Thread: Testing an alternator

On later model cars that can also really mess with your cars computer.

As Bob said not advisable especially on new cars. Buy a multimeter. A cheap one is all you need.
Measure battery volts, start the car and read volts again.
These days it is more common the battery dies that the alternator.

I've got a BA falcon I've had for 12 years and 115,000k.
It looks like a shed that's been through three cyclones but in all that time all it's cost me is wheel bearings and a radiator.
It does however spit a battery every three years almost to the day - and I only use Century batteries.
The alternator however has never missed a beat.

The way I was taught to test an alternator 30 years ago was this way


Using a multimeter measure the voltage of the battery
Start the car
You should see the voltage increase slightly
Turn on the headlights (high beam)
The voltage should remain stable or increase slightly if the battery was low

Also back then an alternator puts out 13.8 volts and a car battery is normally 12ish volts

These numbers may have changed with modern batteries and alternators may the theory should be the same.


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What makes you think the alternator is faulty

It's only true for petrol (and many gas) engines. A diesel will quite happily keep ticking away...

Furthermore... disconnecting and re-connecting the alternator while the engine is running always has the risk of damaging the alternator/regulators diodes! (It's generally negligible, but the risk is still there.)

Just to make it very clear as mentioned above, DO NOT DISCONNECT THE BATTERY IN A BA FALCON WITHOUT A MEMEORY SAVER CONNECTED

Smidsy,

Century do batteries in a range of grades, and I would suggest that in the lower grades, 3 years is a reasonable life span, you might get more from a higher grade battery, or if the battery is used conservatively (most starts result in a 15min or longer run to replenish charge). Things to try to avoid for long battery life include:

frequent short trips (particularly with lights required) because the battery may not fully recharge before the next start cycle,
leaving vehicle outside in cold frosty weather causing sedimentation in the cells and increasing self discharge,
removing battery and leaving on a concrete floor increases sedimentation in a similar manner to leaving in car in the open on a frosty night,
leaving connected in a vehicle not being used regularly allows the car standby usage to slowly discharge the battery to a point where it is difficult to recover.