Possible alternator problem, advice needed.

[chris-y]

Hello all. Firstly, I apologise, I bought my VXR8 yesterday and would normally have introduced myself first but I have a problem I need to ask for help with.

The journey home from buying the car was 330 miles, the dash board voltmeter read about 14v as you would expect for the first 200 miles. Then I noticed that it had dropped to 13v.
To cut a long story short its usually reading about 13v now, occassionally popping up to 14, sometimes less that 13. I used a digital multimeter between the positive post under the bonnet and chassis ground and got 14v, then seconds later 12.8v, which it stayed at since.

My guess is that the alternator is failing due to normal wear and tear (car has done 109K miles), my questions are -

Would you agree, or is there something else I should be checking?

Where is best to source an alternator, does anyone have a part number, I wondered if its a common item readily available, or special to our cars.

Thanks for any help you can give, I'll introduce myself better later with a pic of the car.

[chris-y]

Thanks for the replies so far

I didnt know about engineering mode, a quick google search has educated me on that one, I'll have a play later and report back.

There are no warning lights on or any messages on the dash when this happens, or any other time.



Jumping ahead a bit here, is the alternator reasonably easy to change from the top, I dont have access to a lift or ramps. Any tips welcome.

[chris-y]

Thanks for the replies again, much appreciated.

Engineering mode confirmed low voltage, as low as 12.6.

The charge/excitation on demand is new to me but makes sense, and would explain why sometimes its 14v, and so definitely can deliver the expected voltage (no way to know current) and sometimes its lower, quite possibly when I'm seeing low voltage its intentional low excitation by the ECU.

I had a few messages with the previous owner last night who is very knowledgeable and helpful, he suggested charge the battery overnight just to make sure its charged as its been standing around a bit, then dont worry about it if the car isnt displaying any warnings.

I've charged the battery last night, today started at 14v, for about 5 miles then back to 13v, so maybe there isnt anything wrong.
I've heard extra gauges called paranoia gauges, know I know why, if it didnt exist I wouldnt be concerned.


I dont know how old the battery is but I'm aware that these cars are sensitive to battery condition from reading posts, if I had reason to suspect it I'd change it and consider it money well spent. Vauxhall trade club battery prices are good so it wont hurt too much.
I'll see if I can find out how old it is.

Thanks again.

[chris-y]

I wondered if it would be a specific type of battery after learning about the more complex charging system, an old fashioned lead acid battery would be happy with 14v at all times but my limited understanding is that some of the modern batteries need very specific charging.

Its changed its behaviour now, and I'd say that whats happening is quite unlikely to be a fault alternator and more likely to be the ECU controlled on demand charging.

I went out in the dark last night and it showed 14v throughout, which makes sense, if it decided demand was higher and it should excite the windings to provide higher output.
Oddly though today during daylight its been 14.5v all day which I didnt expect, it seems a bit high to me when thinking about normal older cars.

I think the answer though is that the ECU is probably making the decisions and the gauge is a bit pointless because there isnt an OK or No good reading, it depends on whats required at the time to maintain the battery.

I think I might be best to take the previous owners advice and dont worry unless the dash displays a fault, which it never has so far.


Good advice on the battery though, having read about these cars sensitivity to battery health its something I would change as prevention when the cash is available. I know not to just buy one that fits.

[chris-y]

While we are on the subject of batteries, the Halfords website recommends this one -

https://www.halfords.com/motoring/bulbs-blades-bat...


The price is reasonable, cranking amps are high and I have a trade card so would get a bit of discount.

Not that I need to buy one immediately but does this look a good choice to those of you with more knowledge than me?

I also thought about trying the Vauxhall main dealer because batteries at trade club prices are usually surprisingly cheap, any thoughts, has anyone tried them?


I know I'm off on a tangent here but I'm keen to learn about the cars and be prepared for whatever comes up.

[chris-y]

I'm not worrying about it at all now, its clearly just charge on demand. At least I've learned something about the car with your input.

I havent been able to get battery life/charge to display in engineering mode though, just voltage, I'll have another go tomorrow, but in reality I dont think there is a concern at the minute.

[chris-y]

Thats fantastic info, it explains everything.

Thanks for that, much appreciated.