How can you prove an alternator is faulty!

salbern, Jan 26, 8:46am
Hi team. How can you prove that an alternator is faulty! I know the battery will be flat but how can you prove it is not charging!

Additionally if the battery is flat and needs to be jumped, and the alternator is not charging will the car still run (94 mitsi).

tks
jono2912, Jan 26, 8:46am
Test the output.
bdogg1, Jan 26, 8:55am
Put a multi meter on your battery terminals. Put your meter to volts DC. The reading should be between 13.5 and 14.2 approx. If it was a faulty alternator it would be charging less volts than above and will continue to drop as time goes by. If it's charging at correct volts but your battery is still draining, then your battery is stuffed.
salbern, Jan 26, 8:58am
Does that mean that if the battery is stuffed the car will run then, and if the alternator is stuffed after time (i.e. when the battery is flat) the car will not run!
bdogg1, Jan 26, 9:03am
Yes your car will continue to run without a battery. Thats how they continue to run when you disconnect the jumper leads after jumping it with a flat battery. If you have to drive it with a poked battery, just dont stall it.

You can drive a car with dying or no battery but cannot get far if you're alternator falls below 12volts approx.
bdogg1, Jan 26, 9:11am
If youre wipers and indicators are going slowly compared to before, chances are you're alternator is dying.
wrackman, Jan 26, 9:11am
PLEASE DO NOT run your car with no batt in it as it could kill your alternator .Most of the time the warning on the dash will stay on if your alt is dead
cuda.340, Jan 26, 9:19am
easy test & a great way to introduce kids into the field of auto electrics. get the kid to lick his fingers & place them on the neg terminal of the battery connection. then get them to put their tongue on the large alternator terminal. did the kid jump & is the kid crying! yes! alternator works fine. is the kid just standing there & look ok! alternater is stuffed. a great test & good for entertaining your mates. try it sometime, you'll be surprised how easy it is.
salbern, Jan 26, 9:46am
Thanks for the info. That will be this weekends job then
dft7322, Jan 26, 10:03am
funny scary thing was that I am a night shift worker& I got the mechanics at my work to jump start me. that was fine until I got on the start of the motor way& my lights started 2 fade then nothing not even my wipers would work or hazzards heres me thinking it a flat stuffed battery so i buy a bran new one again the same thing happen on motorway next night and yes it was the alternator, i broke down on the middle of the motor just outside of kaiapoi at 1.30am 3 hours before the major earth quake hit chch .
oakie, Jan 26, 5:48pm
Drive it up to a garage door and turn headlights on, let it idle and then rev slightly. If the lights don't brighten up then it's not charging!
intrade, Jan 26, 6:26pm
a alternator charges the battery and for charging you need a higher voltage then the battery has when fully charged.if alternator is not charging then you will only use the battery power but not replace the used power = battery will be empty soon. also the battery will mesure below 12.6 volt and the car will start cutting out at 11.8 volt cough and splutter due to not enough power suppy to keep ignition fuelpumps etc powered.
mrfxit, Jan 26, 7:48pm
Agreed,
Running an alternator without a battery is ASKING for trouble.
Alternators are fairly delicate about shorts & running an engine without a battery, you stand a very real chance of blowing the diode pack in the alternator.

Generators are a different matter & there hasn't been any fitted to any vehicles for many years.

A simple volt meter across the battery terminals will tell you promptly if the alternator is charging or not, regardless of battery condition (except a total dead short, in which the alternator is probably stuffed as well by now).

check voltage
Engine off,= 11.5v to 12.5v
Engine on at idle 12.5 to 14.8v
1500rpm = 14.2v to 14.8v

Most modern systems are setup to supply 14.8v at idle on a low charge battery
the_man11, Jan 30, 5:45pm
Not always the case for what one of uz said, the voltage wouldnt always be dropping if your alternators gone, If the regulator in the alternator has then your car will be OVERcharging and reading more in the 15-19.8V, this is a sure fire way to melt a CPU and any other cables floating around, So dont drive a car when the alternators not right, cause your either gonna melt stuff that should be melted, or push start when u stall and or f**k it even more
tshop, Jan 31, 2:49am
Here's a tip;Dont remove the bolted on charging wire.Unplug the plug there's 2 or 3 wires on it, cut the plug off the loom & then join the tails all together, then plug it back in. This will bypass anything that's sucking the charge of the alty', if its still going flat then the alty' definitely is shot.
lookoutas, Feb 22, 5:08pm
Good shit here!
My daughter has just had an alarm fitted. (To lower Insurance excess) and when she came home for Xmas, the doors kept locking and unlocking, and the boot popped open twice.
She went to use my computer to pay them but I said "No wuckin fay" and told her to ring and say she would pay when they fixed it.
The guy on the end of the phone was an arsewipe, and it was fortunate for him that she only told me afterwards. However, the tech who fitted it had a better attitude.
I had disconnected the boot release to alleviate that popping open again on the motorway when she returned home, and they promptly replaced the whole unit, so she paid - and it did the same.
The response now extended to more than a week, and they fitted a diode somewhere in the system, which they said was going to fix the problem, and it hasn't.
Which brings us to the present time. They said it was a fault in her car as the alternator was charging at 14 volts, and going by the above posts, this is normal. I'm thinking they just wanta start charging her for extra work.
Of course, I have asked a sparky mate who is totally against alarms, sez he rips out more faulty ones than he fits. And he sez bullshit, bullshit, bullshit, tell them to stick it and he'll make up a mobilising system, which he swears by. And I'm sort of thinking he's tunnel visioned on the neg side, but I trust his opinion & expertise. But then there's the insurance angle.