alternator light comes on sometimes?

[Day-mow]

as the name suggests my alternator light comes on when i shift at redline a few times in a row.
its never done it before. but after putting a new alternator on about 2 months ago its done this some times. if i turn the car off and turn it back on it doesn't happen. why is this any ideas? i've not really checked it but going to tonight was gonna make sure connections good but other then that i have no idea.
hthanks

[shayne93]

Is your P/S hose leaking onto it. Used to happen to me in my old AU Ute, battery/alt light was an early sign of alternator going to **** it self

[Day-mow]

Yes I know this but I replaced ye unit 2 months ago. And it's never done this before.

[mik]

another possibility, when you give it heaps, and the engine is twisting in the mounts, the wiring or a plug has a fault, and when it moves or stretches slightly because of the engines movement it`s causing the light to come on.

[burnz]

i was going to say brushes, thats the norm for a intermitant...
other wise im stumped..

the only thing i recomend if you love to red line, is underdrive the alternator..
as it is now overdriven 65%!!

[Day-mow]

Originally Posted by burnz

the only thing i recomend if you love to red line, is underdrive the alternator..
as it is now overdriven 65%!!

im not 100% sure what you mean by underdrive?

Originally Posted by mik

another possibility, when you give it heaps, and the engine is twisting in the mounts, the wiring or a plug has a fault, and when it moves or stretches slightly because of the engines movement it`s causing the light to come on.

ahh yeah so just double check all wiring etc?

[burnz]

Originally Posted by Day-mow

im not 100% sure what you mean by underdrive?

ok.. at 6000 rpm your alt is doing around 9,900 rpm..

underdrive means just that say 25%.. 6000 rpm crank speed could mean 4000 rpm for the alterantor

[Day-mow]

oh hecktik. how do i change that?

[burnz]

Originally Posted by Day-mow

oh hecktik. how do i change that?

i wouldn't unless you live on the rev limiter,
your pully ratio is setup for idle~2500 rpm for good charge, above this is a waste..

underdrive and your battery will go flat unless you drive 90% of your time above 4000 rpm (motor sport)

[Day-mow]

Oh. So then really I should be looking for loose connection etc?

[mik]

Originally Posted by Day-mow

Oh. So then really I should be looking for loose connection etc?

you have to start somewhere, check the plug in the back of the alternator is in properly, the screw on fitting is tight, i`d even check battery terminals are clean, any wiring that`s flopping around, or anything out of the ordinary even the engine block earth is secure, you know just a 5 minute check, if you find something ..............good, see how it goes, if you find nothing maybe think about other avenues.

[Day-mow]

sounds good.
cheers mang

[mongrelEF]

Also have a good hard look at your belt tensioner, if the spring's a bit weak it could be allowing belt slip when flat out.

[Day-mow]

true. i need to replace my belt anyway. 200k km. pretty sure its tensioning quite well though.

[Day-mow]

well this is still a problem.i've changed the belt so maybe it is the tensioner. but its only when i push it after about 4,500 consecutively and my volts start going down from about 14 where it sits normally. and then when you calm down and drive "regularly" again it goes back up for 14.

any further thoughts?

[dieseltrain79]

Poor battery terminals ?

[4ford]

If you could rig up a multi-meter to show the alternator FREQUENCY at the battery terminals, whilst you are flogging this car, then that would tell you if the alternator RPM is actually slowing at these high engine RPMs, which would then give you the decreasing output voltage you speak of...
Would only take a couple of wires jury-rigged into the cabin on a tempory basis.

Remember, the alternator light on the dash is a simple light that shows up only when the battery voltage exceeds the alternator output voltage....simple.

If the alternator RPM IS slowing, then its drive mechanism is at fault!

Good Luck...

[mongrelEF]

Alternatively, just stop flogging it

[dieseltrain79]

Maybe the light is on from overcharging ? Due to spinning to high revs?

[4ford]

Ehhh...the light would not illuminate in this case, you would simply overcharge and boil the battery, followed very shortly by battery failure.

4ford

[dieseltrain79]

Wouldn't the charge light come on due to high voltage and not only low ?

[BHDOGS]

Could it be a battery on the way out?

[Day-mow]

i have an XR6 so i can see the volts coming down then the light comes on. ease up and it builds back up to 14.
battery shouldn't be on the way out its about 18 months old.

4ford. what setting would i need to do this?

[mongrelEF]

The charge light only comes on when the field is not active - not charging.
Assuming it's not getting belt slip, you're probably experiencing brush bounce. At extreme RPM, the brushes can bounce off the slip rings and collapse the field = no charge. New brushes would likely alleviate the problem until they wear down.
Bottom line - nothing attached to almost any factory street engine was designed with high RPM in mind. It'll do it, but won't like it.

[Day-mow]

Ended up being a faulty alternator.
Although I replaced the auto tensioner pulley as well