Alternator Voltage Drops when the Car Engine is Hot? What to Do

If you have a voltmeter installed in the car giving you the ability to monitor the alternator/battery voltage, depending on the condition of the car, you may observe a drop in voltage, say from 13V to about 12V or thereabout as you drive the car.

While the car may run fine and with no warning lights on the dashboard, it is perfectly reasonable to wonder if this drop in voltage is normal or whether it could be a sign of an alternator about to fail.

So, this post lists reasons why the alternator voltage output may drop as the car engine warms up, what you can do to fix it, and what voltages you can expect from an alternator in good working condition.

Why the Alternator Voltage Output may Drop when the Car Engine is Hot

Yes, an alternator can fail and its output voltage drops leading to loss of charge to the battery when it is subjected to excessive heat in spite of the inbuilt features – the fan and vents that are are intended to keep its temperatures down.

You may also be interested in this post. 7 Causes of overheating in alternators.

Is this Normal for the Alternator Output voltage to Drop?

In general, you can expect the alternator voltage output to vary depending on the battery voltage and driving conditions. That being said, if the alternator is in good working condition with firm, clean, corrosion-free, and solid positive and ground wire connections then the alternator voltage out (measured at the battery) will be in the 13.5 to 14.5V DC range.

A lower voltage drop is worrisome and requires further investigation. The next section lists some of the reasons why the alternator voltage output may drop.

Why the Alternator Voltage may Drop when the Car is Running – Other Reasons

If the car alternator voltage output drops significantly (about 12V and below) when you star driving then check the following:

#1. Confirm the accuracy of the voltmeter reading especially if you’re using a cigarette lighter plugin voltmeter. Some of these meters have not been found to be as accurate. Therefore, take the voltage reading using a good quality multimeter (set to read DC volts) to confirm if indeed the voltage drop is that low.

#2. Inspect and confirm that the alternator belt is firmly connected and does not slip as the alternator rotates. If it does, check that you’re using the correct type of belt and that the belt is still in good condition. A slipping belt can affect the ability of the alternator to supply the required voltage.

#3. Inspect and check the wire connections from the alternator to the battery. It is possible that there is a loose, broken, or damaged wire connections. Wire connections can look intact from the outside yet its only a few strands holding the wire interior. You can conduct a voltage drop test to rule out any voltage drops as a result of the poorly connected alternator or battery cables.

Using Voltage Drop Measurements to Confirm Alternator Ground Connections

What you need: A good quality multimeter set to read DC voltage


  1. With the multimeter set to read DC voltage, connect the positive lead of the multimeter to the alternator case or ground connection and the negative lead to the alternator battery’s negative connection.
  2. Next, start the car engine and rev it to about 2000 RPMs, while holding the RPMs, switch on the car headlights and radio. Take a reading of the voltage drop. The voltage drop should be up to 0.2 V or less for a well-grounded alternator.

If higher, inspect, clean, and make sure that the ground wire connections are clean and firm.

#4. The alternator may be faulty. If you’ve confirmed the accuracy of the voltmeter reading, that the right belt has been installed and wire connections at the alternator are firm and clean then have the tested at a reputable auto parts store to check whether it is an alternator fault or not.

An alternator can fail for various reasons. For example, the regulator may be faulty and unable to hold the voltage output as required, or the brushes may be worn and need to be replaced. These are just a few of the faults you can experience with an alternator.

Closing Thoughts

The alternator voltage should be about 13.6V – 14.5V when the car engine is running. If the voltage is much lower then there is likely a problem. Confirm the accuracy of the voltage readings using a good quality multimeter and check the alternator wire connections to make sure they are firm, tight and that the cables are intact.

It is also possible that the alternator is faulty. You can have it tested at a reputable auto parts store.

Leave a Comment