Many people with 1996 to 1998 Mustang Cobras find their Intake Manifold Runner Control systems to be a huge pain. They get gunked up, the cables fail, or the motor fails. They also develop vacuum leaks. As a result, many people delete them. However, simply deleteing them from the car will cause a DTC and a CEL, and it will cause the PCM to retard spark at high RPMs, robbing the car of significant power. The best way to deal with these problems is to get a custom tune for the car, but many people don't want to go through that hassle and expense. The Accutach Co. IMRC Delete Relay Circuit is for those people.
The IMRC Delete Relay Circuit simulates the stock IMRC controller. When the PCM commands the IMRCs to open, the relay closes, and send a signal back to the PCM's IMRC monitor input to tell the PCM that the IMRCs are open. Since the PCM thinks the IMRCs are still there, no DTC or CEL is set and the spark advance stays as it should, resulting in no loss of power.
Accutach Company has designed this circuit to provide years of reliable service. We use a high quality automotive relay, oil & fuel resistant Raychem DR-25 shrink tubing and automotive rated wire. Installation consists of splicing 4 wires at the IMRC controller connector on the engine harness. Download the IMRC Delete Relay Unit Installation Guide to learn how it is installed. See below for background information regarding IMRCs.
IMRC Controller Background Information
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 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 2 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. If you would prefer to make one of these units yourself rather than order one from us, please read the how-to IMRC Delete Relay Unit.
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.)
You can make this IMRC Delete Relay Unit yourself using a standard automotive relay, or you can buy it from Accutach company. If you choose to buy it from Accutach, here is a diagram that shows you the wire colors we use and how to splice them into the IMRC controller wires in the engine harness (or into the PCM harness* in the passenger kick panel):
* If you wire this into the PCM harness in the passenger kick panel, you can take power from the Red Hot-in-Start-or-Run wires on PCM Pins 71 or 97.
Here's a photo of the finished product: