Bypassing the MCU on a 4.2L (258) AMC Eagle

by Terry Warren

    There were two different types of SSI (Solid State Ignition) Systems on the Eagle. The 1980-81 models did not have a computer, nor did they have feedback controls on the carburetor. These features were not introduced until 1982. It has been my personal experience that the stepper motors on the carburetors have a tendency to fail—If the stepper motor dies, the car will not run right. Replacements are available, but I did not have the money to purchase one at the time.

    My pleas for help on the AMC-List were answered by list member Glenn Allen. He mailed me all the diagrams and information (even a wiring harness) I needed to bypass the computer controls and use a non-feedback type carburetor. I used to own a 1980 sedan, and when it finally died, I stripped what I could from it. One of the things I kept was the non-feedback carburetor. I had to swap the throttle bodies to get the throttle linkage to line up correctly, but since both carbs are Carter BBDs, they do interchange.

NOTE: I do not live in a state that requires emissions testing—this procedure may not be legal in your state.

    Take a look at the following diagrams. Pay close attention to the 4-wire connector on the Electronic Ignition Control Unit (ECU) and how it connects to the 3-wire connector on the distributor. Also, notice the 4th wire that goes to the coil. In order to perform a bypass of the MCU, a wiring harness is needed to connect the ECU to the distributor.

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    As far as I know, the computer (MCU) controls the stepper motor, ignition advance, and the Pulse-Air System. It is important that the wiring on the computer-equipped car is not mutilated so that re-connection of the computer is possible. It will be necessary to build a jumper harness that looks similar to the picture below. I’ve seen it written on the AMC-List that a harness from a Ford Pinto will also work (I haven’t yet had the chance to check it out for myself); perhaps other Ford model have these harnesses? Notice that the dark green wire is not connected at both ends—a spade connector will need to be attached to the end of this wire.

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    Unplug the 4-wire connector at the ECU and plug in the 4-wire end of the jumper harness to the ECU. Unplug the 3-wire connector at the distributor, and plug in the other end of the jumper harness to the distributor.

    Take the dark green wire with the spade connector and plug it into the D4 slot of the 4-wire harness plug you disconnected from the ECU. Make sure it’s the dark green wire and not the light green. It should be a tight fit.

    One last note: The computer was responsible for the engine timing. Bypassing the computer will have a radical effect on the timing. Therefore it will be necessary to readjust the timing using conventional methods.

    That’s all there is to it.

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