How to test an Alternator in 7 simple steps 

An alternator has two key roles within a vehicle, it charges the battery and keeps the electrical components running. If the alternator becomes faulty, the battery light on the dashboard may illuminate, however, if the light doesn’t come on there are other signs to be aware of:

If the alternator belts aren’t turning freely or have slipped on the pulley there may be a noticeable burning rubber smell

If the alternator fails it may cause bearings to fail in the engine and there may be a noticeable rattling sound

If the alternator isn’t running efficiently, the electrical components in the car may be weak or fail i.e. lights, electric windows, radio etc.

If the alternator fails to recharge the car battery the vehicle won’t start

If any of these signs are apparent, you can seek advice from a garage or if you feel confident you can purchase a Multimeter for as little as £20 and test the alternator using this 7-step guide. 

Step 1 

Set up the multimeter:

The Multimeter must be set so it points to 20 DCV (direct current volts). Ensure the vehicle is parked in a safe location and the handbrake is engaged, then lift the bonnet. 

Step 2

Locate the battery:

Every battery has a positive and negative terminal. The positive terminal will be marked with a ‘+’ and the negative terminal will be marked with a ‘-’. If you are unsure which is which, refer to the owner’s manual. 

Step 3 

Attach the multimeter:

Using your set multimeter, connect the red wire to the positive battery terminal (+) and then connect the black wire to the negative battery terminal (-). Once connected, check the reading on the multimeter, this should display approx. 12.6 volts meaning the battery is properly charged.

Step 4

Check the functionality:

To test the efficiency of the alternator, turn on the engine. 

Step 5

Check the voltage:

With the engine running and the multimeter attached, an alternator in good working order should display a reading between 14.2 volts and 14.7 volts. This is a sufficient enough voltage to recharge the battery, if the reading is outside of these parameters it will indicate that the alternator is either overcharging or undercharging the battery. Although the battery can be sufficiently charged with 12.6 volts; if the reading is below 14.2 the other electrical components will not work effectively. 

Step 6

Load test:

To fully test the alternator, turn on all of the electrical components in the vehicle that you are likely to use simultaneously including the radio, lights, fan, charging leads etc. Monitor the reading on the multimeter when engaging the electrical components looking for any fluctuations. The overall reading should not fall below 13 volts.

Step 7 

Switch off the engine:

As a final test, switch off any electrical components and turn off the engine, check that the multimeter reading is still at 12.6 volts or higher.