If your Audi TT’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 TT 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.
Audi TT 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.
The battery is primarily there to start your TT. 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.
A bad alternator will usually make itself known when the engine is already running. If you see your TT’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: Audi TT 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 TT’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 TT’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: Audi TT
You can diagnose whether your TT’s alternator is bad pretty quick at home. If you have a voltage meter, that’s a fantastic place to start.
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 TT’s alternator is working or not.
The following was taken from bad alternator diagnosis page on this site:
- Set the multimeter to read voltage. Specifically, you’ll need to set it to 20V DC.
- With your TT 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.
- 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.
- After firing the engine up, you should get over 14.2 volts. If you got 14.2 or more volts, the alternator is charging fine. If it’s still showing in the 12’s, it’s time to replace the alternator. If it’s producing a voltage level in the 13’s, go ahead and take it into a parts store and have it tested. It’s probably bad.
- 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 TT’s alternator is bad. It’s still worth having it tested, but be prepared to replace it.
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: Audi TT Battery Light
Your TT 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!