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Battery and alternator light is on but voltage and amperage readings good. I changed my alternator a couple days ago I’ll put a new one in and the battery and Alternator light came on as soon as I started it and it is staying on I let in run for a couple hours to see if it was going to die but it has not yet has not lost any power it drives good why is the battery and alternator light on still?

Premium Member
4,137 Posts
What's the voltage reading when you turn on the ignition while viewing the battery page in the engineering mode ? Then after starting the engine, what's the voltage reading on that same page ? When you installed the *new* Alternator, was it the ACDelco OEM unit ? Just for my knowledge, how do you read/know what the amperage output is ?

Have Bar Will Travel
716 Posts
Do you have a GT model G8? Did you fix your alternator with a remanufactured one? Did it have the slipper on thepulley? (IE does it freewheel in the opposite direction)

The GT model car uses a different alternator than the other models - it is not the standard GM alternator either. IT is a Delco part - it is a mitsubishi sourced alternator. It is not the common GM V8 Alternator.

As such without the slipper - it will provide variance output when the car idles. The GT model idle floats around more so than others (unless you tuned it out differently) and because oft he DOD system. This is what the slipper is for - specifically when DOD activates there is a pulse in the drive accessory drive and this damps it out.

anyway - only 3 alternators turn off the light. The OE stock unit, the replacement part number, and the PPV alternator. If you use the one off the GXP model - the light will come back on. Or so people say I've not tried it.

Another potential issue if that's not the case. using a multi-meter set to AC volts - check your voltage at the battery terminals (or really anywhere). Not digital multi-meters read RMS volts so it spikes and stabilized in a very short bit. So probe it - watch it settle. it should settle to below 1VAC. or thereabout.

If it is above 1Vac - the light will set. Rectifier circuits aren't working in limits.

and the final more difficult find - if you have a scan tool that will read the whole car (not all do) you can read the body control system and it will tell you if the computer sees Alternator out of limits. (that's the AC question above) but it will also show if the alternator isn't putting out command voltage. The computer sets the field effect on the alternator (that 2 wire lead oyu attached on the top back) to vary it's output. If the computer commands 14.2 but the output is 13.5 or whatever. it will set the light. Either the alternator is bad our your 2 wires connectors is damanged.

when I replaced mine last summer the 2 wires had wrapping that had come off completely - so while there I retaped them. Just to be sure it didn't get damage. My alternator was putting out too much AC. read 1.3VAC.
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