Can't Get Enough
Join Date: Sep 1999
Location: Cotopaxi, Colorado
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Re: More MPI swap questions
One relay is the fuel pump relay. These are standard Bosch ISO relays. The positive wire that goes to the fuel pump comes from this relay, and the computer controlls the ground
The other relay is the A.S.D. relay, or automatic shoudown relay. The pcm controls the ground side of both relays. The A.S.D. relay powers up the injectors, coil, and one side of the alternator field.
Building your harness is really not that difficult. There are only 60 wires. Most of them can be eliminated, like some curise control functions, and brake input if you have a standard. I like to disign the harness to become completely modular. I use terminal blocks like for a speaker which I use to mate the two harness together. One for fuel pump out, another for keep alive memory, another for momentary start, another for key on power, another for the check engine light, another for an extra ground, and brake input for autos.
Going through the Jeep service manuals for the wiring diagrams can be a bit confusing. If you can photo-stat a page from the Mitchell's manual, the whole diagram is laid out on one page, and is very easy to follow. I clamp the 60 pin connector in a vise, and stretch the harness across the floor. This makes checking you circuits with an ohm meter real easy, except if your are color blind, then it isn't so fun.
Another note, don't bother wiring in a fuel pump ballest circuit, it dosn't work, and is much easier to wire around like Hesco's harness.