Automotive: sensing when the motor is running or not?

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Thread Starter


Joined Oct 18, 2010
Remote start alarm systems for cars have the option to use voltage sensing to tell the alarm when the vehicle is actually running to it knows to stop cranking the starter. does anyone know exactly how this technology works and if there is a circuit i can create that will do the same thing and provide me with a negative output to drive a relay that will remain constant until it detects that the motor is no longer running (not just turning off the ignition but say the vehicle stalls it will cut the ground output)

Before the voltage sensing you had to connect a wire to the tach signal. i imagine this would be a simpler circuit. i'm curious as to how this style works as well and how i could connect to the tach signal to trigger a constant ground output as long as the motor is running and shuts off when the motor stops.



Joined Sep 21, 2010
My vehicle has dual fuel petrol & LPG it uses a module to detect ignition pulses to keep a relay activated. If pulses stop Ie engine stalls relay is de activated. have a circuit some where. If you want it PM me. As this thread will probably be closed.


Joined Nov 22, 2011
There are quite a few ways that I can think of to do this.

How much you wanna pay us to know?

Cheapest and most reliable solution is usually what engineers decide on.

You will have to price parts to find out won't you.
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Joined Mar 24, 2008
The All About Circuits forum Administrative Team has elected not to host discussions of automotive electrical system modifications/enhancements due to safety concerns, the potential of legal ramifications and the possible circumvention of vehicle regulations at the state and federal level.

This thread is against the AAC forum rules, Chapter 6, as seen here:

Automotive modifications of any kind are strictly forbidden. Therefore, this thread will be closed.

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