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      07-29-2016, 09:17 AM   #1
GuidoK
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What fuse or power circuit only gives power when the engine is acutally running?

For a project I'm doing I need power from the car but only when the engine is actually running. So I dont mean power upon contact (loads of circuits to choose from), but a wire that only gets live when the engine runs.
I need about 5amp tops.
Its for a 3.0i.
Is there such a circuit present wherefrom I can draw power?
I know the eps only starts working when the engine runs but the eps fuse is live on contact. It probably (I guess) gets data from the ecu over a data bus as it's a pretty complicated system
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      07-29-2016, 12:19 PM   #2
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Any power source that is on when the engine is running will most likely also be on when the engine is not running (all circuits that are enabled with key in the ON position). The engine is only deemed running when RPM > 400 or so, in which the ECU would have to trigger some other system to enable a relay.

The EPS control unit receives the following signals across the CAN bus.

From the Dynamic Stability Control (DSC): - vehicle speed

From the Digital Engine Electronics (DME): - engine speed and status of terminal 15 (one message) - CAN status - status of Sport switch

From the steering-angle sensor (LWS): - steering-wheel angle - prefix of steering-wheel angle - status of steering-angle sensor (all signals in one message)

From the instrument cluster: - kilometre reading - switch day/night lighting (0=off, 1=on)

The instrument cluster receives the following signal from the EPS control unit across the CAN bus. - activation EPS indicator lamp in the instrument cluster (0=off, 1=on)


If you had a standalone ECU, this would be cake to implement.
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      07-29-2016, 12:30 PM   #3
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Yes I cant use any databus like canbus.

Cars used to have a D+ on the alternator (for example used to control the run engine light) but the m54 engine already has an ecu controlled alternator so that doesnt work. I measured the voltage on that d+ connector but its not DC (dc 7,5v ac 5,8v) so some kind of complex signal that also isnt set on/off with the engine.

On modern campervans they sometimes use a D+ simulator (d+ is used there also to switch a fridge on/off) and such simulators work on the battery voltage (so they swich on/off at 13.5v or so), but that is way to unreliable for my project.
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      07-29-2016, 02:29 PM   #4
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Install a vacuum switch connected to a relay, with power to the hot side of the relay from an ignition-switched source. Engine running - relay supplies power, engine off - power off.
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      07-29-2016, 02:35 PM   #5
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And when you are full throttle, what do you do? There really is no simple solution. In the past, I've used a voltage differentiator to measure battery voltage. When voltage spiked from ~11-12v to 13+, then I knew the alternator was running, thus the engine was running.

Beside that, you'll have to get creative.
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      07-29-2016, 02:49 PM   #6
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If that doesn't suit his needs, then he can use something else for the trigger like an airflow sensor or capacitive pickup. It's not that complicated to determine if an engine is running, independent of computer signals.
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      07-29-2016, 03:26 PM   #7
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Quote:
Originally Posted by WestCDA View Post
If that doesn't suit his needs, then he can use something else for the trigger like an airflow sensor or capacitive pickup. It's not that complicated to determine if an engine is running, independent of computer signals.
I dont know how. To get a signal from example the crank sensor still requires active electronic circuitry (which has to be reliable) and not interfere with the dme.
I thought about using the o2 sensor heater element, letting that switch a relais that then starts to feed its own coil (because the heater element switches off), but I dont know if the o2 sensor heater wire is also live when a warm engine is started. And this circuit is also active when the engine shuts off but contact is still on (engine stalling). That is something I could probably live with.
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      07-29-2016, 04:57 PM   #8
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I wasn't referring to tapping into critical existing circuits - with most sensors, I wouldn't take the risk of interfering with normal computer function. I meant an add-on sensor/circuit to detect vacuum, rotation, spark voltage, oil pressure, water circulation, air intake, noise, vibration, temperature or any of the other myriad of things that change state when an engine is 'alive' ...

No idea what you intend on doing, under what conditions its needed, and how critical things like start/stop time and continuity are. Maybe what you need does not warrant the effort or expense - or maybe it does. But detecting the current run/stop status of an engine, and using that signal to conditionally provide a 12V supply, is certainly possible.
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      07-29-2016, 05:37 PM   #9
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ok, but I'm looking for a bit more concrete plan
Everything is possible, but an actual example is something different.
I'm not going to design custom active ecu based electronics for a power feed, because I dont know how and I dont know if it'll be reliable enough with temperature etc.
apparantly its not that simple.

