alternator test

How To Test An Alternator

Is your car’s battery flat more often than not? Engine slow to wind over and hard to start? These could be signs of a faulty alternator and a simple 2 minute test will go a long way to diagnosing the issue.

You will need one piece of equipment for this test though, a multimeter. These can be had for as little as $15-$20 online or at places like Super Cheap and a cheap one will certainly be ok for what we are doing here. In fact, the little ‘Best Buy’ one you see me using in this article cost me like $15 and is what I used to sort the electrics when I transplanted an AU Engine and Auto into my XD so don’t be worried about them not being up to the job 🙂

The first test we need to do is battery voltage at rest (i.e. without the engine running – negative multimeter lead to battery negative, positive multimeter lead to battery positive). For a healthy battery this should be above 12 volts.

alternator test 2

For the next test we need to get the engine started. If your battery is flat try substituting it for a known charged one or jump starting your vehicle from another (assuming you know how to do this safely) or charge the battery overnight with a battery charger.

The optimum voltage with the engine started (and alternator working) is 13.7 volts. The Commodore in the image below has the headlights and a/c on which should explain the higher voltage reading, but this does indicate that the alternator is working fine.

alternator test 1

If after starting the engine the voltage remains the same or increases only slightly and doesn’t make it anywhere near 13.7 volts then we have an issue with either the alternator or the wiring from the battery to the alternator, more on that in a minute.

If you see a large increase in voltage when the engine is started (to 17 or 18 volts for example – not all that common but certainly possible) then the alternator will need to be either repaired or replaced. The battery may need to be replaced also if it has been constantly over-charged.

Getting back to a low voltage reading, we need to determine if it’s the alternator at fault or the wiring between the battery and the alternator that is causing the issue. To do this we need to test the output of the alternator, at the alternator.

We need to find the ‘post’ or bolt where the main battery lead is bolted on. Typically this is at the rear of the alternator and may have a insulation boot on it that we need to move out of the way temporarily.

Find a good earth for the multimeter negative lead (a metal part of the engine is usually earthed or use the battery negative if the lead will reach) and probe the post on the back of the alternator.

alternator test 3

alternator test 4

If the voltage reading here is the same low reading as at the battery (nowhere near the optimum 13.7 volts) the alternator is not charging and needs to be replaced or repaired.

If the voltage reading is around 14 volts or higher (compared to the battery’s 12-12.5) there must be a fault in the wiring between the alternator and the battery. If this is the case it’s best left to an auto electrician to sort out but at least you’re one step closer to having a reliable vehicle again!

2 thoughts on “How To Test An Alternator

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