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

#### zirconx

Joined Mar 10, 2010
154
I am installing a DC-to-DC charger in my motorhome. This will charge the "house" battery from the alternator (it's lithium iron so requires it's own special charger). I will install a switch on the dash so I can control when to charge it, as to avoid putting too much load on the alternator at idle or other times I deem necessary.

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).

Does anyone know a simple way to detect that the car is running? I don't think there is anything in the fuse box that will only be on when the engine is running. The motorhome chassis is a 2005 Ford E-450.

My 2nd idea is somehow cause a delay in supplying power to the switch. Something like this could do it: https://www.amazon.com/gp/product/B07C4ZP23D

Oh I just thought of a third idea - a latching relay that doesn't close until it gets a "crank" signal from the key switch. I would still need a delay I guess, you wouldn't want to draw power during crank. Maybe use two relays, the first one closes and stays closed when it gets a signal from the "crank" wire. The 2nd relay (in series with the first) gets power from the "run" circuit.

Thoughts? Thanks.

#### Ian Rogers

Joined Dec 12, 2012
988
Oil light switch.... If it is other than ground the engine is turning.. Its what use on cranes to run our electronics.

#### Marley

Joined Apr 4, 2016
470
One way to do this is by monitoring the battery voltage. When the engine is running and the alternator charging, the voltage should be around 14V or a little bit more. With the engine stopped the voltage will be around 12.6 - 13V.

Test and see.

It would be possible to make a voltage detecting circuit that drives a relay to control the supply to your DC-DC.

This solution would also disconnect the DC-DC when the engine cranks.

#### Tonyr1084

Joined Sep 24, 2015
6,866
Here's a 4 second delay I built several years ago. It's purpose was to delay powering up a gauge that had a colored light. The color was selectable and a memory line would keep it in memory. But during start, the initial surge, then drop out as the engine started, then power returned made the memory default to the factory original color. Was a pain in the neck to reset the color every time, so I built this out of scrap parts. Yes, that's a 24V relay. At 12V it does click in. If you want a longer delay then change R1 to a higher resistance or C1 to a higher capacitance. And whatever relay you choose will also affect the timing performance. So some experimentation will be needed.

#### Ian0

Joined Aug 7, 2020
4,865

#### zirconx

Joined Mar 10, 2010
154

#### crutschow

Joined Mar 14, 2008
29,511
are you saying use that relay in addition to the charger? That relay is at least $50. My charger is already costing me$200.
Yes.
Even if you built you own delay circuit, the parts likely will cost you at least half of what that split-charge relay costs.

#### Ian0

Joined Aug 7, 2020
4,865
The house battery is a lithium iron phosphate, which needs a different charging voltage than the lead acid-alternator system. Hence the DC-to-DC charger.

https://www.power-sonic.com/blog/how-to-charge-lithium-iron-phosphate-lifepo4-batteries/

Or are you saying use that relay in addition to the charger? That relay is at least $50. My charger is already costing me$200.
Your charge circuit can go on the output of the relay.
Split-charge relays are very common (probably more so in a country that likes caravans), Durite was the first example I found. The one that was installed in my car certainly doesn’t look as though it cost that much. But it has a standard 30A ”automotive” relay as the output.

#### LowQCab

Joined Nov 6, 2012
1,929
Use this Circuit to operate a "Continuous-Duty" Solenoid to connect the 2 Battery systems.
.
.
.

#### Ian0

Joined Aug 7, 2020
4,865
I'd recommend a high value resistor from the FET drain to the TL431 adj terminal to add a bit of hysteresis.

I'm assuming that the purpose of the high-current FET would be to switch the batteries directly without a relay. If so, the source-drain diode might present a problem!

#### LowQCab

Joined Nov 6, 2012
1,929
The Circuit was not originally intended for high Battery-Charging Current,
or connecting separate Battery systems.
That's why I recommended using a "Continuous-Duty-Solenoid",
and, the Schematic provided 2 different FET suggestions.
The big FET is over-kill for driving a Solenoid,
and the small one is for switching a high-Impedance Input.
.
.
.

#### zirconx

Joined Mar 10, 2010
154
The split charge relay is a pretty good idea. I was able to find one on Amazon for close to \$20, I ordered it. I'll only be switching a few milliamps (the turn-on signal for the DC-to-DC charger) through a 140A relay, but it should work.

#### MisterBill2

Joined Jan 23, 2018
11,605
Oil light switch.... If it is other than ground the engine is turning.. Its what use on cranes to run our electronics.
The oil pressure switch has also been used to enable the electric fuel pump , the function being that if the engine stops running the oil pressure drops quickly and the fuel pump switches off . That is a very reliable and simple system with a record of being good for safety controls. And if you use an adjustable pressure switch, you can set it to switch off when the engine is idling. G.M.has used a 2-terminal isolated oil pressure switch for the electric fuel pump forquite a while, but that was quite a few years ago. But those switches might be available as a service part. That could be a simple and reliable way to solve the problem.

Last edited:

#### LowQCab

Joined Nov 6, 2012
1,929
Is there a wire or signal in the vehicle that indicates that the engine is running?

That would be the D+ terminal on the alternator.

View attachment 251922
That used to be sort of a standard, but not for the last ~30-years.
I'd be astounded to see a modern car alternator setup this way,
now they're all computer controlled.
.
.
.

#### Tonyr1084

Joined Sep 24, 2015
6,866
Something I thought of just last night when going out to dinner: I can't move the shift lever out of park unless the engine is running. You would have to test whether you can shift out under all possible circumstances, but whatever the mechanism is - there's something that doesn't release the shifter unless you step on the brake pedal AND start the car. Something to look into.

#### Tonyr1084

Joined Sep 24, 2015
6,866
Just tried this on my truck: Step on the brake pedal, push the "Push To Start" button, before the engine starts - I'm able to shift out of park. So eh, doesn't look like that does what I thought it might. Sorry. Maybe yours is different. Probably not, but possibly.

#### zirconx

Joined Mar 10, 2010
154
I ordered the split charge relay, that's what I plan to use. I'll wire it up before my manual push button switch, so that if the switch is lit up, I know the charger is on.

Thanks all for working through this with me!

#### vu2nan

Joined Sep 11, 2014
216
That used to be sort of a standard, but not for the last ~30-years.
I'd be astounded to see a modern car alternator setup this way,
now they're all computer controlled.
Many thanks, LowQCab, for your comment.

My answer certainly does not apply to a 2005 Ford E-450 and has been deleted.

-Nandu.

#### MisterBill2

Joined Jan 23, 2018
11,605
Something I thought of just last night when going out to dinner: I can't move the shift lever out of park unless the engine is running. You would have to test whether you can shift out under all possible circumstances, but whatever the mechanism is - there's something that doesn't release the shifter unless you step on the brake pedal AND start the car. Something to look into.
What brand of car is it that you can't shift out of park unless the engine is running. The first time the engine stalls while driving you will be stuck out in the roadway , locked in park, unable to be pushed out of the traffic. Probably you are not even able to start the car in neutral! And hope that you never ever run out of gas!