The Rockauto newsletter had some very good info in it this month and thought I would share in case some of you don't get it.
The First Things to Check
When the battery goes dead, the condition of the battery and the alternator are two of the first things to check. However, do not assume that if the battery is good then the alternator must be bad and the cause of the problem. My 1979 Chrysler 300 still has its original, never disassembled alternator. I have lost count of how many new batteries and electrical problems the car has had over the decades. Here are a few tips on how to use common tools to diagnose charging system problems to prevent unnecessarily replacing the alternator. These are general suggestions that may not apply to every car. Rely on your car’s repair manual (found under “Literature” in the catalog) for vehicle specific instructions.
Using a multi-meter, measure the voltage across the battery posts with the engine off. It should be 12 or 12.5 volts. If it is less than 12 volts, then the battery needs to be charged, something is draining the battery or the battery cannot hold a charge and needs to be replaced.
If the battery voltage is around 12 volts, then start the engine and again measure the voltage across the battery posts. It should have increased to 13 or 14 volts if the alternator is working and charging the battery.
If the battery voltage did not increase with the engine running, then verify the alternator and battery have good electrical connections. Make sure the battery cable terminals are tight and free from corrosion. Look for loose connectors or frayed cables. Use the multi-meter or a test light to verify the body of the alternator is grounded (zero resistance between the alternator and negative battery post).
If the battery voltage is well above 14 volts with the engine running, then the alternator could be producing too much current and overcharging the battery. Maybe the voltage regulator (internal to some modern alternators) is bad. Or the battery is weak and a vehicle computer has temporarily raised the voltage limit. It might still also be bad connections or loose wiring. Some vehicle lights being brighter than others can be a symptom of this type of problem, because the alternator produces more energy to overcome the resistance of a bad wire or connection.
The condition of the alternator belt is the next thing to check. Of course make sure the belt is not broken. Also look for a belt that is loose and slipping. It would probably be making noise.
If in step 2 the voltage across the battery terminals was 13 to 14 volts with the engine running, then the alternator is properly charging the battery. That is good news, but there is another alternator electrical problem to check for. I know from personal experience with a Ford 5.0L. that sometimes an alternator can charge fine when the engine is running but drain the battery when the engine is off. It probably has something to do with the rectifier diodes in the alternator not properly blocking current from draining away from the battery. This problem may be intermittent. It certainly was on my Ford 5.0. Disconnect all the electrical connections from the alternator when the car is put away for the night. If the battery never goes dead with the alternator disconnected but goes dead when the alternator is hooked up, then that might point to an alternator diode problem. RockAuto sells replacement diodes for some alternators. The diodes are buried so deep inside many modern alternators that replacing the entire alternator is often the most practical option.
If the battery goes dead when the car sits (engine off) with the alternator electrical connections disconnected, then something else is draining the battery. Check first for the most obvious battery drains such as any lights left on (headlights, trunk light, ashtray light, etc.). After that it gets harder to track down the problem especially on modern cars. Newer cars have systems that are designed to stay on for various amounts of time after the engine is switched off. In the next newsletter, I will offer some tips on tracking down specific circuits that are draining a battery.
Finally, there are the mechanical alternator problems. Unusual screeching or grinding sounds might come from a bad bearing in an alternator. However, the noise might also be coming from a loose belt, misaligned pulleys or worn out belt tensioner. Use an automotive stethoscope to verify where the noise is coming from. Avoid damaging new alternator bearings by making sure the belt pulleys are aligned, the belt tensioners are good, the belt is not over tightened, the alternator mounting bracket is not cracked or that there is not some other mechanical problem.