6/16/09:
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, I have been able to reconstruct the circuit.
Chuxsvt on Corral has verified that this circuit works in his car. Ohters have verified it as well.
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.
9/7/09: I added two 1N4148 diodes to protect the PCM IMRC control output from transient voltage spikes cause by the relay coil.
9/12/09: 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.
8/19/15: Originally, I had a coule of typos on this page (had a diode backwards) that made the circuit not work. I fixed that, but after hackers completely screwed up my site a few months ago, I lost the corrected page and restored the bad page. I have now fixed it. If you have copied the old page to your computer, please replace it with this page.
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.)
I decided to build one unit for testing, so here is the build documentation.
Acquire the parts. First get a standard automotive 12V under hood rated SPST or SPDT relay. I used a Bosch SPST relay. The connections will be the same with either type of relay. You can get a relay socket with pigtail, or, as I did, just use 4 standard female spade lugs. Get a 75 Ohm, 1/4W or higher wattage resistor, and two 1N4148 diodes. Cut 4 different color automotive wires long enough to run from the IMRC controller connector to where you want to mount your IMRC Delete relay. Not much current flows through the circuit, so you can use wires as small as 18 gauge. I chose Red, Blue, White and Black 14 gauge wire because that is what I have. Cut a 5th piece of Red wire that is about 6" long. Get various sizes of shrink tubing.
Solder one end of the 6" Red wire to the diode end with the band on it
Shrink wrap the solder joint and the diode body to prevent a short circuit when the circuit is put together.
Solder the other diode to one end of the Blue wire, solder the diode end with the band to the wire.
And shrink wrap it so the solder joint and diode body are covered to prevent a short in the harness in the same way you did the short Red wire above.
Now pull the insulation off of the female spade connector, and crimp and solder the two exposed diode leads together to the connector.
Slide shrink tubing over the diodes, the exposed diode leads and the solder joint on the connector. Heat it to shrink it together.
Cover the female spade connector with shrink tubing.
Slide a piece of shrink tubing over the short Red wire and the long Red wire before doing the next step. Then crimp and solder another female spade connector to the other end of the short Red wire and one end of the long Red wire. Slide the shrink tubing down over the solder joint and shrink it. Then put shrink tubing over the female spade connector, and shrink it down.
Crimp and solder the resistor to another female spade connector and solder the other end of the resistor to one end of the black wire.
Shrink wrap the solder joints and resistor and then shrink wrap the connector.
Solder one end of the white wire to another female spade connector.
And shrink wrap the solder joint. Then shrink wrap the connector.
The wiring is now done, we just need to connect them to the correct terminals on the relay. Install the connector with the two red wires on the lug numbered 86 on the relay. Install the connector with the red and blue wires on the lug numbered 85 on the relay:
Now install the connector on the black wire on the lug numbered 30 on the relay.
Install the connector on the white wire onto the relay lug numbered 87.
Tie wrap the wires together so that the mechanical strain is placed on the white wire wich has no components soldered to it.
Encase it in a black plastic sleeve and wrap with black electrical tape.
Solder and shrink wrap the wires to the IMRC control wires as shown in this diagram. Solder the white wire to the engine harness W/O wire. Solder the red wire to the engine harness DB/Y wire. Solder the blue wire to the engine harness BR wire. Solder the black wire to the engine harness GY/R wire.
I have tested the circuit on the bench and it works exactly the same as an IMRC controller works.