BMW S50B30 coil connections, need help!

[mefmotorsport]

I just came to wire the coils to my ecu loom. there's 3 pins, I know which one is signal and +12V but the middle pin (marked 4a) is a bit of a mystery to me.

BMW wire it to ground some distance down the loom once all the 6 have been grouped together but what is it for? There is no resistance to be measured between this pin and either of the other 2 or between it and the eht plug end.

I know the eht requires a return path, (maybe there's a capacitor in series hence no resistance?) Wouldn't it make more sense to earth the middle pin locally near the coil if its the return path rather than back down the loom?

Any thoughts anyone?

By the way, Happy Christmas everyone!

Martin

[mefmotorsport]

I just checked my BMW info CD and it shows 4a as the eht ground on the secondary of the coil. It also states that the resistance of the the secondary winding cannot be measured with a multimeter. (it also shows a diode in series with the eht to the coil)

I still think I should ground it local to the coil though.

Martin

[mefmotorsport]

I decided to ground pin 4a locally to the coil, in fact to a tag under one of the mounting bolts on each coil. There is a thick ground lead that goes from one coil to the back of the cylinder head, presumably because otherwise the cam cover is isolated from the head by the gasket and mounting fixing rubber washers.

Just a couple more plugs to fit and I should be starting the engine today!

Martin

[mefmotorsport]

Well, it finally runs, it started up first time! I cant run it for long as there's no water in it yet but it's sounding good, maybe very rich but at least it's running at last!

Martin

[Antral]

I knew it would

[stevieturbo]

Good stuff. Often the additional "ground" wire is for missfire detection. How it works Ive no idea.

[Jon K]

Martin I had many, many hours looking into the logic of that pin and grounding and on certain models theres actually a 100 ohm 2W resistor between the common 6 coil grounds and the engine block.

I just ended up grounding them all to the engine block and all is well.