How to Replace an Alternator

Alternator problem?

If the charging light is lit on your dashboard or your battery won’t stay charged, chances are you’ve got a bum alternator. I’ll show you how to test yours without removing it from the vehicle and how to replace it if it’s bad. This hour of work will save you about $100 in labor, and even more if you’re a smart buyer (see “Alternator Buying Tip,” below). You’ll need a digital multimeter to perform the test, and you’ll probably need a battery charger to bring the battery up to full charge.

Test it first

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Photo 1: Connect the multimeter to the battery

Set the multimeter knob to DC volts (20 or less). Then touch the red lead to the positive battery post and the black lead to the negative post. Note the voltage reading.

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Find the state-of-charge

You may need to charge the battery before testing the alternator if the battery is less than 50%.

Start by testing the battery’s engine-off voltage (Photo 1). Then refer to the battery voltage chart below to find your battery’s “state-of-charge.” If the battery is less than 50 percent charged, you’ll have to charge it up to 100 percent with a battery charger before testing the alternator.

After you remove the battery charger, turn on the headlights (engine off) for four minutes. Then shut off the lights, start the engine and test the battery voltage again. If the alternator is good, you’ll get a voltage reading of 13.5 to 14.5 volts. If it’s not that high, turn on the headlights and the blower motor and raise the engine speed to about 2,000 rpm. If the reading still doesn’t hit the mark, let the engine run for five minutes and repeat the test. If it fails this time, replace the alternator. If the alternator does produce the correct voltage, move on to the diode test.

The diodes are the electronic part of the alternator that convert AC voltage to DC. Switch your multimeter to the lowest AC setting and reattach the test leads to the battery. With the engine running, you shouldn’t see any AC voltage. If you do, you’ve got a bad diode and you need a new alternator.

Replace the alternator

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Photo 2: Disconnect the cables from the alternator

Depress the latch clip on the electrical connector going to the voltage regulator and wiggle it out. Then loosen the locknut to the “BAT” cable and remove the ring terminal.

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Photo 3: Replace the alternator

Rotate the belt tensioner (if equipped) or loosen the tensioning bolt near the alternator. Then slide the belt off the alternator pulley. Replace the alternator.

Start the alternator swap-out by disconnecting both battery cables from the battery. Then remove the wires and cables from the back of the alternator (Photo 2). Next, remove the alternator belt (Photo 3). Then remove the two alternator retaining bolts. The bolts are really long and you’ll be cranking for a long time. If there was ever a time to invest in an air-powered ratchet, this is it. The bolts are usually different lengths, so note where each bolt came from. With the bolts removed, lift out the old alternator and drop in the new one. Then reverse the procedure to reinstall.