mmanzano

ey guys im trying to wire a the pcm of a 93 mark viii to the ignition switch of the 67 mustang, i need to know were to get the signal for my relays from the ignition switch, i have them hook to the run, but there is no power to it on crank, so no power to computer = no start, plz help me this is my first time doing a pcm engine swap

67mustang302

Multimeters are your friend. Check wires for continuity and wire to one that gives continuity on crank and run. Alternatively you can find a 12v switched source other than at the ignition switch itself that does the same. A basic digital multimeter that has an audible continuity check is fine, it just tones out if the circuit completes...so you can check the circuits even without power (if the battery is dc'd).

Is this for the coil side of the relays? With EFI it's extremely important to have clean electrical sources that are free of interference. The coil side of relays can come from a standard 12v switch (usually), but with the switch side (power) it's a good idea to draw directly from the positive battery terminal itself. For things like fans and fuel pumps etc, pull a bit farther upstream, like a distribution block that's tapped of the starter relay. If the ECU is wired directly to the battery (ALL wires, hots and grounds for the ECU) and everything else comes of a distribution block, it tends to help with noise filtration and will keep you from having to chase gremlins down.

On my XFI I went a step further and the 12v switch power for the ECU actually comes from a relay that gets power from the battery, and the relay is switched on from a 12v switch source...so the 12v switch source comes on with the ignition (crank and run), that powers a relay that provides power to the 12v switch to the ECU. The reason for that is that EFI has a voltage offset calculation for the injectors that varies the pulse width of the injector as the electrical system voltage changes, to account for variances in the field strength of the driver coil in the injector. In FAST's XFI, the voltage is read from the 12v switch source wire to the ECU, so I wanted to guarantee it was 100% clean (plus it's a good idea to take every measure to isolate EFI as much as possible from vehicle electrical systems).