Jump start procedures

Why would you be using a spanner with jump leads? ;-)

And the voltage in - via the warning light - acts as a control voltage to the regulator. Without this, some alternators won't charge.

It might well to some electronics. I certainly wouldn't take the risk. You carry on. ;-)

Agreed - I spent a whole morning trying to work out why my previous car wouldn't start - eventually worked out that it was a fuel cut-off switch

- I hadn't even realised it had one, and I don't know how I managed to accidentally trigger it. They ought to mark it with some kind of symbol so you realise it's a switch.

Now I drive an old banger... (the previous one was only a year younger, but was a ghia) there are advantages, nothing complex to go wrong (and the insurance is far cheaper!)

In news: snipped-for-privacy@davenoise.co.uk, Dave Plowman (News) decided to enlighten our sheltered souls with a rant as follows

You *know* what I meant :-P

Voltage regulators are self contained. They require no external "control". What I think is confusing you is the fact that an alternator won't charge without "some" voltage present. This is because something is required to produce the field effect in the first place.

A warning light is in effect a bulb connected to two +ves. When the alternator produces a voltage, both sides of the bulb are at 12V and it doesn't light. When the alternator doesn't produce a voltage, the potential difference between the +12v side and the side connected to the warning lamp terminal is sufficient to light the bulb.

I will do.

The point about alternator voltage regulators is that they work by modulating the current in the field coils only. That way they don't have to deal directly with the very high currents that modern alternators can produce. The output can be taken directly from the sodding great stator coils via nothing more than some sodding great diodes.

The down side is that the response time tends to be quite slow. When some big load gets turned off and the regulator finds it suddenly needs to throttle-back the output, it takes a little while for the field to collapse, so the stator coils carry on producing far too much current for that time. It's called load-dumping. Normally, the battery absorbs all the spikes, but if the battery isn't there, then something else has to. The voltage just goes up and up until something on the system actually draws the extra current.

Often, it's the over-voltage protection network of some electronic device or other that "breaks over" and tries to short out the spike, blowing it's fuse in the process. With the fuse blown, the voltage carries on going up until the next device decides it's had enough etc. etc. Eventually, the spike subsides and fuses stop blowing. If you're unlucky, a fuse or two won't blow quick enough and you'll be minus some random control box.

When attempting to jump-start the way you suggest, the load dump that's most likely to cause it will be the removal of the jump leads. The donor battery will be drawing a high current at that time since it's only just started a car.

BTDTGTTS (not on my car though :o)

Cheers,

Colin.

It's not confusing, me, no. Just try starting your car, disconnecting the battery with no load other than the ignition, etc, revving the engine and measure the alternator output voltage. And check on where the smoke comes from.

Car is back and fixed. Jump start didn't do it because the distributor was faulty. This is now replaced and things are ok.

What smoke? THe smoke in your mind?

Just do it Conor. You've already had an explanation from Colin about residual magnetism and how it effects a lightly loaded alternator.

You might also look up the maximum voltage a 5 volt regulator of the type found in an ECU will handle.

Try it and see. The open voltage on the battery posts will approach 60+V very very quickly and you will blow shitloads of fuses.

The message from Chris Street contains these words:

Clamps, not posts, Shirley?

Well if the fuses blow then they'll have been loading the alternator anyway & it'll all be fine :-)