How to tell if your alternator is going bad?

A digital voltmeter, which is a useful tool to have, will tell the story. The voltage should be around 14 volts DC with the lights off and the engine running (slightly higher when cold, lower when hot). It is not unusual for the voltage to be low when idling with the lights on - in the 1970s and earlier it was almost universal, now it is just common. Finally, measure across the battery with the engine running and the voltmeter set for AC volts. You should see under 1/10 (0.1) volts AC. Higher voltages - like half a volt or more - are a good indication of bad diodes in the alternator.

Never remove the cable from the battery with the engine running. If the alternator has bad diodes it will damage the thousands of dollars of electronics in the car. I tested it once on my 1970 Volvo (no electronics!) and measured 60 volt spikes with an oscilloscope.

When you replaced the battery you must be sure that the ground

connections are clean and tight... The battery connection, the chassis

ground, the engine ground, the firewall ground connection. All of

these will be a contributing factor in how well your system is charging.

Alternator output at an idle should be 13.2V + and at 2000rpm it

should be no more than 14.2V.

Measure the voltage across the battery with a Sears digital multimeter at engine idle. If it reads 14.0 Volts DC or more (typically), the alternator is charging ok.

any of the auto part stores will bring a tester out to your car and check it for free.the old way was take the positive cable off your battery if it died it was ur alternator not sure if its safe with that new of a car good luck

Yes, the lights shouldn't dim at idle. Rectifier or regulator or commutator brushes are dying.