Looking for engine running signal

alinton · Post by **alinton** » Sat Jun 20, 2015 8:05 pm

Hey all, I'm installing a fancy alarm, and it wants a 12v signal that's there when the engine is running, i.e. not when ign is on solely.

Perhaps one of the wired at the ECU?

Any ideas appreciated.

A.

H22turb0 · Post by **H22turb0** » Sat Jun 20, 2015 9:05 pm

must be a way

first thing i thought was the battery will be 14v when engine running so some kind of sensor to detect over 12v

mercutio · Post by **mercutio** » Sun Jun 21, 2015 3:46 pm

take a feed from the alternator?

Post by **Merlin** » Mon Jun 22, 2015 9:46 am

Download the service manual as it will show want you want to know

http://www.ludegeneration.co.uk/wiki/in ... ce_Manuals

89 UKDM BA4 4WS (resto job!) · Post by **wurlycorner** » Mon Jun 22, 2015 9:50 am

Oil.pressure switch is the signal that most ancillary's (e.g. abs etc.) use to determine if the engine is running or not.

alinton · Post by **alinton** » Mon Jun 22, 2015 11:43 am

Oil pressure! That's a great idea.

I have the manual already, I'll have a look where that goes.

Thanks!

alinton · Post by **alinton** » Tue Jun 23, 2015 12:20 pm

I'm blowed if I can find the oil pressure switch!

Any help please? And/or where the wiring from it goes, i.e. if it ends up at the ECU?

(H22A3)

Ta

Post by **Merlin** » Tue Jun 23, 2015 12:46 pm

I don't think you can use the oil pressure switch as when the switch activates all it does it ground a circuit (the power for the bulb comes from the other side), there is no voltage in there.

Unless anyone can find anywhere else where that wire goes and has a different function?

Post by **Merlin** » Tue Jun 23, 2015 12:55 pm

What about 4WS? Surely that will only get power when the engine is on?

89 UKDM BA4 4WS (resto job!) · Post by **wurlycorner** » Tue Jun 23, 2015 1:02 pm

Merlin wrote:What about 4WS? Surely that will only get power when the engine is on?

That gets power from ignition I think. That's going on;
Ignition on, goes through self test (you can hear the relays clicking)
Without looking at the wiring diagrams... I think it then just sits there until it's initially activated by the ABS (speed signal, but possibly something else as well?). I think the ABS only gives those outputs when it realises the engine is running, which I think it determines (indirectly) by oil pressure?

Looking at that oil pressure switch diagram you posted up (good stuff!)...
It's a normally open switch, but I think 'normal' is with engine running?
If so, that means with engine running, the switch will be open, which means there will be floating 12v on that line (it's not earthed, so the light doesn't illuminate)
When the engine stops (or oil pressure drops) the switch closes and earths that part of the circuit meaning you'll have 12v on the input side of the bulb and 0v (or near as) on the output side of the bulb.
That also makes sense as being the 'fail safe' way of designing the circuit (you would want oil pressure to 'break' the feed).

If that's right, take a feed from the output side of the bulb (or at the oil pressure switch itself) and you have the feed you need.
Make sure the input side to whatever you're connecting it to has a high resistance though, otherwise you'll have your oil pressure light fully or dimly illuminated all the time...