Originally Posted by
Jon K
That's exactly what I needed to rule out Javier - I am reading MUCH literature and finding that the aftermarket ECUs insist to use the fuel pump relay as a +12v source for the "coil". However, I have multiple coils and a completely different circuit than distributed cars, and so I was given a warning by another MS user to isolate the coil +12v from the rest of the car by using fuel pump so as not to burn it up if ign key is left "ON". I thought to myself well, this doesn't happen with OE system so... and I wanted to make sure. I have about 3 diagrams saying the 12v comes from various sources... I'd rather ask the EXPERTS (thanks javier, no thanks Crustam) what their thoughts were on it.
Re aftermarket ecu's:
Jon I've done quite a few retrofits of some common programable 3D (fuel/ignition) ecu's such as Microtech, Wolf and Greddy. Most of my setups have consisted of retrofitting direct fire coils to cars that had used traditional distributors.
My take is the following:
All of them use thier main relay (usually fuel pump) for the +12v feed to the coils (they all switch the coils on or off by the transistors switching 'on' and grounding the -12v side of the coil). This is a safety feature that allows you to pull one relay to isolate all the engine electrics if needed.
All coils are earth (-) switched be it by points (kettering) or electronic (transistor) ignition system, this reduces the load (amps) handled by the switch.
In the days of points (kettering) systems if you left the ignition on with the engine off you could burnout the capacitor (used to stop the points from arcing, increasing their service interval) or burn out the primary winding in the coil. most cars had a resistor in line with the coil that dropped the voltage going into the coil to around 8 volts, this reduced the risk of coil frying but didn't stop it all together.
But fear not those days are well gone now!
In a computerised system things are very different, the coils aren't triggered (grounded) by the ecu unless the ecu receives a valid rpm input from the crank (or cam) angle sensor.
So basically even though there is power to the coils the circut is incomplete (not grounded by the switching transistors in the ecu). Once the engine is cranked over the C.A.S. signal tells the ecu to switch the ignition transistors 'on' (to ground) in the correct order at the correct time (using the ecu's ignition / rpm map).
If the engine is not rotating the ecu DOES NOT receive any crank trigger reference or rpm signal, therefore the ignition transistors will be 'off' (open circuit) and the coils will not be 'charged' by the ignition system.
Logic tell us that if a switch is 'off' no current flows.
HTH
Ross
BMW parts professional
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E28 M535i (MMM M88!)
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