New alternator - low volts?

[Yozza]

Hi Folks,

I hope you good people can assist as I'm scratching my head on this one. I am working on a project (been in the garage for far too long and I'm keen to get it on the road this summer). I put a new battery on her and started up for the first time (yippeeee). However, I checked the volts at both battery and alternator and they are about 12.7v regardless of revs. I changed the voltage regulator on the alternator - no joy, bit the bullet and got a re-manufactured alternator from GSF and still no joy

The only part I had to transfer from the old alternator to the new was the capacitor (2.2uF 100v), I'm not sure what this is for? Could this be the problem as it's the only thing that hasn't been replaced? Other than that any ideas?

(Reference: It's a 90A alternator on a 2.1 DJ)

Thanks,
Yozza

[Oldiebut goodie]

First port of call is your earths need cleaning up. Second is - is your alternator being excited? (Has the charge bulb on the dash blown?)
Cap is only for electrical interference suppression.

[Yozza]

Thanks,
It has had replacement earth braids on the engine in the past but I'll make no assumptions and check the connections.

Not sure if the alternator is getting excited - how can I check?

And you're spot on regarding the charge bulb in the dash, it doesn't light up at all - that sounds like the crucial clue???....

[CovKid]

Well put it this way, you've probably bought an alternator when there was nothing wrong with the old one.

[Oldiebut goodie]

Replace the bulb - without that working the alternator will never charge. (Don't be tempted to replace it with a led either - that will not work). It should then work Mr. Two Alternators Yozza!!!

[AdrianC]

And you're spot on regarding the charge bulb in the dash, it doesn't light up at all - that sounds like the crucial clue???....

Quite possible. Can't be arsed to trace the wiring diagram in the Bentley, but I know that on some cars the current for the alternator excitation (the bit that "switches the alternator on") passes through the bulb. No bulb, no excitation, no charge.

Disconnect the small connection from the alternator, and get the multimeter out. You should get +12v (on the wire, not the alternator) with the ignition on.

[Oldiebut goodie]

Modern ones are self exciting I believe, what will they think of next? Also some have a capacitor inside already for suppression.

[Yozza]

Thanks for all the constructive replies.

Can I quickly check if the bulb is the cause by temporarily swapping it with another dash bulb (are they all the same)?

Thanks,
Y

[Oldiebut goodie]

There is only a bulb in the headlight light - I can't remember if it is the same as its along time since I poked around in there. I think that they are 1.2w but don't quote me on that! Easy enough to take out and check though.

[Yozza]

Thanks (again), I will laugh (at my expense - literally and metaphorically) if the solution turns out to be a bulb.

[ghost123uk]

Watching this because I am going to stick my neck out (and my head above the parapet etc). I maintain (though have not tried it yet, but will next time I have the lid off) that these alternators are self exciting and will work just fine with the blue wire totally disconnected (or no bulb fitted).

Odd though that a new alternator fires the motor up (suggesting earths are ok) unyet only outputs 12.7 volts (or = nothing). Perhaps check your meters accuracy on a known good vehicle ? I have seen some meters that are a whole volt out of calibration.

[AdrianC]

Odd though that a new alternator fires the motor up (suggesting earths are ok)

Eh?

unyet only outputs 12.7 volts (or = nothing). Perhaps check your meters accuracy on a known good vehicle ? I have seen some meters that are a whole volt out of calibration.

But it's giving 12.7v with the engine off, too. So the battery's healthy, just not being charged. If the voltmeter was a volt out, then the battery would be REALLY, REALLY fully charged, but still not being charged.

[ghost123uk]

Odd though that a new alternator fires the motor up (suggesting earths are ok)

Eh?

Sorry, badly worded, I had only just got up What I was trying to say was that the alternator is securely earthed to the engine, and that the engine (ok, gearbox) to chassis earth must be OK or it would suffer during cranking. Indicating (imho of course) that it won't be a bad earth causing the lack of charging.

unyet only outputs 12.7 volts (or = nothing). Perhaps check your meters accuracy on a known good vehicle ? I have seen some meters that are a whole volt out of calibration.

But it's giving 12.7v with the engine off, too. So the battery's healthy, just not being charged. If the voltmeter was a volt out, then the battery would be REALLY, REALLY fully charged, but still not being charged.

Ah, I hadn't seen that the OP had tested the battery without the motor running.

We will have to wait and see if it is an excitation issue or a dud new alternator then.

[California Dreamin]

Definitely exciter type.....no blue wire= no charge.

Martin

[Ian and Lins]

Watching this because I am going to stick my neck out (and my head above the parapet etc). I maintain (though have not tried it yet, but will next time I have the lid off) that these alternators are self exciting and will work just fine with the blue wire totally disconnected (or no bulb fitted).

Odd though that a new alternator fires the motor up (suggesting earths are ok) unyet only outputs 12.7 volts (or = nothing). Perhaps check your meters accuracy on a known good vehicle ? I have seen some meters that are a whole volt out of calibration.

With you on this. Not theory but I ran our van for over a year (daily drive) with a blown LED alternator bulb and had no charging problems.