Best of Deals Car Reviews Repair Shops Cars A-Z Radio Show

2009 Volkswagen Jetta - Battery light with RPM

battery light comes on and off depending on rpm’s, mostly on low idle. Had belts replaced, but still lighting up. what can it be?

When you had the belts replaced you should have had them also check the battery and alternator . That is your next step.

Thank you. How safe is it to drive like this? I’m trying to schedule it to shop.

The car is “safe” to drive… it won’t blow up or anything.

It may run your battery down so far the car will no longer run but it will just shut off and leave you by the side of the road. THAT situation you can judge for yourself as safe or not.


The first thing to check is that the belt tensioner is still putting enough pressure on the belt to keep everything turning. These tensioners can get weak and no longer keep the belt tight enough.

If that is ok, then you need to have the charging system checked.


Thanks for your input. My car is still @vw service due to battery light issue so frustrating! I’ve already had secondary air hose w sensor replaced, throttle body cleaned, alternator belt and a/c changed for preventative And needed maintenance thru all of this they ignored my request for them to check alternator…finally they did and replaced it but battery light still on!!! What else could it be? VW can’t seem to get it right…help!

Bad tensioner or the fusible link is blown. or the new alternator is not working like it should.


That light comes on when the alternator isn’t producing as much electrical power as the rest of the car is asking for. There’s 3 possibilities

  • the alternator isn’t producing the amount of power it should be for that rpm
  • the rest of the car is asking for more power than it should be (due to an electrical system or battery problem most likely)
  • the circuit that performs the alternator light function is not working

It’s possible for a shop to easily to measure the alternator output vs rpm. It is also easy for a shop to test the battery performance. Less easy to do, but they can also measure the amount of electrical power the rest of the car is using to check for shorts etc. Removing fuses for unneeded circuits and seeing if that solves the problem is another diagnostic option, although not the preferred one. If the tests show the alternator is matching the full power vs rpm curve, the battery is good, and there’s no abnormal electrical drain in the car’s electrical system the problem must be the last one, the alternator warning light isn’t working.

There’s one other remote possibility: The alternator warning light may be used as a back-up brake warning light. That’s the case on some cars to use the alternator warning light as a brake fail safe warning lamp. In which case the problem isn’t the alternator, but the brakes. VW should know about this though if that’s the case for your Jetta, so I doubt that’s the problem.