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post #1 of (permalink) Old 03-27-2001, 02:15 PM
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More MPI swap questions

Can anyone tell me what the two releys are for on the hesco wiring harness for thier mpi swap. I also need to know where to splice these into a
mpi harness out of a 95 cherokee. Also need to know how to wire the fuel pump into the 95 cherokee harness. Do I just use the original wires or do I somehow have to rig it. Thanks and all help and ideals are welcome.....this is the last bit of info i need before going mpi on my 89 4.2

89 YJ 4.2, AX15, Procomp 4" Lift, MSD6a, and just gerttin started......
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post #2 of (permalink) Old 03-27-2001, 03:07 PM
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Re: More MPI swap questions

The two relays power the harness (the computer, ignition, etc.).

One is switched with the ignition and the other gets constant power to maintain the memory in the computer. If you have the complete harness from the '95, the harness will get power from somewhere. I would check near the battery, maybe a block with some relays or something. All of the other other connections will go to specific components for ignition, fuel pump, crank sensor, oxy sensor, injectors, etc.

On the mpi kit, the wiring it too simple. Each plug or lead is clearly marked to where it goes. This was probably the easiest part of the kit. As to the '95 fuel pump, there should be two wires running to it from the harness. If those were cut (since the pump is in the tank) it may be harder to identify.

Let us know what you find out. A friend of mine is considering the same thing and I have been thinking it will be pretty easy.

258 4.0 Head/MPI,NV4500,D300,4.10's,HS9500
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post #3 of (permalink) Old 03-27-2001, 07:19 PM
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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.

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post #4 of (permalink) Old 03-27-2001, 10:19 PM
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Re: More MPI swap questions

thanks for the info. Hey dorfs is there any way you can email me a copy of your wiring harness setup. exspecially interested in the wiring of the releys mentioned above. Thanks

89 YJ 4.2, AX15, Procomp 4" Lift, MSD6a, and just gerttin started......
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