Quick Wiring Question - Xantia

addo · Unread post by **addo** » 17 Jul 2011, 10:29

Simple one! Is there any risk in starting/running the car with the alternator output lead disconnected?

I am presuming not, as it seems generally that you can drive a car on a dying battery once the alternator has failed. Correct, or am I misinformed?

Thanks, Adam.

mongoose100 · Unread post by **mongoose100** » 17 Jul 2011, 11:46

Adam, I've done it many times when swapping dead batteries etc and had no drama...

In saying that, I'm not saying yes or no here

Unread post by **myglaren** » 17 Jul 2011, 13:13

Generally considered inadvisable as the diode pack in the alternator can be damaged - the battery is a low impedance sink.

Gibbo2286 · Unread post by **Gibbo2286** » 17 Jul 2011, 14:42

Is it me, did I mis-read the op? I thought he was thinking of running the car off the battery with the alternator wire disconnected as he'd got home with a duff alternator, if that's what he meant the only thing he needs to be careful of is short circuiting the loose wire.

Unread post by **myglaren** » 17 Jul 2011, 14:53

If the alternator is still producing a current then disconnecting the positive lead is the same as disconnecting the battery.

Unread post by **Peter.N.** » 17 Jul 2011, 15:08

In reply to the initial question, I would say - no - until the battery goes flat of course.

Peter

wheeler · Unread post by **wheeler** » 17 Jul 2011, 20:48

As for starting for a minute i wouldnt say its a problem. As for running it i've always been led to believe this is a big no no, if the alternator is operational & generating power then the energy its generating has to go somewhere. I think worst case is the alternator can heat up & go on fire.

DickieG · Unread post by **DickieG** » 17 Jul 2011, 21:27

wheeler wrote:I think worst case is the alternator can heat up & go on fire.

Minor point then

Unread post by **Peter.N.** » 17 Jul 2011, 22:54

That is not so. Unless there is a load on the alternator is not producing any power, well only just enough to keep it energised. When the battery is fully charged the voltage rises to around 14v and it almost stops charging, otherwise it would cook the battery. It will continue to power anything that is running on the car but in daylight hours that can be very little.

When the voltage reaches about 14.5 it stops charging altogether so if you run it without the battery connected the voltage will rise to a point that the regulator provides no field current to the rotor and thus no output. the alternator sees being disconnected in the same light as being connected to a full battery.

Peter

Unread post by **CitroJim** » 17 Jul 2011, 23:14

I'd agree Peter and if you disconnect the warning light wire as well I believe this will stop excitation completely.

I base that on having once observed a break in that wire preventing it charging.

Unread post by **Peter.N.** » 17 Jul 2011, 23:28

That is so, it won't charge if the bulb has gone either.

Peter

addo · Unread post by **addo** » 17 Jul 2011, 23:37

Sounds like I'm good to go. Either the alternator is carboned up with a mix of LHM and brush dust, or the starter solenoid is crook again.

I'm suspecting the former. Joy, oh joy. Intake manifold off again!