battery "cut out" switches

[roger001]

In order to protect the alternator diodes when installing a battery cut out the norm is to provide a path to earth via a resistor, the connection to which is made when the cut out is in the off position.

Does anyone know is this adequate to protect the ECU or are other measures necessary

[JohnB_SPY8808053]

I'm not sure - my Dad's a real Wiz with car electronics though. We are going to work on my wiring diagram this weekend so I can ask him about it then.

John B.

[rutthenut]

Originally posted by roger001
... Does anyone know is this adequate to protect the ECU or are other measures necessary

I don't know of any problems with fitting the 'full' cutout of this sort (which is the type I've used). If the ECU power supply is wired correctly from the same power source (battery/alternator) then it should have zero power when the switch is thrown.

As I understand it, the load supplied by the resistor is to prevent the alternator circuitry, which may otherwise try to increase output rapidly whilst the unit is still turning but no balancing supply or voltage sink is detected from the battery circuit. This tends to blow the diode pack or some other componentry in there.

Having the resistor allows the alternator to survive this short period of disconnection from the battery, but whilst the engine (and hence the alternator drive belt) is still turning.

If there any issues with the ecu in this case, I'd like to know about them!

I realise the possibility of blowing all manner of electrical components if welding is performed using certain types of equipment (mig, tig, whatever - I have no idea which is which), but I assume that is due to the high voltages that may be put through the vehicle chassis and componentry.

[Stratos]

Originally posted by roger001
In order to protect the alternator diodes when installing a battery cut out the norm is to provide a path to earth via a resistor, the connection to which is made when the cut out is in the off position.

Does anyone know is this adequate to protect the ECU or are other measures necessary

Roger,

My cutout switch gets tested frequently, and I reckon if it was going to cause a problem with the ECU I would have know about by now.

Before I load the car onto the trailer for each rally, I start the car and then go and switch it off by pulling the cut-out switch, because I know that at most rallies the scrutineer is going to do that, and if it doesn't work he won't let me start the rally.

And the same probably applies to Gary Lomas, and I know his car has been running the same engine and ECU for the last 9 years!

[rutthenut]

Originally posted by Stratos
My cutout switch gets tested frequently, and I reckon if it was going to cause a problem with the ECU I would have know about by now.

Before I load the car onto the trailer for each rally, I start the car and then go and switch it off by pulling the cut-out switch...

I tend to use my master switch to shut off the car almost all the times that I use it - although I have to admit that's not at all often these days!

[David May]

As I remember it, the use of a resistor was advisable in the old days of coil ignitions and dynamos when an amp or two was enough to keep the ignition firing, which could be leaked through the various warning lights.

The famous 'load-dump' (which used to damage early electronic systems) is caused when the alternator is heavily charging and suddenly has its load removed (by a cut-out switch or bad connection). Being relatively slow to respond, the voltage can rise to as much as 80Volts for a brief period before it gets back into control, enough to break down many semiconductors (diodes, transistors etc.)

For many years now, all sensitive car electronic components are fitted with a 'load-dump' suppression circuit (usually a 27Volt varistor diode) to absorb this dangerous energy.
The latest hi-power alternators may have such a circuit built-in although in this case the peak may still reach 50Volts.

Only a very poorly designed circuit (that has not been fully homologated) will suffer any damage, so modern ECUs are OK.

Dave May