Is there a wire or signal in a vehicle that indicates the engine is running?


Joined Sep 24, 2015
03 Hyundai Sonata; could not shift out of park unless you started the car first. If you stalled while in any gear or neutral you could still push the car. But if stalled and you put it in park you were stuck in park until the car was started. I never had the experience of a stalled car so I couldn't tell you if it was possible to attempt to start it (and fail) and still take it out of park.

09 Toyota Venza, same scenario.
17 Tacoma, same scenario.

Many rental cars could not be started unless you were in park or neutral. If parked and in park, you had to step on the brake pedal before the car would start, and you couldn't take it out of park unless you've started the car. I've had only one experience where I stepped on the brakes, pushed the start button and quickly shifted out of park before the engine started. So in that instance (and possibly all others) you COULD get the car out of park.

But this leads me to think that a simple microswitch installed in the shift lever housing could detect when the shifter is in park. Only then could you could get a signal that you were NOT in park. But that wouldn't necessarily mean the engine was running. The best thing I've read so far was an oil pressure sensor light switch on the engine block. When the oil pressure is low or at zero the switch closes, which would indicate no oil pressure; which is usually a pretty good indicator of an engine not running. When the pressure comes up the oil pressure switch opens, and thus turns off the light. However, those systems are opposite of what I think the TS wants. But it can still be used via a relay to trigger a switch for a high signal when the engine is running. But then again, it's possible the switch is powered the moment the key is turned, thus, once again, giving a false indication.

It's complicated. Perhaps an auxiliary crank shaft position sensor would indicate when the engine is either cranking or running. So maybe the best solution is a delay circuit, sort of like the one I posted in post #4. Messing with the capacitance and resistance will alter the time delay of the relay energizing, which in my case was about 4 seconds. Long enough to crank the engine without activating other circuitry before or while the engine is cranking.

Been reading other posts similar to this one and a lot of information has gotten confusing for me. I'll have to re-examine this thread and try to determine exactly what the TS wants to accomplish. So if I'm a little off base, give me a chance to catch up.


Joined Sep 24, 2015
I am trying to decide how to supply wire to the switch. The easy answer is use the "run" power circuit (receives power when the key is in the "run" position). But this means when I turn on the key I immediately have a 30-40A load on the battery. It will turn off of course when I turn the key to start. But I don't like the idea of drawing 40A for a few seconds each time before I start the vehicle (this is assuming someone has forgotten to turn off the manual charging switch I mentioned I will install).
OK, so the complaint is the pre-draw on the battery before starting. As the TS mentioned, it DOES turn off during the start cycle, so the battery is not being drained for both starting AND charging the secondary battery(ies). In some cases it can be a good idea to pre-warm the battery by draining a little current. Warming the battery in cold weather has long been a practice for starting a car that otherwise the battery is too cold to fully deliver its current. While a lot of people aren't aware of that practice, it IS something I've heard of before.

I had a high compression engine on a 72 Nova. With a weak battery the car often would not start. In southern California, warming the battery was not needed, nor was it useful. However, by temporarily shorting the ignition coil (Induction Discharge Ignition - or IDI, not to be confused with CDI, which is Capacitive Discharge Ignition); by temporarily shorting the coil the starter could get the engine spinning over at a full pace. Then allowing the ignition to take over the engine would start. But that's sort of off topic.

HERE's an idea: A vacuum sensitive switch. If the engine is running it has vacuum. If it's not running - no vacuum. THAT'S IT! Vacuum switch.


Joined Sep 24, 2015
Here's a vacuum switch kit. Has a micro switch and can be configured for Normal Open operation or Normal Closed operation.

READ THE REVIES BEFORE YOU BUY. There are others on the market. But the reviews seem a bit negative. Still, a diaphragm and a micro switch, sounds like a vacuum switch to me.

Even an old manifold absolute pressure (MAP) sensor could be configured. However, that's usually a 5V sensor with three leads. Power and ground with a variable output. You'd need to build a comparator circuit to know when the vacuum is of significant output. But there can be a downfall to this as well; under heavy driving loads the output of the MAP will drop, possibly below some set threshold. Which might be something you'd want to consider as well. Reducing the load on the engine during long hill climbs by NOT charging the aux battery. Food for thought.
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Joined Jan 23, 2018
Any vehicle with an electric fuel pump has some scheme to switch the pump off when the engine is not running. And not all engines pull enough vacuum to provide adequate functioning of a vacuum switch. Oil pressure is fairly reliable but using it would indeed require adding a normally open pressure switch if the system does not already include one. Another option could be an AC coupled relay picking up assorted electrical noise from the fuel injector drive circuits. The benefit of that is it will not require any actual connection, only a pickup wire wrapped around an injector power wire. Of course, that would also be active when starting, so some time delay

Of course, back in post #18 the TS has already stated their choice, so why are we still responding????