Is my alternator working (well enough)?

[amrbose14]

Earlier in the year I replaced my battery. Thought nothing of it - it was 10 years old and in recent years the car has had only occasional use and often short journeys.

Recently I had a (new but) flat battery again. Hmm.... and started to worry about the charging circuit.

If the battery is reading say 12V when the car is off, then it rises to ~13.5V with the engine on. If I turn the fan on full and headlights on full beam that drops to maybe 13.3V.

My conclusion is that the battery is charging, but the reading is not as high I was hoping for from what I've read in a few places (14V or greater).

Is that enough, or what other people see? Perhaps I just need to accept that my 28 year old alternator is OK but rather weak?

As I plan to have the inlet manifold etc out in the near future, I will have most of the bits that are in the way out. I could put in a new (reconditioned) alternator, but they're not cheap (seems like ~£300 exchange).

cheers,
Chris

87 220T.

 

[944Scott]

Doesn't sound too bad to me, maybe you have another problem.

Anyway I spotted this chap on ebay

http://www.ebay.co.uk/itm/Bosch-Alternator-Porsche-928-944-968-/171366479420?pt=LH_DefaultDomain_3&hash=item27e63cf63c

 

[edh]

It's not that expensive to get your alternator rebuilt - find a local Bosch agent

I see 14V+ on mine

 

[bmnelsc]

Your voltage is low on charge but you really can't trust the gauges 100%. I would clean the cable ends, check the main grounds (engine, etc) and then get a shop to test the alternator. If you're lucky, cleaning the grounds will solve the problem but at least you'll know what the problem is after a proper test. I say that the gauges can't be trusted 100% as the 944 instrument panel also suffers from faulty grounds and then the instruments aren't accurate.

 

[amrbose14]

I should also have added that the 13.5v is at idle. But this does not increase when I raise the revs (I presume the voltage should rise fairly rapidly.

Thanks for the link. That is a lot cheaper.

ORIGINAL: 944Scott

Doesn't sound too bad to me, maybe you have another problem.

Anyway I spotted this chap on ebay

http://www.ebay.co.uk/itm/Bosch-Alternator-Porsche-928-944-968-/171366479420?pt=LH_DefaultDomain_3&hash=item27e63cf63c

 

[tref]

I had an "interesting" alternator problem on a 944. If you started and do the checks you suggest it would check out fine, as you have found (I would expect the charging voltage to be slightly higher, but as suggested gauges vary.), however in my case the car was fine on short journeys, but after about 25 miles it would suddenly stop charging. The volt meter in the car didn't detect this particularly well, I ended up driving with a multimeter in the passenger seat attached to the battery - this showed the sudden step change as the alternator stopped charging.

 

[Monkeythree]

Agree with above posts - you can't trust the gauge. You need a voltmeter across the battery terminals to see what the alternator is putting out at idle and in normal running conditions. Check the battery terminal connections and the battery earth strap connection. I would also check your belt tension to ensure you are not getting belt slip.

It could also be nothing to do with the alternator and simply a carp battery or your usage cycle hasn't kept the battery healthy. Could have had some sulphation of the plates which will lower the charge acceptance and then it's just ever decreasing circles from there until the battery is dead (not flat, dead).

 

[amrbose14]

All the above is good advice - but have you checked that there's nothing draining the battery? I had similar flat battery issues that I eventually traced to the glovebox light not going out.. (this also made my alarm fail to set properly). If you have a multimeter with an amps range you can connect it between the battery and battery cable (with everything supposedly switched off) and if you see more than a fraction of an amp that the alarm system & clock might draw you can pull fuses one at a time to identify which circuit is drawing current. It's easier if you get a purpose built probe set for your meter like this one.

Hope you find the cause anyway, and please post the result!

 

[T3rra]

13.5 is good for stock. Should be limited by the vvoltage regulator. You can fit a higher type. Iv got 14.2 or 14.4 in mine.

Start up I get strong voltage at battery and dash and autonator

Dash is check at the cigarette lighter. I find my dash reads less and there for display then at the battery.


Do you have the ducting and cover, as heat then affect the autonator output

 

[zcacogp]

ORIGINAL: tref

I had an "interesting" alternator problem on a 944. If you started and do the checks you suggest it would check out fine, as you have found (I would expect the charging voltage to be slightly higher, but as suggested gauges vary.), however in my case the car was fine on short journeys, but after about 25 miles it would suddenly stop charging. The volt meter in the car didn't detect this particularly well, I ended up driving with a multimeter in the passenger seat attached to the battery - this showed the sudden step change as the alternator stopped charging.

Interesting - what caused that?


Oli.

 

[tref]

Not the foggiest idea, I simply changed it and added it to the pile of "must figure out what is wrong with this" components!

 

[zcacogp]

ORIGINAL: tref

Not the foggiest idea, I simply changed it and added it to the pile of "must figure out what is wrong with this" components!

Oh Tref, I would have expected more of you than that!


Oli.

 

[amrbose14]

I should have been clearer. I am measuring voltage at the battery terminals. I don't think that it is gradual drain problem particularly, it's a good idea to check what the current is with eninge off. And I'll check the terminals. Thanks. It doesn't sound like I should be rushing into changing the alternator. I think some evening blasts are in order, that the evenings are lighter, to dust off the cobwebs and give her a good run.

Chris

 

[scam75]

I get about 13.6V across my battery terminals with the engine on and idling and minimal things switched on. 12v with ignition off.

I can leave my car 2 weeks and it will still start so that seems a reasonable amount of voltage you have for a standard alternator.

Incidentally the output should not rise with revs, this would cause many issues, none of them pleasant!!

I would conclude you most probably have something draining your battery whilst you are not in your car. Glove box light maybe?

Cheers

Stuart