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Alternator Help


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Hi All,

The other day driving my '87 90 V8 to work a slight burning smell was noticed then the battery warning light illuminated.

Upon closer inspection it was observed that the alternator rear mounting bolt had worked loose and had fallen free. Today I replaced the mentioned bolt and the light distinguished - but only on idle, when the revs are raised the light comes back on.

I measured the voltage at the battery on idle and it is 12.6 - 12.7. At 2000rpm it is 13.8. From my limited knowledge I think this is too low. So has the alternator had it?

If a new alternator is needed, can anyone shed any light as to which alternator is needed? I see there are many varients, obviosuly I can match up the 'fittings' but how can I identify what rating I need - i.e 65amp?

Cheers.

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Hi All,

The other day driving my '87 90 V8 to work a slight burning smell was noticed then the battery warning light illuminated.

Upon closer inspection it was observed that the alternator rear mounting bolt had worked loose and had fallen free. Today I replaced the mentioned bolt and the light distinguished - but only on idle, when the revs are raised the light comes back on.

I measured the voltage at the battery on idle and it is 12.6 - 12.7. At 2000rpm it is 13.8. From my limited knowledge I think this is too low. So has the alternator had it?

If a new alternator is needed, can anyone shed any light as to which alternator is needed? I see there are many varients, obviosuly I can match up the 'fittings' but how can I identify what rating I need - i.e 65amp?

Cheers.

So, your battery is good, but your alternator is probably working. Did you replace the belt and re-adjust the tension correctly after you put the bolt back in? Also, can you measure the alternator output at the back of the alternator itself? Lastly (and unlikely) could the wiring on the back of the alternator have touched anything hot will the bolt was out?

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So, your battery is good, but your alternator is probably working. Did you replace the belt and re-adjust the tension correctly after you put the bolt back in? Also, can you measure the alternator output at the back of the alternator itself? Lastly (and unlikely) could the wiring on the back of the alternator have touched anything hot will the bolt was out?

Thanks for the reply.

I believe the belt is correctly tensioned - I reset the tension when I replaced the bolt. I can check the alternator output, I'll do some research on how to do that - unless you can advise?

As for the wiring touching anything hot, I'm pretty sure it hasn't - there isn't a lot of 'free' wire for it to get near to anything and as the other mounting bolt was still in place I don't think the alternator moved... much! I also gave the wires a quick inspection when I replaced the bolt and didn't notice any evidence of damage/burning.

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I can check the alternator output, I'll do some research on how to do that - unless you can advise?

On the back of the alternator there will either be a bolt with one or more thick brown wires attached, or a square black plastic plug under a wire clip, with one or more thick brown wires attached. The first type is easy, just a meter from here to the engine or in the second case flick the clip off, pull the plug, remove the cover and refit the plug (without the clip!) then same as before. If you can get SWMBO or a Spearos-minor to start and rev the car this'll make it much easier.

Please be careful working next to the running engine or you'll get "rotating parts"! :blink:

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Ok, just been and checked the output at the alternator. It is 12.7v @ idle and 14.1 @ 2000rpm.

Does this give any clues as to what (if anything) is wrong?

Cheers.

Pretty certain it's not the output of the alternator (14.4V is perfect). The only way for the alternator light to come on is if the voltage at the battery and the voltage at the alternator are significantly different. Since one side of this is a brown / black wire from the alternator to the lamp, and the other side is the ignition live (though there is a resistor across the lamp too, to make sure the alternator works), and we know the alternator is putting out plenty, that leaves us with two alternatives:

1) the connection between the alternator and the battery isn't very good (but your battery would be dead by now :ph34r: and 0.3 volts over the length of the wiring isn't significant); or

2) the electronic pack in the alternator is beginning to go (which isn't supported by the voltage either) :unsure:

OK, last two tests*: start her up and turn every electrical load you have (lights, radio, fan, heated window, sat nav, neon underbody lights :blink: etc.) on. Take a reading, then give it a rev and see what happens to the voltage. I guess this would be best done at the alternator.

Also, turn all the gizmos off and see if you can measure the voltage on the thinner brown/black wire on the back of the alternator at idle (probably 12.7) and revs.

* Then I'm stuck ...

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Thanks for the replies.

Pretty certain it's not the output of the alternator (14.4V is perfect). The only way for the alternator light to come on is if the voltage at the battery and the voltage at the alternator are significantly different. Since one side of this is a brown / black wire from the alternator to the lamp, and the other side is the ignition live (though there is a resistor across the lamp too, to make sure the alternator works), and we know the alternator is putting out plenty, that leaves us with two alternatives:

1) the connection between the alternator and the battery isn't very good (but your battery would be dead by now :ph34r: and 0.3 volts over the length of the wiring isn't significant); or

2) the electronic pack in the alternator is beginning to go (which isn't supported by the voltage either) :unsure:

OK, last two tests*: start her up and turn every electrical load you have (lights, radio, fan, heated window, sat nav, neon underbody lights :blink: etc.) on. Take a reading, then give it a rev and see what happens to the voltage. I guess this would be best done at the alternator.

