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How does DME know when to stop cranking?

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How does DME know when to stop cranking?

Old 02-04-2019, 09:27 AM
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Default How does DME know when to stop cranking?

Just a question out of curiosity since I've not had any trouble yet. When I press the Start button how does the DME know when to stop cranking? I've worked on standby generator control boards in the past and they usually look at engine RPM or generator output voltage to know when it's able to run on its own. My car has always done a simple 1-2 second crank and away it goes. Was curious if I ever did something like draining fuel lines or something and it were to need a few seconds to crank before getting pressure, how it would know how long to engage the starter.

This is on a 2009 535xi, but obviously applies to any BMW with a Start button.
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Old 02-04-2019, 10:06 AM
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Cranks until it runs. If it won't start, it'll crank for a long time until you take your foot off the brake, or push the button again.
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Old 02-04-2019, 10:37 AM
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Right, I get that it ďcranks until it runsĒ, what Iím asking is how does the dme determine that itís running? Is it looking at rpmís meeting a particular threshold? Alternator producing a minimum voltage? Etc.
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Old 02-04-2019, 11:16 PM
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thanks man, now I am stuck with this pointless curiosity too... as if I didn't have enough to worry about.
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Old 02-05-2019, 06:48 AM
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Originally Posted by kd7iwp View Post
Right, I get that it “cranks until it runs”, what I’m asking is how does the dme determine that it’s running? Is it looking at rpm’s meeting a particular threshold? Alternator producing a minimum voltage? Etc.
Let's ask the question this way. How did the starter "know" to stop functioning before DME's existed? The answer to your question lies there.
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Old 02-05-2019, 07:17 AM
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Of course, itís really easy for a human to be trained to know when to let go of the key. As a Software Developer I am saying that it can be complicated to tell by sensor data alone since the situation can vary over time. Iíve had engines in the past that sometimes needed to be cranked for quite a few seconds because they were stumbling and not fully catching. That scenario would be difficult for a dme in my opinion. The dme has to weigh whether the engine can run on its own against running the starter motor for too long. I donít think thatís an easy problem to solve so am trying to learn from others who might know, how the dme comes to the decision that it can disengage the starter.

Remember that the dme canít ďfeelĒ the engine stumbling or hear how the exhaust sounds. It has microphones in the knock sensors but engine wonít be knocking at startup necessarily. It also can tell if a cylinder is misfiring but I donít know how reliable that is at the low startup speed.

Last edited by kd7iwp; 02-05-2019 at 07:20 AM.
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Old 02-05-2019, 07:41 AM
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Pretty simple. Crank Position Sensor. Whilst cranking, the pulses from the CPS are slow. When the engine starts, the pulses speed up and the DME then stops cranking as it now 'knows' the engine is running.
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Old 02-05-2019, 08:31 AM
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i would say Tach signal. the RPM's jump once the engine is running.
however, any of the sensors on the engine could be used singularly, or together, to let the DME know when the engine is running.
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Old 02-05-2019, 09:39 AM
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Similar to what audiophool said. Not sure at what speed the engine cranks, but each crank only lasts a split second.

So I would assume the DME needs to see constant engine revolutions over a given time (say 1-2 seconds) AND engine revolutions at or above idle speed of 500rpm.

This data would signal to the DME that the engine is now running on internal combustion and not the starter motor anymore.

Last edited by dingolfing; 02-05-2019 at 09:44 AM.
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Old 02-05-2019, 10:46 AM
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Cranking speed is much lower than idle speed. Pulses from the CPS (it reads a toothed wheel using Hall Effect (with signal conditioning)). Say the pulses give a 500Hz signal whilst cranking, the signal jumps to 1500Hz once the engine starts running. The DME reads the higher frequency pulses and says "Hey! I'm running now, turn off the starter!"
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