Its for installing dual wideband lambda and narrowband converter btw.

A D+ simulator (which dre99gsx basically describes) that on its turn switches a contact on feed might work.
I dont want to blow my wideband sensors when for example I have to jumpstart my car. That would be pretty bad.
Jumpstarting my car because you cant drive with the result that you cant drive.

I figured out that the connector on the alternator (which on old cars is a D+ signal) on the m54 alternator is a bsd feed (bit serial data), so that is definately useless. But I saw that there is a second pin that isnt used. I'm going to measure that first to see if thats a dummy or some other feed.
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      07-30-2016, 10:03 PM   #10
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Jump starting the car shouldn't cause any issue with your sensors, but if you're worried about them getting killed by an over voltage spike, why not just power them normally and use surge protection? You can use two zener diodes with their anodes (end opposite the stripe) connected together and the other ends connected to the positive and ground of your sensor to protect them.

https://www.amazon.com/Zener-Diodes-.../dp/B00M2E5U6G
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      07-30-2016, 10:41 PM   #11
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No its not that I'm afraid that the sensors die of overvoltage.
If I use voltage detection to heat the sensors, and it gets triggered by overvoltage (say by jumpstarting or by a battery charger), the sensors are hot while I start the car.
That's not allowed to happen as the cold shock (from the first few cold combustion strokes) will destroy the sensors.
Thats why the lambdasensors only get heated while the engine is running.
The heater will heat the sensors to about 700C I believe so they run quite hot and wont survive a blast of cold air.
Not having them heated is also not an option because a drop of condensation will/can form on the sensor, causing it to die also.
(because of that condensation, lambdasensors arent allowed to be mounted in certain positions)

So I want it to be pretty foolproof that the sensorsheaters wont get triggered before the engine runs.


The second pin on the alternator is a dummy btw, I measured.

there are now 2 reasonable options.
The best one is that the original heatercircuit switches a relay that then feeds the relaycoil (output to relay coil) and that relay switches a contact voltage (position 2 on the key).
But in that option the ecu must heat the lambdasensors every time when the engine starts, so also on a hot start. I have to measure if that is really the case.

The other one is a voltage triggered switch (d+ simulator) that switches a contact voltage (pos 2). In this scenario I cant start the car when its charged (jumpstart) and the key is in pos 2. I have to go from 1 directly to start.
when there's a batterycharger on the battery and the key is in 2 (so power is drawn to a lot of systems) I dont think the charger has enough power to spike the voltage above 13,5V. When its off or in 1 it can spike above 14v, but if it then comes on, it switches a dead wire (no contact pos2).

I've measured and with most systems on (all lights, stereo, aircon/heater etc) and on idle, the clamping voltage is still almost 13,8V.

In the manual it looked so simple, just 1 sentence: "switched means 12v only when the engine is running"

Other brands of o2 lambda controllers (for instance innovate) just say to use normal switched 12v (so key switched) and always start the car like you normally do. (so from off pretty fast to starting) But that would mean you cant have it on ignition for a while (if you want to do something with lights, radio, nav or whatever) and then start the car. Bang, pay for 2 new widebands (and in my case not be able to drive as I plan to completely ditch the narrowbands).
Maybe the innovate controller has an extra timer build in before it starts to work, but I need the narrowband output to be ready once the ecu expects a signal otherwise it will throw a code.
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      07-31-2016, 12:13 PM   #12
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Got it! Take a look at the output of the ping sensors with your DMM and see what level of AC voltage is coming from them when the engine is running. They're piezo electric voltage generators that generate an output voltage proportional the their input stress level and feedback the acoustic signature from detonation in the cylinders to the DME. There's only output there when the engine is running, and if it's high enough I can come up with a threshold detector circuit that will close a relay when there's an output from the ping sensor.
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      07-31-2016, 01:21 PM   #13
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Messing with the ping sensors (when the ecu has to DSP those signals coming of the same sensor) is a bit risky. I also rather not use a circuit with lots of active components (exept maybe diodes) as that can cause reliability issues. (transistors, chips or an arduino or so). And it gets quickly out of my 'capabilities range'

I already got a point of one of the alternator coils that has a square wave varying with rpm (this is at idle).:

(dont mind the ancient scope, its all I had)
I tested it and it only carries the signal when the alternator is rotating and it's not influenced by battery or ecu.

