VXR8 voltages all over the place

[FishmanUK]

I seem to have some strange behaviour going on my 2008 LS3 over the last few days

Start the car from cold and the voltage jumps up as usual to ~14.5V.

Drive it and it stays there for a variable amount of time before dropping down to 12.7/12.5 or 12.0/11.8v (measured from gauge/engineering mode). Sometimes the voltage will go back up to ~14.5 for a bit.

I have not changed the battery in the 2.5 years I have had the car so it could be the original one. I decided to drive in engineering mode this afternoon and monitor battery charge and it started at 52 and built to 53% before the voltage dropped. I did also get an "alternator" warning pop up. It cleared itself and the voltage went back to 12.7 for a bit before back down to 12.

Has anyone experienced this before and is like likely to be knackered alternator?

The car is my daily driver and starts first time and if I didn't have a voltmeter I wouldn't have even noticed anything was up till that message popped up.

Thanks

[SteveMJ]

I saw something similar on mine last week.

I drive mine to Cambridge form welwyn each day and takes me about an hour with the engine running at ~2000 rpm for almost allthe time.

I checked my charge before I've left and it reported 83%, on hour later when I parked it reported 85%. I think there is something strange about the BCM' software algorithm.

Steve

[FishmanUK]

Thanks Steve,
Was that just a temporary drop in voltage or a more repeatable and obvious problem?

Hoping it is just the battery is old but not convinced it is that.

[R8heron]

Mine did exactly this a few months ago, would start fine alternator would kick in fine and meter would go up to around 14 - 15v as usual but then slowly drop to 12, battery charge was 60+ but I noticed the day before it happend that the sound of the alternator was a bit louder than normal. I had a car electrician guy come round and check it and he suggested a new alternator which I did and problem fixed. And then my battery died a few weeks ago as well

[FishmanUK]

R8heron said:

Mine did exactly this a few months ago, would start fine alternator would kick in fine and meter would go up to around 14 - 15v as usual but then slowly drop to 12, battery charge was 60+ but I noticed the day before it happend that the sound of the alternator was a bit louder than normal. I had a car electrician guy come round and check it and he suggested a new alternator which I did and problem fixed. And then my battery died a few weeks ago as well

Thanks

Just had a poke around with multimeter. Battery from cold is 12.2v with engine off (and looks like a PITA to get out if I need to swap it), mirrored at the charging points under the bonnet and the alternator. On warm cranking the voltage bottomed out somewhere in the 10s I think.

On startup it goes to ~14.5v but the voltage is not stable. Drove round the block and it was down to 12v, by the time I opened the bonnet and got the multimeter on to the alternator output it was back at 14.5v but again not stable. After some time it dropped to 0/12v and the alternator warning popped up. I caught voltage at 11.8 at some point also

There is a bit of a grinding sound that probably wasn't there before which I guess is alternator.

Is there anyway to rule out the BCM sending bad signals to the alternator in the remote chance it is that ?

[SteveMJ]

You are welcome to compare with mine if that helps. I have a digital voltmeter too.

I'll text you (assuming I still have your number) as it may be a while before you get this message.

Steve

[R8heron]

I found this thread on the g8 forum,
http://www.g8board.com/forums/archive/index.php/t-...

go's into detail on the problems you are having including how to check whether the bcm is to blame, hope it helps

[FishmanUK]

nice find and epic problems the guy had.

I guess there are more electrical tests I could do but the chances of getting them wrong increases with complexity so best to replace the alternator and battery and see where I am.

Access to alternator doesn't look too bad so hopefully a fairly simple job (for someone who knows what they are doing)

[mattcov]

If you're not sure of the battery, I'd change that first. Its quite easy to change - only takes ~20 mins. I had a similar prob a month ago - the volt meter was all over the place, then it displayed contact dealer, it played up with the air con and auto decided which gear it wanted to stay in.

Changed the battery (think it was the original one) and fine since - its always started first time, but now starts much quicker with the new battery. Its this battery: http://www.amazon.co.uk/gp/product/B005XVUQ6Y?psc=...

It might be the alternator, but I'd try the simplest and cheapest part first.

[FishmanUK]

New battery in and a 10 mile trip saw constant ~14.5v so I guess that rules out alternator warming up and not working.

Driving home from work after about 6 miles saw voltage drop down to 12 for a bit then back up to ~12.5v. I am hoping for the best in that the car thinks the battery is full and charge was not needed. Hopefully over the next couple of days it will all settle down, if not possibly time for that alternator or to investigate other possibilities.

Thanks all

[350]

I'd also had some unusual voltages, I wasn't sure if it was charging properly at the time or if it was normal.

After a 120 mile trip I stopped for fuel and when I came to restart I just got a click. nothing else apart from the odd warning message.
RAC diagnosed a dead battery. Since I got a new battery fitted it does turn over a lot quicker and seems to idle better too, but I'd not had the car for long enough to realise the signs the battery was not at its best (not that there were many!)

[FishmanUK]

You would have thought 120 mile trip and failing to start a hot engine means dead battery and possibly alternator.

With new battery my car seems to be running a lot of the time 12v and voltage across battery with engine shut off was 12.1v just a minute ago. Battery charge level however meaningful that is is heading downwards. Maybe new battery just bought me some time...

[FishmanUK]

Just to update

New alternator in and all sorted