Suspect Bad Alternator Wires? (What Tests you Should Do)

A bad or faulty alternator is one of the reasons why your car battery may not be charging.

Note, that this is just but one of the possible causes. There are others too – a bad battery, weak or loose cable connections – note this list is not exhaustive.

Even when you narrow down the problem to the alternator, you’ll need to check the alternator wire connections as it might well be why the car battery is not charging and not the alternator itself.

So, how do you go about checking and testing the alternator wire connections?

This post lists what tests you can do to check and confirm whether the alternator wire connections are good or not and what symptoms you can look out for.

Symptoms of Damaged or Loose Alternator Wires

Note: The symptoms listed below are only indicative and may be attributed to other faults other than damaged alternator wires.

Additional tests may be required to confirm that there is a fault with the alternator wire connections.

#1. The car battery may be drained flat because the alternator is not able to charge it giving the impression that the alternator is faulty.

If there is a break, damage, or corrosion to the wire connecting the alternator positive output to the battery then the battery may charge partially or not at all.

The battery can then drain overnight or cause the car to stall when driving because there is no alternator power or battery power to keep the car’s electrical systems powered.

You may also be interested in Why your car’s battery may not be charging fully

Test: Conduct a voltage drop test on the alternator ground and the alternator’s positive output. The voltage drop reading should not exceed 0.2V.

How to conduct the voltage drop test on the alternator ground

What you need: Good quality multimeter.

Testing the Negative ground and Positive circuit voltage drops

  • Start the car engine. Set the voltmeter to read voltage. Place the black probe of the multimeter connected to the negative terminal on a known ground connection for the car. Look for a stud on the car’s metal chassis.
  • Next place the red meter probe connected to the positive terminal on the alternator ground connection. The voltage reading should not exceed 0.2V. If higher, confirm that the connections are clean and make firm connections to the respective terminals.

If the voltage drop is still higher than 0.2V even after confirming the cable connections, then there is probably a broken or damaged cable connection that you need to identify and replace.

To check the voltage drop on the positive circuit, repeat the above procedure but this time place the black negative probe on the battery post marked negative and the red probe on the alternator output terminal.

#2. The output voltage from the alternator when the car engine is idling is low. The output voltage of the alternator measured with a good quality multimeter across the battery terminals should be in the 13.5 – 14.5 V range when the car is idling.

A lower voltage may be the sign that there is a break in one or more alternator wires.

#3. The replacement alternator still not charging a known good battery even after the initial diagnosis points to a faulty alternator.

The Alternator wire connections

The car alternator has a plug connected to it. Depending on the model of the car, you may have 3 wires:

  • one wired to the charge light on the dashboard that turns off when the alternator is charging the battery,
  • the second supplying contact 12V from the battery to the alternator regulator, and
  • the third, supplies 12V to the alternator when the ignition is switched on.

Additionally, there is a heavier wire that runs from a stud on the alternator (usually labeled “Batt”) to the Battery and finally the connection of the alternator to the car chassis.

Will an Alternator Work with Only One Wire?

If there is a break or partial damage on any of these wires then the alternator will not work properly.

Depending on the wire affected, the battery light may not work as expected or the alternator will not charge the battery.

What can Go Wrong with the Alternator Wire Connections

#1. Terminal connections in the connector plug are broken, corroded, and not making clean contact with the alternator socket.

#2. Loose positive cable running from the alternator to the battery

#3. Broken or cracked plug as you remove or fit a new alternator. Tip: Take note of the positioning of the wires in case the connector breaks. It will save you hours trying to figure out the correct placement of the wires.

If this is not possible, for newer models, you can contact the car dealer. For older models, you may seek support from others who have the same car make and model from a forum for your particular car.

Final Word

Your alternator wires may be damaged if the battery is partially charged or not charging at all or the alternator voltage is low.

To check the alternator wires, you can conduct continuity tests on each and or voltage drops tests for the positive and negative ground circuit.

The voltage drop should not exceed 0.2V for a good solid connection.

Leave a Comment