How do you check your alternator?

Thu Jan 20, 2005 6:23 pm

Anyone know how to check an alternator is working correctly?

My sport is having problems holding a charge, a new battery just lasted me 2 weeks before dying!
I know that I have to check its not the alarm,leccy aerial etc that is draining the battery but I also want to be sure it isnt my alternator.

Thu Jan 20, 2005 6:31 pm

I think if you put a multi-meter across the poles of the battery it should read about 14-14.5 volts with the engine running.

Thu Jan 20, 2005 6:49 pm

That is one way
with the engine off place a multimeter inline from the battery terminal to the positve lead you will find if ther is any current drain
if ther is it means i am afraid a new alternator or at leate a new voltage regulator or stabalising capacitor so the battery is recieveing Ac Voltage in stead of a fairly smooth dc output if you are near the surrey are i will check it for you
Regards Ben

Thu Jan 20, 2005 8:01 pm

when you have the ignition on, is the battery warning light coming on? I know this might sound a bit stupid, but the alternator was causing no end of problems on the 316i, the alternator was charging but the battery wasn't recieving it, a new earth strap put that right...

Thu Jan 20, 2005 9:41 pm

i have a spare alternator that is basically brand new if thats any use.... im not sure if they are the same on M10 as the M20

Fri Jan 21, 2005 2:03 am

No warning light cam on to warn me that my battery was going to die.

With the engine running should the battery read 14-14.5volts even if the battery isnt fully charged when I put it in? Im thinking that maybe there is something like the alarm or leccy aerial draining my battery continuously so if my battery reading is below 14-14.5volts it still might not be my alternator?

Fowler, im not near Surrey and will have to do this myself with the multimeter so can you please explain what you mean by this?:

with the engine off place a multimeter inline from the battery terminal to the positve lead you will find if ther is any current drain

Fri Jan 21, 2005 4:49 am

I had this problem on my 318i, it was the main batt/alt/starter cable shorting to earth which in turn burnt the brushes out on the voltage regulator on the alternator.

"with the engine off place a multimeter inline from the battery terminal to the positve lead you will find if ther is any current drain"

With the ignition off, If you disconnect the positive batt terminal and connect the multimeter in line between the batt terminal and the cable with it set on amps, this will show if there is a load/drain on the batt with the ignition off.

Also in the Haynes manual it says that on early models if the charging warning light has blown then this will stop it from charging but on later models they put a resistor in to stop this.

Things I have come across on other cars which can cause this problem is bad earth leads from the engine to body and body to batt, the heated rear window, alarms

Hope this helps

Fri Jan 21, 2005 5:31 am

Just out of curiosity, does the battery light come when you first switch the ignition on, before the engine is started?.
I'm sure a multi-meter should still read 14-14.5v providing the alternater is charging correctly. Can you see the cell's in the battery, is the fluid low in the battery?

Alternativly, if you live near a Motor World, an Auto Electrics or even a Halfords they should be able to test it for you. It should only take them 2mins.

Fri Jan 21, 2005 12:07 pm

what about disconnecting the battery once started and then turn on the lights etc and see if it sounds funny

Fri Jan 21, 2005 12:35 pm

Best way to find a power drain

Put the battery on charge and make sure it's fully charged. Put it back in the car.

Then put a multimeter across the terminals and you should see it slowly dropping if you have a drain. Get someone to start pulling fuses out one by one until you discover which one stops the drain. When you find it look up what circuit that fuse controls and investigate this circuit.

Sat Jan 22, 2005 1:25 pm

The battery was bought and fitted only 2 weeks ago when i bought the car.
Just took it off the charger today and it read 13.9v DC, put it on the car and started it up. I turned on the lights, heater, radio(with leccy aerial which is wired to back of the radio) separately and they all gave a slight drop, biggest drop was from the full dipped headlights and indiglo dials which dropped it to 13.3v DC.
After about 10 mins with the car just idling i noticed that the battery now read 13.6v DC so something is slowly draining the battery.
The only thing that i havent checked is the alarm but only a thief can totally disarm an alarm?

Is it ok to take out fuses one by one to see which cicuit is causing the drain? I mean, can it damage the car in any way?

Sat Jan 22, 2005 1:46 pm

fowler wrote: so the battery is recieveing Ac Voltage in stead of a fairly smooth dc output.

I think you will find you have that the wrong way around. Your battery would be most unhappy if you give it an AC voltage.

The Alternator itself is an AC device, but this signal is rectified and regulated before it's sent to the battery.

Ian.