Exchange alternator or DIY

Alternator is a Bosch. Started from cold and a volt meter across the battery terminals gave 1.8 volts. After a two mile drive it went down to

14.9 and 15 on a fast idle. The Haynes manual says it should be 14.2 volts and drops to 13.2 volts when lights and rear de-mister is on.

Is the alternator dicky and needs replacing? Will excessive voltage ruin the battery?

Probably

Eventually

How confident are you in the accuracy of the meter you are using?

Chris

Is 15 volts excessive?

It is a mid price unit. I'll may try another volt meter, and see if they are the same.

Well it depends on th ebattery & the meter. It's high , but if the initial readind was 1.8V then I'd suspect the meters accuracy 1st.

You could always bang another regulator in the alternator. Cheaper than getting a new/exchange unit.

Is the 1.8 volt number correct or a typo?

I suspect that the car would not start if the battery reading was 1.8 volts, but if that number is accurate, the alternator would be putting out maximum in order to charge the battery.

The alternator should be checked with a fully charged battery.

Typo. 15.8 v The battery was checked when cold and was approx 13v.

The alternator is over-charging slightly, and battery may have a tendency to run low on electrolytes due to the over-charging. Keep a supply of distilled water handy, and check the battery fluid level often. You may also see a corrosive buildup around the battery terminals. Invest in those fabric washers that fit over the battery terminals, and spray the terminals with battery terminal protectant.

Thanks. The battery is sealed, requiring no topping up. No corrosive build up - yet.

What battery technology was the vehicle intended to have? Many from about

1999 on use slightly different technologies to standard lead-acid ones. These do have a higher charging rate of around 14.6 volts; that's not a long way from your meter's 14.9.

It wouldn't be the first time that Mr Cut and Paste Haynes had been wrong ;-)

It would be my opinion that if the alternator regulation had failed, you would be seeing more than 15 volts at a fast idle and with a fully charged battery.

The first signs of such failure is usually an increase in headlight brightness when revving the engine after a cold-start, usually followed by frequent bulb failures. If you don't have these problems I wouldn't be too worried.

Chris

Many modern batteries simply aren't designed to cope with overcharging and only have a safety vent.

You would indeed - when it failed on my SD1 with a fairly modest alternator by today's standards it went up to 17 volts.

On mine the weird indication was the disappearance of the LED OBC display

- the makers had obviously anticipated this possible scenario and included over voltage shutdown. But then it was made by Smiths rather than Lucas.

Thanks. I had the impression the info in Haynes was a more general to charging voltages than model specific. No increase in headlight brightness when revving the engine after a cold-start, or frequent bulb failures (well not out of the normal). The lights are steady.

The car is a 1998. The Alternator is a 80A Bosch 0123325012. Looks like this and some specs:

The Battery is a 2 year old sealed maintenance free Varta, which is supposed to be a good battery.

Haynes say that a maintenance free battery if being charge after a discharge state (less than 12.2 v) is best to be charged at a dealer or out of the car. They say it requires a higher charge voltage up to 14.9 v. They state these batteries can take up to 3 days to be fully charged when in the vehicle. Well that is what they write in the manual, which again may be very general.

Is it possible a voltage regulator can vary the voltage, as I read two different readings?

My clock/outside temperature LED display disappeared. Did your LED display come back?

This is making me think the alternator regulator is temperamental. Or is it?

Yes. The regulator fault was intermittent. Perhaps heat related as it never happened until the car had been running for some time. I eventually made up a lead to plug the DVM into the cigar lighter socket so I could see what was happening at all times. It was one of those faults you couldn't make happen.

The only way to be sure would be to do something like I've said. A voltage of between about 13.5 - 14.4 would be normal depending on engine speed, battery condition and load. But a sudden jump outside this wouldn't be.

You don't nead the dealer for that though, the alternotors quite up to it

Yup, but you don't need fully charged, it's at 50% after a 3 or 4 hours driving.