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I recently installed a new aftermarket alternator in my 2006 Lexus is250 and checked the battery to make sure it was good.

After installation, the charging system warning light is on and I can't figure out what the problem is.

In testing, the voltage across the battery terminals are a constant 13.85V, at idle and at 5000rpm. So the battery is charging, but not optimally, as I understand the voltage should climb to around 14.2V at higher rpm. Could this be regulator related? If so, does it indicate the wrong regulator, a blown one?

Cheers, Juan

  • Welcome to Motor Vehicle Maintenance & Repair! What is the battery voltage with the engine off? – Pᴀᴜʟsᴛᴇʀ2 Nov 12 '18 at 15:00
  • Thanks! It's around 12.4V. I took the battery in for testing, all good on that front. – Juan Nov 12 '18 at 15:25
  • Check all of your connections to the alternator. Ensure there's no corrosion on any of them. Secondly, it could be you got a bad alternator. Just because it's charging doesn't mean it's doing it correctly. If the connections look good, take the alternator back and see about a replacement. – Pᴀᴜʟsᴛᴇʀ2 Nov 12 '18 at 15:40
  • One test you can do is to actually measure the current output of the alternator. That means you need a meter or shunt capable of 120A or so. As long as the battery is not fully charged then you should see 30A or 40A or more at 2000 rpm, the workshop manual will tell you exactly the numbers to expect. If the current is low then it may mean the diode pack or the regulator is faulty... – Solar Mike Nov 12 '18 at 16:20

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