A bridge rectifier, a capacitor and a relay to switch the current is all I need. (maybe a voltage regulator like a 7812 or so)
The only thing I dont know is if a bridge rectifier will still be funcioning properly at 10khz or so(the max frequency it could ever reach tops). I dont want it to get hot or anything (so I dont know if it gets hot at higher frequencies, even with a minor load, about 100mA for the relais)
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      07-31-2016, 03:40 PM   #14
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Half wave rectification should be sufficient for what you're doing. Just pick a relay with at least 75 ohm coil resistance or you'll need to increase the capacitor value proportionally. Something like this should do.
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      07-31-2016, 04:32 PM   #15
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But can a 1n4002 handle that frequency? It's not a schottky diode I think?

A stock low amp automotive relay has a coil resistance of about 100-120ohm (they take 100mA or so), so that wont be a problem.

I'll probably head down to the local electronics store to see what they have on stock.
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      07-31-2016, 08:09 PM   #16
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I've used the 1N400X series diodes above 30KHz with no issues. It's only got approximately 20pF capacitance even at the low voltage you'll be running it, so you shouldn't have any problem. If this was an application where you needed a really fast reverse recovery you might look at a Schottky, but you don't need it for this.

http://www.onsemi.com/pub_link/Collateral/1N4001-D.PDF
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      07-31-2016, 10:28 PM   #17
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Well now, if you are going to go that crazy, just use a LM2907N and configure it output 5v logic to run a relay once you have any frequency at the input pin.
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      08-01-2016, 06:03 AM   #18
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thats also an interesting approach too. that way any drops in voltage are excluded.
Can a LM2907 drive an automotive style relay directly? (12v, 120mA)
I want to use a generic relay to keep the posibility of a roadside/local garage/local autostore repair.
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      08-01-2016, 07:30 PM   #19
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50mA is the max sink current on the 2907 so you'd need to find a higher coil resistance relay or use an external transistor for the additional current sink. Collector voltage is rated at 28V so you can use a 12V relay.
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      08-01-2016, 07:58 PM   #20
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First I'm gonna try it with rectifieing with some schottky diodes and an elco.
See what kind of voltage I get (I hope 12v, the scope I was using was pretty dodgy).

I'm tapping into 1 of the stator coils. That tap is also used in the bosch rectifier
Here seen as W~


I wonder what its for. probably for checking rpm or so (alternator rpm) The rectifier is controlled or checked by a BSD bit serial data interface which according to WDS pretty much monitors the complete alternator, so it checks/regulats overvoltage, but also monitors (and maybe intervenes) for belt breakage etc.
Pretty advanced for an engine launched in '99. Before this project I never knew the alternator was monitored like that. I always thought that it was a pretty basic device.
From what I understood its a variant on LIN
http://cache.nxp.com/files/analog/do...00.pdf?pspll=1
(in that document the terminal I use is called PH)

If I look at that lin document, the ph voltage is lower than 12v though, and can get boosted/regulated somehow (in that document they talk about 3-5v... that seems a bit odd for a coil coming from an alternator).

If this all fails I also thought about looking at the throttlebody voltage (usually a tb only goes on/moves when the engine goes on) but I dont know what happens with dsc etc, or looking at the oil pressure switch. But in the oil feed there's also a valve, so it could hold its pressure when turning the engine off. Depends on where the sensor sits I guess. Its easy measureable.
I looked up all the ecu connections in WDS but there is definately no dedicated 'engine on' pin. I found out that earlier ecu's (m52 engine) did have that pin though.
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      11-11-2022, 08:33 AM   #21
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I realise this is a super old post, but I had this requirement of needing a signal when engine running, so I made a circuit with an ATTiny85 for voltage detection/check









OLED Display screen and NeoPixel are optional - mainly used for testing. OLED shows voltage, charging status and running status. NeoPixel is RED for “normal” state, GREEN for charging state, and BLUE for running state

I've revised my design a little by adding an output signal selector - so you can choose between +ve output signal or -ve (GND) output signal. I added this to make it more universal since some external hardware require negative triggers and some require positive triggers - I'm awaiting delivery of this revised design pcb.

Let me know if interested and I can send you the gerbers to have JLCPCB or whoever make the pcb and then my code to make your own.
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