An effective method to determine whether it's your battery or alternator that's gone bad is to connect jumper cables from a running vehicle's battery to yours. After a few moments, try starting your vehicle. Once it's started, remove the jumper cables — if your engine stalls, your alternator has probably gone bad.
“If the [battery's] voltage is low, replace with a new battery or charge the battery to the recommended 14.2 to 14.7 volts,” he says. “Then try to run the vehicle and see if the voltage after is outside of that range. If it is, that's a sign of a bad alternator.”
A corroded or defective alternator diode will faultily continue charging the circuit even when the car off. This, in turn, will drain your car battery and cause the car not to start.
If your engine won't turn over or takes far longer than usual, it's time to grab the jumper cables and attempt a jump-start. If your engine starts and stays running but won't start again later, it's likely a battery problem. If your vehicle immediately stalls, it's probably a bad alternator.
“With the car running, you can turn on all the heavy loads; turn on the heater blower motor, turn the lights on, etc.,” he tells us. “Then check the voltage at the battery terminals. That'll give you a general idea of the health of the alternator.”
Make sure your alternator's positive and negative terminals are clean. Put the multimeter's black cable to the negative terminal and the red cable to the positive terminal. Look for an ideal alternator reading of around 12.6. Start the car, and look for a reading of between 14.2 and 14.7.
Alternators can go bad suddenly, or slowly over time. If your alternator is slowly going bad over time there are some warning signs you can look out for.
One of the most common problems you're likely to experience with an alternator is a failure in the bearings. There are needle bearings in the alternator that allow the rotor to freely spin inside the housing, and those bearings can break down over time as a result of exposure to heat and dirt.
If you're hearing a grinding sound in your car, this could indicate that the alternator is going bad. The grinding sound may be caused by a worn-out bearing. Your car can also make a whining sound when the voltage regulator is sending signals to the alternator to charge more than is necessary.
Parasitic Drain - If the electrical devices in your car or the lights on your car are left running even when your ignition is off, they will suck the juice out of your battery slowly but dramatically. This is often eerily described as parasitic drain.
Why won't my car start but I have power? If you attempt to start up your car but the engine won't turn over and the dashboard lights up, then you most likely have an issue with your battery. You could have a damaged or corroded battery terminal, which is stopping the engine from starting up when you fully turn the key.
The alternator translates the mechanical energy generated by the car into potential energy for the battery to store—like a charger for your car's battery. If the alternator is kaput, the battery in the car will soon follow and can stall the car.
Alternators can average anywhere from $100 to $350, depending on the model. In most cases, costs will average between $350−400 for the alternator replacement if no other parts need to be changed. Add another $20 to $50 to your bill if the serpentine belt has to be replaced as well.
You can check the alternator by simply starting your car and then disconnecting the negative battery cable. Be careful doing this, though, because there are a lot of moving parts in your engine compartment that could cause injury.
If your alternator fails when you're far from home, you can typically drive up to 200 miles on battery power alone. That's assuming your battery is in good shape and fully charged. Of course, it's best to get the alternator repaired or replaced as soon as you can.
Contrary to its purpose, an alternator can drain a battery. It's rare, but it's possible. A draw on the battery only occurs if there's a problem with either a component in the alternator itself or connections to it.
It requires a good deal of energy to crank the car's motor, and if a battery is not replenishing its power properly due to a faulty alternator, it will become drained and ineffective. You will hear a clicking noise while attempting to start the car, and the engine will have difficulty turning over.
Most alternators will last between 80,000 and 150,000 miles — or about seven years. However, your driving habits and the area where you live could wear it out prematurely.
Failing alternator bearings:
The alternator has several bearings which allow the alternator's rotor to spin freely. If these bearings begin to fail, you will notice a loud whining noise, which will increase when you accelerate or the car is under heavier load.