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In an older car with a key system, the operator manually turns the starter motor off & on.

I have a push to start system in my car (where's there's a push button to turn on the ignition & starter motor). I was wondering how is the computer able to "know" when to turn the starter motor off?

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    As the revs increase (ie as they rise > 600) would be a good guess... – Solar Mike Jun 26 '17 at 16:30
  • I don't know for sure either, but agree with @SolarMike on this. – Pᴀᴜʟsᴛᴇʀ2 Jun 26 '17 at 18:07
  • Not all key systems are of the type that the operator manually turns the starter motor on & off. For example, on my 2011 Toyota Yaris with stop & start system, turning the key very momentarily on a really cold winter day could start the car. The reason was that the car computer contained a system for keeping the starter engaged even after the operator doesn't turn the key anymore. No doubt the system was there because of stop & start. – juhist Jun 30 '17 at 16:57
  • Oh, and push systems always don't have the system to keep starter engaged. I have driven a Volkswagen Passat that started by putting the key to a special location and then pushing the key. So, the key was the push button. The car didn't have a system to keep the starter engaged. So, you had to push long enough for the engine to start. These two comments show your assumption is wrong: push vs turn isn't the same as "keeps starter engaged" vs "doesn't keep starter engaged". Push is just the user interface, the rest gets decided by the car computer. – juhist Jun 30 '17 at 17:00
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On Toyota's with a push button start the ECM will power the starter relay for 30 seconds or until the engine reaches 500 RPM for 500ms. Other makes may use a different strategy.

A43-5 (STAR) - Body ground Input/Output P - Body ground Outputs voltage to the starter relay

Outputs voltage (12 V) to the starter relay when starting the engine. (Max. 30 sec. Turns off when an engine speed of 500 rpm or more continues for 500 ms or more.)

HINT: When Hi is selected, a start request signal (STAR) will not be output even though a start request signal (STSW) is input. Shift lever in any position other than P or N → shift lever in P or N (at 20°C (68°F)) 10 kΩ or higher → 93.75 to 136.36 Ω - Receives neutral start switch (P, N position detection) signal

Monitors whether the neutral start switch is on (P or N) or off (other than P or N). Applies voltage via a resistor (large) in the certification ECU (smart key ECU assembly). When the neutral start switch is on, due to the starter relay coil resistance (small) using some of the voltage, the voltage at terminal STAR will be low (2.7 V or less), and when the neutral start switch is off, the voltage will be high (9 V or more).

Perform the following operations except while the engine is cranking:

Shift lever in any position other than P or N → shift lever in P or N(at 20°C (68°F))

30 kΩ or higher → 93.75 to 136.36 Ω Neutral SW/ Clutch SW

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A pre-engaged starter motor is always engaged with the flywheel when the engine is below a certain rpm e.g. below an rpm at which the engine will run. When the solenoid connects power to the starter it spins the engine. When the engine starts and goes faster than the starter motor the starter gear is disengaged and this causes a break in the solenoid circuit.

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