Ignition coil grounds have resistor to ground? 1

[JonathanK]

Trying to figure out the purpose of this. My 1992 BMW does not have this resistor on the car or in the schematic. My 1995 engine does. I am hunting down ignition coil noise and I want to figure out whether this is a good thing to implement or not.

The resistor is 240 ohm, 2-watts. I doubt it'd fail but if it did, that would be less than ideal. What purpose does it serve and should I run install one? Currently I do not have a resistor there.

Thanks!

Jon

 

[patprimmer]

It looks like a coil on plug system.

The traditional reason for a resistor in the coil circuit is so it can be bypassed for starting and correct for voltage drop caused by the draw of the starter motor.

I have seen resistors fail on older cars however a failure is rare.

If your coils are rated to have extra resistance in line, I would be reluctant to hook them up without the resistance for fear of a smoke leak developing. I expect it really depends on the coils used.

Regards
Pat
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[bradrs]

Are there other wires going to the coils that aren't shown in that picture? Maybe a 12V, and then another wire to the ECU? My first thought would be that that common ground is a feedback to the ECU, and not the main coil current.

 

[JonathanK]

Yes sorry - this is just the grounding diagram.

The coils have 3 terminals. +12v switched by key, output from ECU to coil (sinking output, pulls to ground) and the ground wire pictured above.

The only thing I can think of is that its just to move the ground up. Not sure what it's trying to accomplish. Current limiting? 240 ohms is not much, though. And further, the part number for the coils is the same regardless of whether the car has that resistor there. The ground back to the ECU is confusing me also.

 

[bradrs]

My guess is that it is NOT to move the ground up. You don't have enough current through a 240 Ohm resistor to charge a coil. 50mA isn't going to work for any automotive or bike coil I've ever used.

The coil is charged by the other 2 wires. 12V power, and the other one "grounds" it to create the charging current, just like most other standard ignitions from that era.

My guess, that "ground" wire isn't a ground wire. It is just a small current feedback signal, and the 240 Ohms is there to create a voltage using that current from the coil. The voltage is then measured at the pin on the ECU. The current on this wire might be generated from the coil secondary, when the coil fires.

Whether or not you need it all depends on what ECU you are using. If it is what I guessed, then its just a feedback to ensure proper operation. With an aftermarket ECU(preferably one you can write the firmware for), you could get clever, and possibly measure that to determine misfires, etc.

 

[dicer]

If all the items that are above the intersection point also represent a resistance, then it is a voltage divider circuit.
And there will be a voltage at that main intersection point. Inother words it will be above ground.

 

[LionelHutz]

It almost looks like the secondary ground for the coil where the ECM is reading the voltage to determine if the spark actually jumped the plug. The resistor still seems too high for that too though.

You were quite vague about how the '92 is wired but I believe you are meaning the '92 does not have that feedback wire to the ECM and that the coils are solidly grounded. If so, you could try the resistor and see what happens.

 

[MacGyverS2000]

I can't see the diagram clearly enough... but if the ECU was measuring the voltage across that resistor, I would expect to see a measurement point heading back to the ECU. If such a point is there, it's not labeled clearly (though I can't read a single value off of that heavily compressed pic).

Dan - Owner

http://www.Hi-TecDesigns.com

 

[bradrs]

Dan, there does appear to be a measurement point heading back to the ECU. The ECU is the slightly larger block in the center of that diagram. There is a single wire heading back to that block, from the resistor. That wire appears to be shielded, so that there are 2 pins from the ECU that go to the feedback(1 for signal, 1 for shield).

I can't imagine this being anything other than a feedback, since it is connected to all 6 coils in parallel. I would hope it isn't the coil control signal, because firing all 6 coils simultaneously generally seems like a bad idea

 

[Ron07663]

That systems monitors secondary ignition current through the 240 ohm resistor to determine if misfire has occured.