New battery, New alternator, car runs fine but I still have a check battery light

Hey guys, my car pooped on me yesterday so I went out and got a new alternator and battery. I made sure to install them both properly and after the initial jump due to my battery being drained from the bad alternator all was well. However my check battery light is still on. I took it to auto zone and they tested both the battery and alternator and they said it’s working like it’s supposed to. I took my car out on the highway and when I reach higher speeds the light will turn off. After some time slowed down or idled the light will turn back on. I’ve checked all of the connections. I’ve also disconnected the negative terminal for a bit to see if that would reset the warning light. Maybe it’s the wiring? I’m unsure and would like you guys to give me some feedback. Thanks!

Check the negative ground from the battery to the engine block, my first thought.


I’d guess your serpentine belt and the tensioner needs to be replaced. I’d also guess this was your problem all along.


Check the PCM for fault codes, there are other charging system problems that can cause the charging system light to illuminate.

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Damn… when I picked up my alternator I figured I should grab a new belt but I didn’t. Makes sense though considering the belt might not be moving well with the alternator to create enough juice.

Yep. I’m going to bring it to a shop tomorrow so they can check the codes and give it to me straight. Thanks

You can buy one of those system testers for under $20 even at Walmart. Plug it in to the accessory port and will tell you the battery voltage at rest, and then while you are driving. Complete with red, yellow, and green lights for quick reference. Should be over 14 when driving. I have one in each car just to check once in a while since they don’t have volt meters in most cars anymore.

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Update: the mechanic told me it was a wiring harness that was worn out so I got that replaced and the battery light is off for good. Thanks for the help guys

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Good for you. One bit of advice, suggest to not run engine w/battery disconnected. Battery acts like big capacitor, filtering out alternator’s voltage spikes. Potential damage to sensitive car electronics.

Worn out wire harness. Inconceivable.

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The way I read this is a wire was grounded where it shouldn’t have been and caused the problem.

I had a '99 Monte Carlo that needed an alternator. The new one crapped out too. I looked closely and found the positive battery cable had been rubbing on something (Since the car was new) and the insulation finally wore through. As I moved the cable I actually saw sparks! After I fixed the cable and another new alternator the problem was gone.