[carter_46]

Hi guys back again with another problem that's giving me a headache I had my alternator light pop up the other day and she finally whent flat .
I removed the alternator and had it tested turns out it was kicking out its 14 volts.
I reattached it and while I was at it changed the belt as it was a Lil worn then charged the battery started he back up no problem and was kicking out 13 volts while running.
After 5 mins of having her running the light popped back up again now I'm confused if any one has any advice I'd be great full thanks jamie

Sorry one more thing when the lights on the dash it ticks over slightly over 1000 weather it's supposed to do this to compensate I'm not sure

 

[M32Guy]

You need to check the battery voltage when the engine is off then at idle. After that turn every light and electrical item on you can think of, full beams, fog lights heated screen/seats the lot then check it again.

Compare these results to the voltage at the back of the alternator buy putting the positive side of your multimeter in the 13mm nut and the negative on a clean part of the alternators casing

 

[carter_46]

Thanks I had the alternator checked at a workshop that specialises in them that's why I'd got the head ache I only checked the battery at idle

 

[M32Guy]

You still need to check the voltage at the back of the alternator and compare it to the voltage at the battery in order to find out where the problem lies

 

[carter_46]

So I need to test all scenarios alt v bat sort of thing ?

 

[oberlisc]

From my understanding of the alternator you have to put it under stress like iGaz says, but also the light is like a gate, its closed when the alternator is charging, but if the catch on the gate is broken the gate will be open giving a false reading when the alternator could be charging properly, also a slipping belt can make the light come on.

 

[M32Guy]

So I need to test all scenarios alt v bat sort of thing ?

Normaly you wouldn't need to check them all but if your not sure what your looking for then do them all and post the results up here

 

[g4vvq1983]

Hi Firstly i am not familiar with your particular car, but the principal is about the same on most, these days the alternator is controlled by the engine ECU, and the charge light doesn't appear to come on until the battery voltage gets down to about 8 volts, even if the voltage is 14 volts it doesn't necessarily mean the amps are there....but its a good place to start....basic tests are make sure the belt is tight and turning the alternator, make sure the connections are clean and tight, make sure the charge light comes on with ignition, then goes off when running....with a fully charged battery when engine running about 1500 rpm or above, the voltage across the battery should be 13.9 to 14.7 volts, i would try that when the charge light comes on....if its not charging the battery will go flat, if its starts up normally when battery is up you can rule out the battery and starter really.....if the red light comes on you almost certainly have a charging fault....so dont waste your money on changing other stuff, as the Man above says check the voltage at the output lead and the pos batt terminal, should be the same, or do some voltage drop tests, i.e. check voltage reading from positive battery terminal to alternator output terminal, should be less that 0.4 volts, then check the voltage from the negative battery terminal to the body of the alternator, should be less than 0.2 volts, do those tests with the engine running at about 2000 rpm...

The normal circuit is the output lead, the thick one, goes straight to the positive battery terminal, the neg batt terminal is common to the body of the alternator, the small wires on the alternator, one cones from and ignition feed and the other goes to the ecu when it finds an earth, thus letting current flow through the rotor winding....the ecu does the regulating, sometimes the alternator has also a built in regulator....hope that helps....sounds like you have some kind of intermittent charging fault....check the earth connections on battery and engine....from Fred in Essex.

 

[carter_46]

Hi guys thanks for all your advice I finally fixed the problem turned out it was an intermittent regulator fault on the alternator after scratching my head and cursing a fair few times lol

 

[g4vvq1983]

Hi guys thanks for all your advice I finally fixed the problem turned out it was an intermittent regulator fault on the alternator after scratching my head and cursing a fair few times lol

Sounds like you cured it, i was just gonna say as per you opening message above, when its not charging it could idle at a higher speed and when charging it puts an extra load on the engine and slows it down a bit...if you have ever tried turning an alternator with the field winding energized you you see what i mean....so from that you can see that if turning the alternator under load puts a small load on the engine then the engine uses more petrol...so you dont get something for nothing, if all the lights are on it loads the alternator and reduces the MPG, only a little buts that how it works....you dont get free electricity out the battery to work the lights, the power comes from the alternator, even if it was coming from the battery the battery has to be charged so its the same thing..sure you wanted to know all that....from Fred in Essex.

 

[andymanhole780]

Just a piggyback off this thread but it is normal for an alternator to take a second to raise the voltage? I mean when I put load on it drops and then takes a second for the alternator to bring it back up.

 

[g4vvq1983]

Well it doesn't usuallybut as long as its not a problem i shouldn't worry, it's probably the regulator switching, that might be in the alternator and might be in the ECU. it could do that if the ignition warning light isn't working, maybe.....Fred.