continue to experience problems, please contact us so that we may assist you.

Does leaving a chassis sit for a while without running the vehicle affect the batteries?

Yes. It is imperative that vehicles do not sit for over a week without running the motor or being driven. Vehicle overhead loads that exist on the chassis, even when the vehicle is not running, will deplete the battery beyond damage level. It is important that the battery or batteries be put on a trickle charger or solar powered dashboard-cigarette lighter trickle charger if the vehicle will not be run for extended periods of time.

Does APS provide a warranty?


My vehicle battery exploded while using an alternator from another manufacturer. Could this happen with your product?

Battery problems usually are caused by discharge beyond recommended limits.

Most modern alternators contain internal regulators that stop them from spiking or sending too much voltage to a battery. However, even modern alternators cannot sense whether a battery has been depleted, and they will continue delivering a charge to a battery that has been discharged beyond recommended limits. The problem here is that a lead acid battery in this state cannot be recharged due to sulfide buildup*.

If sulfide buildup becomes heavy enough, then attempting to charge the battery can lead to bubbling, bulging or even explosion. Because of these risks, it’s important to avoid over-discharge of lead acid batteries. As a rule, a lead acid battery is considered damaged in terms of capacity and service life when it is discharged below 1.69 volts per cell (10.14 VDC for whole battery).

*Sulfide buildup results from sulfation, which begins when specific gravity falls below 1.225 or voltage measures less than 12.4 (2.06 vpc). Sulfation hardens the battery plates reducing and eventually destroys the ability of the battery to generate volts and amps.

Does your system provide for a separate regulator housing?

On voltage systems rated at 12VDC we do not recommend external regulation for two reasons.

First, it requires that the OE chassis wiring be cut. This violates the integrity of the OE chassis digital wiring and harms the communication between the PCM and alternator.

Second, with modern, reliable IC circuits, a far more stable regulator can be designed closer to the power source without using components such as resistors and capacitors, which are subject to heat and vibration damage.

Do you recommend a specific battery?

Yes. We recommend the Hawker PC1700 AGM.

We thought we had an alternator problem, and then we changed out the batteries. Now the alternators are working fine. What’s happening?

We find that batteries are the primary source of chassis power problems. In most cases of power problems, the underlying issue is that the battery or batteries have been discharged beyond damage level and cannot be recharged. For a complete discussion on this topic, see the first question above.