I want to install a circuit in my car and I need a 12V power line after I start the engine. I don't want this line to be powered when I turn the key on ACC/ON positions, only after the engine has been started. I looked in the fuse box but all the fuses were powered with 12V when I was turning the key on ACC/ON. I need only 40mA to turn on a relay. Where should I look for to find the wire I need?
You can achieve what you intend to quite simply by using a relay to which you attach the trigger +ve pin to the ACC/ON ignition feed and the trigger ground pin to the oil pressure switch. This has long been the method of wiring up solid state fuel pumps in race cars so that should the engine stall, they will not run.
The attached diagram shows a circuit where a fuel pump is connected in this manor. You'll note that this includes an additional priming button to manually prime the fuel lines prior to starting the vehicle. If you never need to have your device running with the engine off, you can omit this circuit.
You don't say how old the vehicle you want to put this in is however I'm going under the assumption that it is something newer than 2000.
The best choice you have for an analog method of telling if the car is running is if the battery voltage is over 13 volts. That means that the alternator is running and that the system is charging.
If you have some programming experience then I would say you want to hook into the ODB-II diagnostic port and read the RPM out of the computer.
Most dashboard tachometers are not mechanical so I wouldn't suggest modifying your tach, if you have one. Even if you don't have one the car computer keeps track of the engine speed so that it properly meters fuel to the injectors and adjusts timing.
Essentially you would have a small circuit powered by the accessory circuit that senses current in the run line. If that current is present
For the issue if you detecting power at the fuse box, because of the battery being the energy source any wire attached that isn't a ground will have energy on it. When you are checking at the fuse box all of those wires always have current on them because they are hooked up to the positive side of the battery. And then you are hooking them to the negative side of the battery through the meter so you see voltage. If instead you were to attach an ammeter you would see nothing because there's nothing flowing through that circuit unless it is complete.
If you have the right tools and skill set, I think the easiest (though not most accurate/reliable) solution would be adding a 12VDC relay near your battery, using the battery + line as control and adding the appropriate resistor to between the relay and the + terminal.
From my initial estimates, this would be a ~10 Ohm resistor, but it does depend on the specific relay you choose, and the actual draw of the relays switch system. (This is based on a 200 mA draw on the relay itself.)
The inaccuracy will be dependent on how sensitive the relays control line is (at what point it will stably contact), and your batteries voltage.
Using this method, your new circuit would stay on for up to several minutes after each drive, and may be slow turning on when you first start your car. Additionally, when your battery (or alternator) becomes old and weak, the circuit will start having problems and a really bad battery will absorb too much voltage, thus deactivating your circuit. Finally, depending on how sensitive your new circuit is, you could have damage caused by the relay chattering on/off as the battery voltage passes a small range after start and stopping of the alternator/engine. This would mainly be an issue if you plan on running computers or really sensitive low-voltage equipment.
There are definitely more accurate and cleaner (in regard to electrical flux) methods that you could use, but if you know what you’re doing, this could solve your problem with ~1 hour of work and under $50 (possibly - don’t take that as a price quote or exact wrk estimate... I don’t know exactly what you’re trying to add.)