There was a description of a relay based circuit that would fool the 96-98 Mustang Cobra PCM into thinking IMRCs are still in place when they have actually been deleted. The web site where it used to be, http://home.hawaii.rr.com/hfpc/imrc/, has now been gone for a long time. With the help of 20psi on ModFords, and Erich on Corral, I have been able to reconstruct the circuit.
I need someone to test this circuit for me since my IMRCs are long gone, physically and in the tune.
I analyzed the stock Cobra IMRC circuit in the Ford Emissions Manual, and I learned that there are On-in-Run Vbatt power and Power Ground pins going into the IMRC Controller. There is a control signal that goes into the IMRC controller: When that signal is high, the IMRCs are closed, when that signal is low or grounded, the IMRCs open. There is a IMRC Monitor output and a Signal Return that goes to the PCM to tell the PCM if the IMRCs are open or closed when they should be. If the IMRCs are open, this signal will be less than 1.6V and if they are closed this signal will be greater than 1.6V.
There are a pair of switches in the IMRC controller that close when each IMRC opens. When the IMRCs are closed and both switches are open, the IMRC Monitor input is pulled to a high voltage inside the PCM, so when the PCM reads it it gets a higher than 1.6V voltage reading. When one IMRC switch closes (one IMRC bank opens) then the IMRC monitor input is pulled lower through a resistor to signal return (another ground), but it is not pulled down to 1.6V. When the other IMRC opens and the other switch is closed, a second resistor is added in parallel to the first resistor. From Ohm's Law, we know that that cuts the pull down resistance to half of the individual resistor value, which pulls the IMRC Monitor input to below 1.6V.
Thanks to Erich on the Corral forums, I got an IMRC controller circuit board to play with. I learned that one of the two resistors is 127 ohms and the other is 182 ohms. When you run them in parallel, you get 75 ohms. I don't know from the circuit board alone which resistor is associated with which IMRC cable. This can be simplified to a 75 ohm resistor and a SPDT or SPST relay. I would recommend using a lower current draw 12 volt relay in order to ensure you do not run too much current through the PCM control output, possibly damaging the PCM, although the PCM output is probably protected in case of a short in a wire. Two diodes are in the circuit to protect the PCM IMRC control output from voltage transients caused by the relay coil.
Here is a diagram that shows the stock IMRC circuit, the IMRC connector and the IMRC delete relay circuit on one page: (The connector diagram is shown from the pin side of the connector that plugs into the IMRC controller module.)
Again, I need a volunteer to build and test this to validate that it works.
If you do, don't forget to let me know at molson at accutach dot com.
Thanks!