Cadillac ATS: Battery Light On – Diagnosis

If your Cadillac ATS’s battery light is on, it means a few things.  It is trying to warn you that there is an issue with the charging system.  The charging system itself is relatively simple.  It’s almost always going to be caused by a bad battery or alternator.

The battery light comes on when it detects that the voltage in the battery is too low.  Most battery lights will come on when the vehicle starts if the computer has detected that the voltage was too low for a quality start.  With an older battery, you will often notice this on the first truly cold day of the year.  

You may have noticed your ATS struggling to start, and then the battery light stays on after the start.  That would indicate a bad battery, but it’s still worth your while to have it tested first.

P0118 Cadillac ATS


Cadillac ATS Battery Light Causes

The charging system in vehicles hasn’t changed that much in the last 50 years.  The engine turns the alternator, which charges the battery.  There isn’t a lot to it, which makes diagnosing it relatively simple.


1. Battery

The battery is primarily there to start your ATS.  It can run off the alternator just fine once it’s started.  This is an important thing to know.  A bad battery usually makes itself known when you are trying to start the vehicle.


2. Alternator

Cadillac ATS Battery Light Diagnosis
A typical alternator

A bad alternator will usually make itself known when the engine is already running.  If you see your ATS’s battery light turns on while the engine is already running, that is a strong indication that the alternator is no longer providing enough wattage to power the various electrical systems and charge the battery at the same time.  A new alternator will rectify the problem.

See Also:  Cadillac ATS Bad Alternator Diagnosis


3. Alternator Belt

Nearly all modern vehicles use a serpentine belt to turn the alternator.  The serpentine belt turns more than one engine accessory (A/C compressor, power steering, etc).  If the serpentine belt goes bad, it will no longer be able to turn your ATS’s alternator enough for it to get an adequate charge.


4. Battery Post Corrosion

The voltage from the alternator is sent to the battery through a wire that connects to the battery post.  If the battery post is really corroded, it won’t be able to do its job of sending the voltage to and from the battery efficiently, and your ATS’s battery light will turn on.

It only takes a second to check it out.  A battery post cleaner or wire brush can clean it up in no time.  Just be careful not to get any of the corrosion in your eye.


Battery Light Diagnosis:  Cadillac ATS

You can diagnose whether your ATS’s alternator is bad pretty quick at home.  If you have a voltage meter, that’s a fantastic place to start.


Voltage Test

The first thing that you’ll want to do is get a good multimeter.  With a multimeter you do a quick test to see if your ATS’s alternator is working or not.  

The following was taken from bad alternator diagnosis page on this site:


  1. Set the multimeter to read voltage. Specifically, you’ll need to set it to 20V DC.
  2. With your ATS turned off, you’ll connect the negative side of the meter to the negative battery post or wire.  Touch the positive side of the voltmeter to the positive battery wire.  With the engine off, you should get a reading somewhere between 12.0 volts and 12.6 volts.
  3. Now that you have a baseline, it’s time to turn the engine on.  Leave the air conditioner off, as well as the headlights and any other accessories.
  5. Now, assuming that the alternator was producing over 14 volts, go ahead and turn on the headlights, air conditioner, radio, and any other accessory you can think of, and run the same test all over again.  If the voltage dropped back into the 12’s, it’s a very strong indication that your ATS’s alternator is bad.  It’s still worth having it tested, but be prepared to replace it.



Battery Test

If the alternator seemed to be charging the battery fine, then it is time to move on to the battery itself.  We recommend taking it to the local parts store.  Almost all of them will test it for free.  If it’s bad, they’ll even install the new one.  


Conclusion:  Cadillac ATS Battery Light

Your ATS will operate for a short period of time off of the battery alone.  But, if the alternator has gone bad, it will eventually leave you stranded.  If your battery light comes on when you are on the road, get somewhere to have it tested as soon as you can.  If you feel there is anything you can add, please leave a comment below.  Good luck!