I can’t find an engine run signal, what can I do?

I can’t find an engine run signal, what can I do?

Engine run signal

An engine run signal would give you a +12V when the engine is running an 0V when the engine is off.

Switched ignition signal

A switched ignition would give you +12V with the ignition on and the engine is not running and 0V when the ignition was off.
It is important to find an engine run signal and NOT a switched ignition signal when split charging because the split charge relay will engage and both batteries will be connected together during engine cranking which can damage the leisure battery.

The solution

If you can find a switched ignition signal you can create an engine run signal by installing a voltage sensing relay (VSR).

A voltage sensing relay works by connecting a negative, switched ignition feed to the input then the out put would become your engine run signal. For the relay to connect the input to the output it needs to detect +13.2V on the input to engage rather than +12V like a traditional relay.

Using a voltage sensing relay (VSR) means you can use a switched ignition because the voltage on the switched ignition would be below 13.2V unless the engine had already been started and the alternator was charging the vehicle battery.
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