How to test the alternator

[carboy2017]

I have a strange feeling that my alternator is playing up a bit because

i) there is a strange whining noise at times when the car is idling
ii) yday the battery went dead for no reason as far as i know

So before I go to a garage I would like to know if I can test the alternator myself and whats the best way to do it

Please note this is the 1st time I will be doing something like this

[Mignon]

Get a digital multimeter and check battery voltage when the engine is running. It should be over 14 volts.

[PositronicRay]

Added to that over 14v though out the rev range, worn brushes can cause odd things to happen at speed.

[GreenV8S]

Added to that it should stay at about the same voltage when you switch on big electrical loads like lights.

[robbocop33]

Don't know if it's true of most cars, but only car i've ever had that had a faulty alternator would make your battery light flicker on your dash intermittently, this became more frequent until it stayed on!
Just something to look out for maybe.

[Peanut Gallery]

I have annoyingly had an alternator fail without bringing on any lights on the dash - until the battery went flat and the car died mid drive.

I now regularly check that starting the heated windscreen (large electrical load) makes the revs dip at an idle.

[Emanresu]

I wish people would stop spouting advice with no other experience than changing an alternator on their car once 25 years ago. It confuses people. If you don’t know for sure, don’t say anything.

No it doesn’t have to be over 14 volts. Some alternators charge as low as 13.5 volts. If you’re getting 13.5-14.5 volts while running it’s ok.

Also remember overcharging can cause damage as well as undercharging.

[gordmac]

If there is a noise you think is alternator see if it goes away if belt removed?

[227bhp]

gordmac said:

If there is a noise you think is alternator see if it goes away if belt removed?

What will that tell you when the belt also drives the air-con, water pump and PS pump?

[gordmac]

It will tell you if it isn't the alternator or indeed one of the other things you mentioned.

[Slushbox]

If the car has a smart alternator, it will tend to charge the battery to only 80%, settle to about 12.5-12.9v then dump 13.8-14V (approx) charge into the battery on engine over-run. (With all the lights/wipers etc off.)

On my last two cars, driving with the throttle open sees about 12.5 VDC on the voltmeter, coasting with the throttle shut sees it go to 14.2 VDC.

One of these will tell you if you have smart alternator. Having one flat battery incident means you should also get the battery tested too. A new battery is cheaper than a new alternator. :-)


https://www.amazon.co.uk/TONVER-Digital-Voltmeter-...