Jump start kils ECU

In addition to Ford Smart Charge (introduced ~1998):

many cars use the ECU to effectively turn the alternator output off when the lights are not in use and the battery is at least partially charged. The alternator is only put back 'on load' when the ECU senses an over-run condition. Obviously this is a fuel saving strategy

BMW have done this on some models from 2006; their system is called Intelligent Alternator Control. Later Disco's turn the alternator off at tickover.

Incidentally, the author of the above link suggests that Ford vehicles with Smart Charge and a flat battery should *never* be jump-started!

Chris

Some of the ecu operations cant be fed through circuits with these components fitted .

ECUs have different operating voltages for some functions , within the electronics of a car you can create a voltage differential of nearly 30 volts if required you can have - feed + feed to a ground , split rail grounds all within the ecu plus ac

"Dave Plowman (News)" wrote: [snip]

Actually it's not. I've just had to replace a wing mirror and the Infinity head unit and amp in the Jeep because erindoors got a jump start.

Fried the memory in the mirror and blew heavens knows what in the stereo. Deader than corduroy flares.

The ECU and the LPG controller are both fine.

Why? The surge suppressor won't pull any current?

It's not the logic that's slow, it's the control of the output voltage by the field current that has an inherent lag.

But Varistors that can absorb 200J repeatedly are serious money, and rarely necessary, jump starting with some load attached is far cheaper.

I don't understand the 2nd bit, but then I don't see how the starter puts a spike back on the 12V rail, the reason it's input goes high is that it's disconnected from the 12V rail. Unless you're doing somethig very strange it won't be driven by the alternator. (Presumably jumping a dead car with a car with a knackered battery may also be a bad idea if you don't follow the instructions)

Not when it's turning. Try looking at the supply with a 'scope.

As i already explained the functions of the ecu will prohibit such

not every wire comming out of the ecu carries +12 volts some carry 3 volts some carry 5 volts some carry dc some carry ac some are 0 volts otheres can be -3 volts -5 volts -12 volts some wires are signal wires .

Some wires are even multifunction

Varistors are only of any use for voltage surges , no good at all for inrush of current.

Agreed but at that point its pulling the load itself once its started the car it disconnects from the flywheel and the load drops off .

When the key is realeased the unit is isolated again via the relay and soleniod system

Did say from the start it requires decent design. Which I'd not expect on a Jeep.

If you were jumping a car with a flat battery, you wouldn't have too many other items switched on. If other items did fail, it may not be noticed until later.

Chris

Isn't a flat battery a pretty effing big load?

Tim

Not necessarily, no. It depends on how long it's been flat & whether or not one of the cells has gone open circuit. Most of the time it's an ideal heavy load, under almost all the circumstances it isn't then it won't start the car on it;s own.

If it was ok before it went flat and hasn't been flat for long, yes. But if it has failed through one or more cells going high impedance or has got sulphated, no.

so really a bump start if possible is safer all round if the battery is low (rather than absolutely dead)

Absolutely. Unless it's a recent twin-battery Transit...

Chris

I thought bump starting, like curiosity, kills cats?

The chances of damaging anything with a jump start where the battery isn't totally dead are extremely remote.