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DX to ZC wiring conversion

Matt Blodgett

The following are the modifications needed to convert an 91 Civic/CRX 1.5l DX wiring harness (manual transmission only) to an 91 ZC motor.

The first area of modification is at the ECU. A DX model will not have any connection at pins B10 and B12 on plug C452 (The middle ECU plug). Therefore, to connect wires at this point you will need extra connector pins to add to the DX plug. These connectors can be carefully grafted from unused pins. Some models will already have wires at B12, in this case you will cut the existing wire off and use that terminal directly. B10 will be empty, a wire for this space can be grafted from unused pins such as B2 or B11. Once you have a place to connect wires at B10 and B12 you will cut the orange and white wires off of pins C1 and C2 on connector C453 and connect them to B10 and B12. NOTE: These wires can be connected in any order because the measurement is resistance, but the book says orange to B10 and white to B12. Be sure to leave enough wire on C1 and C2 for the next step! The next connection is to run wires from C1 and C2 to the engine compartment for later use, label them C1 and C2. The last modifica 89 Acura Integra (RS, LS or GS) ECU or an 91 Honda Civic/CRX Si ECU.

Part two of the wiring modification is in the engine compartment. With the DX motor out of the car and the ZC motor next to it, carefully transfer the DX wiring harness over to the ZC. You should be able to make all the connections. If a wire is too short (TPS and EACV usually), remove some of the flex loom and it ll fit. You will end up with three extra plugs, C117 and C118 which are DX injector plugs and C115 which is the DX s Tandem Valve Control Solenoid Valve. Install the motor into the car and continue.

To make the rest of the connections, the ZC motor must have plugs and excess wire on the injectors as well as the Cylinder Sensor which is on the end of the exhaust cam. To wire the ZC s fuel injectors you will need the yellow and red wires from C117 and C118 respectively and the two wires you ran from the ECU labeled A3 and A7. Assuming the ZC s injector wires are intact with excess wire make the following connections: [Connect the yellow wire from C117 to the ZC s #1 injector which should be a brown wire. Connect the red wire from C118 to the #3 injector which should be a light blue wire. Connect the wire you labeled A3 to the #2 injector which should be a red wire. Then, connect the wire you labeled A7 to the #4 injector which should be a yellow wire.] With all of these connections in place you will need to connect the power leads to each injector. These leads MUST be wires through an injector resistor, which is basically a 6 ohm resistor for each injector to reduce the switching load on the ECU. You may use any Honda injector resistor, as they are all 6 ohms. As old as a first generation CRX Si or as new as a 97 Accord. For my application I used one out of an 87 Acura Integra. The resistor is a small aluminum box, usually with a heatsink, that has five wires coming from it. Four of these wires will be the same color, connect one of these to each . injector, and the fifth wire needs to be connected to the two yellow/black wires from C117 and C118. Now that you have completed the injector wiring you need to hook up the Cylinder Sensor. This is simple, just connect the wires you marked C1 and C2 to the blue/green and blue/yellow wires on the Sensor. The last modification is VERY important! You must reverse the green/white and yellow/white wires on the Throttle Position Sensor, because the DX s turns backwards.

There you have it. It may seem like a lot of work, but when faced with a complete car harness swap, these modifications are a piece of cake.

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