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2008 Saturn Vue Green Line - Detailed overview of a charger issue

Here’s what’s going on with my Vue. It’s a 2008 Green Line hybrid. The alternator charges both the 12 volt system and the 36 volt hybrid system. It’s a “mild hybrid” system in that the hybrid just gives a boost to the gasoline engine.

I’m having an intermittent issue with the battery light coming on and the vehicle not charging. I replaced the alternator but that didn’t fix the problem.

I can start the vehicle up cold and it will be fine. If I drive it six or seven miles, then stop for a few minutes to run an errand, the problem may or may not manifest itself.

Say I go to the post office. When I start the vehicle after getting done there, one of three things will happen.

1.) It will start as normal

2.) It will start as normal, but I will get four “dings” from the warning chime without an accompanying message on the dash.

3.) The battery light will come on, along with a "service hybrid system"message.

I can run three or four errands when I am in town and the problem may or may not occur. On Saturday, I ran to town and made several stops with no issue. When I got home, I started the car just to see what happened right after I shut it off, and the battery light came on. Didn’t start the car again until today, and it started fine.

I had a friend hook a fancy scanner up to it. The relevant codes were P0AF800, hybrid system voltage; and P1A4700, battery energy control module hybrid battery voltage sensor performance.

But something else interesting showed up. With the engine running and the battery light on, the hybrid system voltage showed up as 3276.9. That’s not a typo. Three thousand, two hundred seventy-six point nine. All temperatures and hybrid battery fans checked out normal. But the hybrid battery low current sensor and high current sensor had some wild fluctuations, from zero to negative amperage.

I think it’s related to the temperature of the hybrid battery, or the battery energy control module. If it’s the BECM, that’s an expensive part to try to obtain new, and I’m not sure how easily I could find a used one. The way the system works, if the hybrid system isn’t charging due to a fault with the BECM, it won’t charge the 12 volt system either.

What I really want to do in the interim until I figure out for sure what’s going on, and how much it will cost to fix, is disconnect the 36 volt hybrid system so that the vehicle is only running on the 12 volt system and the gasoline engine, so the alternator will continue to charge the 12 volt battery. One would think that could be done, but I can’t find a definitive answer.

3276 volts? Did your friend borrow this fancy scanner from you local power utility? I’ve never heard of any automotive scanner measuring anything that high.

Just a guess but I suspect your friend had his Voltage meter set for the wrong range and the real number is 32.769 Volts. You need to find a shop that knows hybrids and get a proper diagnosis.

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I think that is a software error or mismatch. The scan tool just displays information from the vehicles computer, it doesn’t measure the hybrid voltage.

I know little to nothing about hybrids, but it seems to me that the module is receiving false information from a bad sensor or circuit. Find what sends that bad signal and you may find a solution.

You stated that the trouble appears after driving for a bit so it seems to me that the problem may be temperature related as things have had time to warm up after a cold start. Looking at the strange number the tester showed I tend to agree with @Nevada_545 thinking there is a possible error in the decimal point position. I think you could find out what is causing the trouble by testing things using a normal voltmeter and see what it shows the voltages really are at instead of some software related number. Check for some possible bad connections. Hopefully it is something real simple to fix.