Alternator working or not ? :(

[paulo5]

Hi

about a week ago. driving back from france. battery warning light came on... luckily i was able to get home on just battery.... Next day battery went flat whilst driving. recharged it fully and same thing, battery warning light on, so evry night after home from work have been charging the battery indoors. until this morning and again this evening the battery warning light is not illuminated when driving... the light bulb is ok. cos it comes on briefly with the rest of the lights when ignition starts. . If the light is not coming on whilst driving does that definatly mean the battery is receiving power and recharging from alternator. i sat outside my house with everything on headlights, air con, radio etc reving engine and still seems ok battery didnt seem to get flat or low on power. could the alternator have come back to life some how??? it is strange as i was just gone get new alternator in few days...

 

[Burnerman]

Trust ye not the red light on the dashboard.....!
To check the alternator, use a multimeter set to 20v DC.
With the engine running, your reading should be around 14.4v and it shouldn't drop much when you put the headlamps on.
Stop the engine. The voltage should drop to around 13v and then decay overnight to around 12.5 v as an average.
This is a rough test for the alternator, but not totally conclusive.....if the rectifier diodes are failing, the battery could be discharged when the car is standing idle.
John

 

[TicklyT]

Gone are the days of the flickering ammeter on the dashboard' and the mental arithmetic attempting to assess the state of the battery from whether driving 5 miles showing +15 Amps, then three minutes showing -10 amps means your battery is running flat - Nostalgia isn't what it used to be!

Basically the warning light extinguishes when the alternator output voltage matches the battery voltage, but the alternator output needs to exceed the battery voltage to recharge it.

It sounds like something has reached the end of it's useful life. Maybe it's the accessory drive belt, the rectifier pack, or even damage to part of the wiring loom. These things rarely fix themselves permanently.

One cell starting to fail in the battery following a few deep charge and discharge cycles may reduce the battery voltage enough for the alternator to match the voltage, and extinguish the light, but what may really have happened is continued use with a faulty alternator has also damaged the battery.

 

[gregers]

what car is it?

the answers already given are more then likely correct,but it may be dependent on car that maybe the pulley has seized but the alternator is ok???long shot but do check.

 

[Mickymoody]

In my experience, when a car is about 10 years old, the battery fails, and causes random faults. New battery required.

If the car is less than that, then the alternator is fubar.

You can have both tested at a garage FOC, such as Halfords.

 

[ericmark]

For the warning light to work the brushes must be OK so warn brushes and regulator must be OK so worn brushes can result in the light coming on if it stops in right place but not making enough contact to charge.

As to voltage that depends on system. Step charging is used with some cars now and that means any where between 13.2 and 14.8 volts can mean it is charging.

How old to remember the third brush and moving it to get right amount of charge. Had to be careful to not leave ignition on as no cut out just a free wheel on dynamo. I have only seen them in museums.

At least now not so hard to change brushes it Bosch one can often change them without taking alternator off. With old Lucas ACR range the machine sensed alternators would burn a hole through centre of slip rings. Battery sensed did not switch so rapid.

One could at one time get voltmeters to plug into cig lighter socket. Never used one myself but looking for a suppressed zero meter for my sons boat as too easy with only two 70A alternators to draw far too much power and end up with flat battery. Also looking at if a relay between both batteries will allow the domestic batteries to charge faster or not? Likely will need special regulator or inverter.