looking to remove stock coil on BT20v COP conversion

[chi-town]

I'm not wild about having to leave a functioning stock coil in the system to run COP on my blacktop. Has any one experimented with removing and replacing it with something else such as a capacitor or resistor?

[morgan]

leave it, sucks but it is what it is. things wont work right without it. iirc, the tach for one

[chi-town]

Yeah I know the tach won't work and the check engine light can come on. I'm looking to eliminate the electrical draw from the coil and the issues of having to ground out the 30,000v + coil to the chassis can't be good for any electronics in the car. I figure the code is set due to the resistance of the primary coil not being sensed so I figure replace it with a resistor. As for the tach does any one know what the actual signal sent is?

[chi-town]

So the coil resistance is .6 ohms. The tach works off of the negative side of the coil with the drop in the field when the coil fires. The question is will the drop out in the field from one cop be enough to trigger the tach?

[gaijin_rokurunner]

Aftermarket ecu allows for sequential COP operation thats the only way u can get rid of the coil...its a freak of nature that it even works when the stock ecu was never made to run them

[chi-town]

Yeah I know was just hoping to by a little time to save up for a stand alone while being able to drive it.

[foonix]

I am running a megasquirt running COP and still had to build a special circuit to git rid of the coil. It is pretty much the same as what you are looking for.

The problem is, a 12v square wave signal is not enough to drive the stock tac. It runs off the voltage spike (called "flyback") that results when the ground to the coil is released. The spike has to be recreated somehow. The only way to create the voltage spike is to use some kind of inductor. The stock 16v coil works, but a smaller inductor (such as the coil side of a relay) can be used. First you'll need a circuit that generates a ground. Then, create a circuit that generates the flyback.

Here is what the flyback generation circuit looks like:

+12v -> relay coil -> 700ohm resistor -> (tachometer signal wire)

When the tach signal wire is pulled low, it charges the coil. When it is released, the inductor will push voltage toward the resistor above +12v. The resistor just keeps the coil from getting hot (by capping the current).

Next the signal circuit. This really depends on the type of coils you are using. I believe the popular conversion uses the +5v IGT signal to tell the coils to charge, and releases it to fire. We actually need the opposite for the tach. We need the wire pulled low most of the time, and allowed to "float" so that the voltage spikes. On the megasquirt I was able to do this pretty easily because it has a separate output for the tach. But, you can probably piggy back a circuit on IGT to drive the tach. Using an NPN transistor to invert the signal would probably work.

The circuit would look like this: http://i.imgur.com/Vf7JDSc.png