Alternator/charging circuit experts?

[Nasher]

Do we have any experts in Alternators and Charging circuits that can offer a grain of wisdom?

A long story, but here goes -

I had a call from Mrs Nasher Friday evening to say she’d returned to her Astra in the car park and it was completely dead – no remote unlocking, and not even an interior light when she opened it using the key. It’s a 55 plate 1.7TDCi.

After accusing her and the kids of leaving something on all day, I drove down and jump-started it off the D3.
It started fine, but then died within 30secs of removing the jump cables.
So I jumped it again and left the cables attached and both engines running for 5 mins to get some juice in the battery.

Off she went, and only made it about 1/2mile before the electric power steering cut out and she managed to limp it into a lay-by before it died again.

At this point I just wanted it at home in the garage where I could look at it properly, so we called the AA who turned up in less than 5 mins, which was amazing for rush hour on a Friday.
When I described the symptoms to the AA man he said he didn’t even have to open the bonnet, if it was a TD Vauxhall it would be the Diode pack on the alternator burnt out, which he proved to me by jumping it again and showing me that the alternator was actually drawing current from the battery rather than giving it.

What he did was disconnect the alternator, and loan us a fully charged battery to drive it home, and she managed the whole 15 miles OK with him following behind.
At this point there had been no dashboard warning lights of any kind at any stage.

Saturday morning I whipped the alternator out, which was a pain because it has a Vacumn pump hanging off the back with 2 x air pipes plus and oil feed and a drain, and found a guy in Portsmouth who supplies exchange units, so dropped the old one off and put a recon unit in.
Stupidly I didn’t put a meter across the battery at this point to see if it was charging, just assumed it was.

She used it Saturday afternoon without any issues, and got to work OK Monday morning, however I had a call from her on her way home Monday to say the battery warning light had come up on the dash. I told her to carry on home and call me if she didn’t make it, but she made it home OK.
It started when I got home, interestingly the battery warning light didn’t come on, but my meter across the battery terminals showed only 11.5V with and without the engine running, so obviously no juice was getting to the battery.

Convinced that the recon alternator was duff I decided to have a play, and soon discovered that even a momentary application of 4K revs turned the Alternator on, and it would then keep charging even when the revs died down to idle, giving me 13.6V at the battery for as long as the engine was running, yet as soon as the engine was shut down another application of 4K revs was required before it will charge again.

I charged the battery up, put it back in the car, and it hadn’t lost it’s charge by this morning, so the original issue hasn’t returned.
Mrs Nasher got to work OK having been told to make sure she hit 4K revs when the engine had some heat in it.

To be fair to the Alternator guy he’s going to rebuild the original unit with a new Diode pack and swap it for the one in the car at the moment, but he can’t explain the very repeatable 4K revs switch on, and why it would then charge even at idle afterwards.

All I can think of is a slight difference in the control circuit between the two units, but would welcome anyones thoughts.

Obviously I’ve got to swap the units over again, but with practice it’s not too much of a pain, but I'm hoping it sort it all out.

Nasher.

[Anchorhandler]

Sounds like an exciting issue ....:-)

No seriously though, it sounds like the alternator rotor is not receiving an excitation voltage before 4k...( thats assuming that model alternator uses an excitation system) not much you can do apart from change the regulator (if possible)...

Simon

[Nasher]

Thanks for your thoughts, I assume you are talking about the units that only work above a certain RPM.
Surely the unit would stop charging when the revs dropped again?
Unless of course it’s duff.

Nasher

[jwalker]

Dunno Nasher, it seems a bit weird. You may already know this but I'll give what I know. You've got the diode pack, the voltage regulator and the ignition light circuit to consider. The alternator is three phase and the diode pack rectifies the three into a single phase. The voltage regulator works by breaking the field current - A transistor is switched through a zener diode, this is turn switches a power transistor which is used to turn the field windings on and off. The zener conducts only at the threshold voltage (charge voltage) for regulation so it causes the power transistor to rattle on and off (very fast) because as soon as the circuit is broken the charge ceases, the zener therefore stops conducting and the charge restarts. This is the regulation.
The ignition light usually conducts one way through the bulb from the ignition switch to the alternator via a diode. But as the charge starts there is then 12 volts (ish) at the alternator end of the circuit so the bulb sees 12 volts at both ends so stops illuminating.

Good luck with it.

[Nasher]

Thanks JW, I understand the process better now, although what’s happening makes less sense now than it did.

I’m hoping that putting the old unit back in with a new Diode pack fitted will sort it out, but even so would like to understand what’s going on.
That will of course also put the original Regulator package back.

I’ve even brought my multimeter into work today to test, as I was surprised last night the car was starting, even from warm, with at one stage just over 11V in the battery.

Nasher

[Anchorhandler]

Think its a good idea to replace the original regulator as you said. The Diode pack is a simple way of creating DC voltages from three phases of AC (like a rectifier) so not sure that just changing this would sort out the 4K RPM charging issue.

Good luck with it though

Simon

[Nasher]

Put the original, refurbished, alternator back in tonight and all appears well so far.

The Alternator guy said he'd found some 'differences' in the original one, and admitted he had thought all Vauxhall units with the pump on the back were the same, so it looks like he'd just given us an incompatable exchange unit.

Nasher.