Alternator or battery issue? (Mechanic noob)

[Trey22]

1996 Ford Explorer XLT 2WD.

Ran an errand, started up fine and 5 minutes later was driving in freeway traffic (around 35mph), and the battery icon keeps flashing on and off erratically (seemed to follow the decrease in RPM's, when I hit the gas it would stop sputtering). I check the battery "meter" gauge needle and it's at the bottom at L.

Pull off the freeway and as I exit and come to a stop, it sputters and dies. Turn the ignition and all I hear is a clicking sound, engine doesn't crank/turn over. Everything was dim (interior/exterior lights, turn signals).

Put on my hazards and waited for AAA to tow it home. The hard part was pushing it into the garage... in the freaking snow.

Tow truck driver said it was probably the alternator. Accurate diagnosis?

 

[7window]

It could be either one. How old is the battery? Charge your battery and take it to autozone or oreilly. have them check it . they will give you better diagnosis. It is also free.

 

[olds]

Battery cables tight?

 

[Zenmervolt]

It's the alternator.

Once an engine is running, the battery isn't strictly necessary since the alternator (when properly functioning) will provide more than enough current to run the engine on its own. So a dead battery will not cause an engine to stall as long as the alternator is functioning properly.

If the alternator stops producing sufficient output to run the engine, the engine will start drawing power from the battery and start draining the battery. Once the battery itself dies, the alternator cannot keep up and the engine will stall and the battery will be so drained that it cannot re-start the engine.

There are some very rare situations where a battery can be damaged (reversed cells, etc) and cause problems on its own, but those almost never happen. In the situation you describe, 9,999 times out of 10,000 it's the alternator.

ZV

 

[mmntech]

It's definitely the alternator. I've had the exact same thing happen on my 7th Gen Civic, which is one of the most common mechanical problems with them.

Take it to the mechanic. They'll hook it up to a machine to test the electricals.

 

[Pacfanweb]

Probably the alternator, but the battery isn't an impossibility.

Quick, easy way to check: Jump it off, and remove the negative cable while it's running.

If it runs fine and the problem goes away, it's the battery. If you have the same issue, it has to be the alternator, since you've taken the battery out of the equation.

 

[LTC8K6]

Probably the alternator, but the battery isn't an impossibility.

Quick, easy way to check: Jump it off, and remove the negative cable while it's running.

If it runs fine and the problem goes away, it's the battery. If you have the same issue, it has to be the alternator, since you've taken the battery out of the equation.

I have always heard it's not good to disconnect or connect the battery while the engine is running.

The voltage drops/spikes aren't good, and the output from a bad alternator might not be good for the electronics either. Could be an interesting waveform.

 

[996GT2]

Most likely alternator, but check the battery cables/terminals too because they're very inexpensive to replace if needed.

 

[Trey22]

Thanks all, I'll be working on it this afternoon and will run the above tests.

 

[sontakke]

Battery is cheap and easy to replace. Test the system with voltmeter and it would be pretty obvious if alternator is not charging. You will need good battery (or jump) to get the vehicle started. Once it is running, put the voltmeter and if it is under 13.x volts, you need new alternator.

 

[mike2fix]

Probably the alternator, but the battery isn't an impossibility.

Quick, easy way to check: Jump it off, and remove the negative cable while it's running.

If it runs fine and the problem goes away, it's the battery. If you have the same issue, it has to be the alternator, since you've taken the battery out of the equation.

Sorry but this one still makes me cringe when I see it, it was valid many years ago with generators but not alternator systems. Disconnecting the battery can cause damage to the system and just generally is not a safe practice.

 

[The Stigenator]

Alternator... charge teh battery up and then check voltage when cars running it should show 14v ish at idle..

 

[Trey22]

Battery tested fine... it's the alternator. Picked up a new one, will be replacing it this afternoon.

Thanks all for the advice.

 

[SpatiallyAware]

Thank you for coming back and letting us know!


Just a heads up, if your battery is older than 5-6 years you might consider going ahead and replacing it. Dealing with a bad alternator is usually pretty tough on a battery.

 

[tortillasoup]

All you need to diagnose an alternator (since more sophisticated testing requires complete removal of the alternator) is to take a Volt/multimeter, hook it up to the battery. Measure voltage, should be above 12V, then turn on the car, idle it, it should be above 13.2V, then turn on the headlights, voltage should be above 13.2V, and then fully load it down with the defrost and everything running, A/C OFF, check voltage, if it drops to 12.x Volts (Battery voltage), you have a dirty electrical ground(s). Another thing to check for when idling the vehicle with no load is to see if it has a whining sound to the car when it's idling, like the sound of a super charger, if it does, it means you have a bad diode in the alternator and it needs replacement.

That's all there is to diagnosing electrical faults like this in a vehicle. Knowing that your vehicle is well over 10 years old, my guess would be you have dirty electrical grounds and the alternator just can't sufficiently charge the battery because of it. Dirty grounds will also prematurely wear out the alternator and hurt your gas mileage due to the continuous charging. One other thing I'd like to mention is that while grounds are 99% of the problem, I did encounter a volkswagen Jetta where the issue was of course dirty electrical grounds but ALSO a corroded/melted/damaged fusebox which sits right on top of the battery. After I repaired that issue, I later found out that the customer went through 4 alternators and 4 batteries before I finally fixed the issue for them.

 

[LTC8K6]

Charging System Light
This light, shown as a battery symbol on your cluster, indicates
that your battery is not being charged and that you need to
have the electrical system checked

The light on with engine running pretty much indicates that the alternator is laying down on the job, barring an unusual situation.