Also, turn all the gizmos off and see if you can measure the voltage on the thinner brown/black wire on the back of the alternator at idle (probably 12.7) and revs.

* Then I'm stuck ...

Ok, had a chance to do as you suggested. The small brown wire gave me 12.7 - 13.4v @ idle and 20.1v @ 2000rpm! On idle it constantly went up and down.

With load (lights, radio, heated screen and heater full whack) the alternator gave me 11.9 @ idle and 13v @ 2000rpm.

Does the brown wire giving over 20v @ 2000rpm give anything away? Seems like a bit of a rougue reading...

The light coming on means that it is finding a path to eath either through the wiring or throught the diode pack built into the alternator. I would say it is likely to be a faulty diode in the alternator itself

I think I read somewhere else that a defective diode can cause the warning light to illuminate so maybe that's it? So next question is, can you buy these diodes and is it easy to replace them?

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So next question is, can you buy these diodes and is it easy to replace them?

The rectifier / regulator pack has gone! You can get the pack replaced at specialist motor electical places for much lower cost than a new alternator.

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The rectifier / regulator pack has gone! You can get the pack replaced at specialist motor electical places for much lower cost than a new alternator.

Ahhh ok - and you have come to this conclusion because of the readin at the small brown wire? Only asking as I'm keen to learn.

I will have a look to see if there's anywhere local who can fix my alternator, however I may buy a new/secondhand one and keep the old as a spare. Trouble is I'm finding it hard to get a part number, is there a way of checking what output mine is - 45A or 65A?

Thanks again Errol :D

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Ahhh ok - and you have come to this conclusion because of the readin at the small brown wire? Only asking as I'm keen to learn.

Yep, shouldn't be 20V, implies that the regulator electrics aren't doing their job. Could be a failed diode, could also be other things in the pack. Either way, that's why you're charge warning light comes on (20V > 14.1V = 5.9V across the bulb, and if the main alternator output is trying to get to 20V then it won't be doing the battery any favours either (though not fatal, in the short term).

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Yep, shouldn't be 20V, implies that the regulator electrics aren't doing their job. Could be a failed diode, could also be other things in the pack. Either way, that's why you're charge warning light comes on (20V > 14.1V = 5.9V across the bulb, and if the main alternator output is trying to get to 20V then it won't be doing the battery any favours either (though not fatal, in the short term).

Ok I think I get it now - The regulator 'wants' 20 odd volts from the alternator (due to it being faulty)and the alternator, of course cannot manage this and is only supplying 14v. Because there is a difference between these values, the warning light is on?

Thanks again for your help, I would buy you a pint but it's a little too far for that so I'll donate it to this forum :)

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Ok I think I get it now - The regulator 'wants' 20 odd volts from the alternator (due to it being faulty)and the alternator, of course cannot manage this and is only supplying 14v. Because there is a difference between these values, the warning light is on?

Thanks again for your help, I would buy you a pint but it's a little too far for that so I'll donate it to this forum :)

Nowhere near that simple. If you look at one of my previous posts, the warning light comes on because the regulator is putting out 20V. Look at page 31 of the link, but the rest is interesting too.

Ta for the virtual pint, the forum will be glad of it!

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Ok had a look at your link - interesting, however I'm trying to apply it to this scenario.

Would be a great help if you could tell me what is the small brown wire?

its connected before the voltage regulator that ensures that the alternator never goes above 14.4v or so. it is connected the output from the 3 phase rectifier pack which changes the ac output of the alternator into a varying dc output ,,, does this help :huh:

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Thanks for your patience ;)

its connected before the voltage regulator that ensures that the alternator never goes above 14.4v or so. it is connected the output from the 3 phase rectifier pack which changes the ac output of the alternator into a varying dc output ,,, does this help :huh:

So this small brown wire is connected to the rectifier where ac is converted to dc (by the diodes)?

Next question is then, if the large (b+ terminal?) takes current to the battery - where does this small wire go to?

Thanks again :P

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The small wire goes to the bulb.

Other side of the bulb is connected to ignition live.

If there is a voltage difference across the bulb, the light comes on.

I really hate to keep re-visiting this and I'm embarassed to keep doing so but...

Going on what you say, in this instance the ignition live is giving 12 odd volts at the light and the small wire is giving 20 odd volts (because of a faulty regulator/rectifier). It is this difference that is causing the light to come on?

Again, thank you for your patience!

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