Accutach Company

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.

Chuxsvt on Corral has verified that this circuit works in his car.

IMRC Controller Background  Information:

I analyzed the stock Cobra IMRC circuit in the Ford Emissions Manual, and I learned that there are On-in-Run Vbatt power and Ground pins going into the IMRC Controller to supply power and ground.  There is also a control signal that goes into the IMRC controller: When that signal is high (or an open circuit), 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 both 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.

Per Perry Herndon on Corral, the IMRC controller will only pull the cables to open the IMRCs.  It depends on the IMRC springs to pull them back closed, it does not push them back closed.

Bench Testing an IMRC Controller:

To bench test a stock IMRC Controller, refer to the IMRC connector pinout below.  Pull the IMRC cables out to ensure they start off in the IMRCs closed state.  With a 12V power supply that can supply more than 3.2A of current, connect +12V to pin 3 and ground to pin 3.  Don't do anything with Pin 1.  With an ohmmeter across pins 5 and 6, you should see an open circuit.  Then ground Pin 1 and your ohmmeter should read about 75 ohms, and the cables should pull to open the IMRCs.  If neither cable pulls or if only one cable pulls, or if your ohmmeter doesn't read 75 ohms, your IMRC controller has a problem.  After you unground Pin 1, you will need to pull the cables back out to the closed state to get the resistance back to an open circuit.

The IMRC Delete Relay Circuit:

The IMRC controller eliminator 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.)

Let me know if it works for you at molson at accutach dot com.

Thanks!