New alternator and battery, still low voltage.

[jamesgmg]

Had the battery light come on on my 306 1.9D and measured the alternator output direct from the alternator terminals and also measuring off battery terminals, getting 11.3v at idle.

Figured bad alternator so just replaced it with a new one. However the new one is giving the same results.

I've checked the big fuses in the box under the bonnet on the wing, I've checked the continuity of the + cable from alternator to battery and the earth from battery to gearbox.

I've now also just replaced the battery as the old one was 8 years old, and now getting 11.88v off the battery and back of the alternator on idle and 12.20v with some revs. So still low...

Any ideas?

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Just to update this:

The battery light on the dash doesn't come on with the ignition, only when I've driven the car up the road does it come on. Wondering about the exciter wire, though don't fully understand this yet.

Can someone confirm if on the 1.9D, the battery light should come on with the ignition and then go out on start?

[Niall]

And what state is the car in when your checking it? Warm/cold? Take it for a drive, get it right up to temp and see what voltage you get then.

[jamesgmg]

Think we might be getting somewhere... the battery light on the dash doesn't illuminate with ignition. Though once I revved past 3k, it came on. At this point I got 14.2v off the battery terminals.

So I think this has something to do with the exciter wire, and the alternator self exciting at high rpm. I don't know how to check this though yet.

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And what state is the car in when your checking it? Warm/cold? Take it for a drive, get it right up to temp and see what voltage you get then.

It's cold, but I don't see what difference this would make?

In any case, I've realised now that I can get the alternator to kick in (self excite?) by revving the engine (>3k rpm), once it has kicked in it stays in and I get 14.2v on the battery.

[Niall]

The car has only just been started and starter motors draw a considerable amount of current. The battery needs to recharged so will "sap" power from the alternator. If that battery was fully recharged after a long drive like i suggested, it should hit around the 14.4 mark.

Take into account things like heated windows and heaters when its cold at first start in the morning as well and you have a considerable drain on the alternator as well. In the middle of winter, seeing 11v isn't unusual until its warmed up and fully recharged.

[nominous]

Stick a wire from the batt (or alternator output wire!) +ve to the exciter terminal on the alternator via a small bulb. Say a 5w indicator one.
Bulb is to limit the current. Should excite it and check with your DVM.
You can't leave it like that tho or it will be on constant.

Then the fault is between the exciter wire and the dash.

[jamesgmg]

Just checked fuse F28 15 amp - battery light, clock and outside temp display, electric front windows, rear wiper and it had blown! Put in new fuse, blew that too.

So I guess I need to track down what is causing that to blow! Will check the rear wiper relay first as had problems with that last year.

[jamesgmg]

Almost solved this! Disconnected the rear wiper motor which shares this fuse and all is working normally

[Niall]

So that means your battery light will be working fine but you will still get the low voltage when its cold which is correct

[jamesgmg]

So that means your battery light will be working fine but you will still get the low voltage when its cold which is correct

The battery light supplies 12v to the exciter on the alternator. No battery light, meant no exciter, so the alternator was doing nothing.

Once I'd sorted the fuse, the alternator is kicking out 14.4v to the battery immediately at cold idle, so perfect!

I didn't know alternators had "exciters" until today!

[Tom]

Needs the battery to tickle its balls before it puts out...