What tools should be carried in a modern car?

Surge.. Well how are we defining that?, a surge of current which by Ohms law would in this instance mean a rise in voltage and to what level would that be?.

Most all cars that are charging or have charged their system battery are limited at 14.4 volts which isn't that high. If this was going to say 20 or 40 volts then yes that I'd consider a "surge" but surely that would cause even more damage then the alleged damage to this rather fragile ECU?...

Disconnected?. So they disconnect the other electric's in that vehicle then?..

I am not satisfied with the explanations on this alleged surge as yet...

Well then -were- are we agreed on there is exactly a problem?..

So are we to assume that because of the current sink of the donor system that the volts are now in the resuscitated vehicle going to be too high for it to cope with or the alternator is now out of control?. If that really is the case then thats very poor system design..

The recovery service would - err recover...

Sometimes that would be to the member's home, for them to arrange repairs, sometimes that would be direct to a repairer. From that point on, the recovery service would have no knowledge of, or interest in, the vehicle, unless it was to a garage that the patrol regularly attended. In that case, professional interest by the patrol might lead to an enquiry as to the fault, but the actual organisation would not know the outcome.

Chris

Across the donor battery so they are I presume clamp diodes which offer a short to the system to clamp the voltage ?. Seems a tad scary if that is what they are really doing;!...

So therefore we seem to be in a state where the alternator of the resus vehicle is running with no control over its output?..

In article , Dave Plowman (News) scribeth thus

Indeed.. I'd expect that the ECU has at least some sort of switch mode input to its power system and each in out line to have voltage clamps to it to prevent over volt spikes affecting it. Similar to output lines after all get this design wrong and it shouldn't be rocket science, thats an awful lot of recalls..

Mind you look at what a c*ck up the WAG coil pack has been;!....

I thought that they were giving assisted "jump starts" then this problem arouse to such a level that they then had to revise their equipment or procedures?..

No, the voltage surges, the current decays gradually.

But the vehicle hasn't got a working battery connected.

Well it depends what else is connected, but if theirs a lot of other things connected then the voltage rise will be lower.

No, when you disconnect the jump leads it's not connected to the other vehicle anymore

You don't think that the field winding has some inductance & therefore takes a finite time to decay to zero?

The alternator which is disconnected from a battery, when you disconnect the jump leads, continuing to supply current & overvolting the cars electrics.

The alternator is out of control when you disconnect a large load, it's only relatively recently that they've had anymore sophisticated control than the old mechanical regulators, when the voltage is low then turn the field on full, when it's high disconnect the field. Until you've got alternators that supply large currents at low rpm it's not a significant issue.

When do zeners start applying a short?

Until the field current drops it has no control.

Temporarily, shortly after the reverse voltage exceeds the voltage rating of the junction, with the short disappearing shortly after the applied voltage drops again. It actually short circuits, more or less, the incoming voltage to its reference voltage. Permanently, shortly after it turns out that the current that the voltage source can supply exceeds the reverse current rating of the junction.

That's not a short circuit though, the voltage doesn't drop below the clamp voltage.

Yup, I think the whole point of them is to avoid getting ahywhere near that point.

In the case of the charge lead snubbing circuits, I'd expect a suitably rated Voltage Dependent Resistor to be more use than a zener.