[chevy55]

Where can I get a good wiring diagram for a 90 YJ 4.2L? I have the chilton's manual but just isn't helping much. Here is a little background and maybe someone could help.
The problem is the jeep wont start. I am only getting half the battery voltage to the coil. So I am assuming that is the problem. I have started tracing wires to try and figure out where im loosing the voltage at. The first day I spent several hours removing all the wires that someone had previously installed under the dash and to the starter. Still not enough voltage to the coil. I decided to uncover the ECM and the ignition module. I was able to get to the ignition module last night and look at the voltage coming to it. If I remember right there are two plugs into the module. One with two wires(yellow and another color) and another plug with three wires. I unplugged the module and checked the voltage with the key on and I was getting a full twelve volts to the Yellow wire on the one plug and to a green/black wire on the other plug. This is where I started getting confused. I checked for continuity between the green/black wire at the module and to the coil and found that that is where the wire goes. Then I checked the voltage at the coil with the ignition module unplugged and the key in the on position and I have a full 12V.???? I started undoing the wiring harnesses and just before the wires from the ignition module go into the main loom someone took some duct tape and wire cutters and spliced three different green/black wires from the main loom to the one green/black wire to the ignition module.

Maybe someone could explain to me how the voltage gets to the coil. Im used to vehicles without an ecm. Im guessing it would go from the battery to the ignition switch, to the ecm, to the ignition module and then to the coil.

Any input would be helpful.
Thanks

[jbolty]

this should help.

[chevy55]

Awesome, thats what I needed. where did you get those?

[jbolty]

Originally Posted by chevy55

Awesome, thats what I needed. where did you get those?

got them off some other forum a while back.

autozone has some diagrams too, but is not complete.

btw, I think those splices with the duct tape are factory. Mine has them too.


I just gutted my wiring harness and installed a howell tbi kit. I will probably want to sell the ecm eventually but not sure what it's worth. If you end up needing one let me know.

[chevy55]

Thanks again, Ill probably take the ignition module down to autozone and have them test it, seems odd to me that when the module isnt plugged in I have 12V to the coil, when the module is plugged in I only have 5-6V. Im guessing it might be no good. I dont even know how I would test to see if the ecm is bad. I would love to get the Howell TBI kit. Maybe Ill look at that down the road cant afford that now.
Can anybody explain what the ignition module does?

[chevy55]

Ok, I had the ignition module tested and it tested as being good. Other than tracing wires to see if there are any problems is there a way to test the ecm?

[myoutboards]

may not be the same as yours, but ill try.

i have a 92 yj 4.0l and 5 speed. i had same problem as you with no spark at coil. my ign fuse kept blowing and it turns out my starter relay was shorted out internally and causing a direct short to ground.

mine had a black/white wire wich carried voltage to the coil and also the fuel injectors and the rest of the ign system. i followed that black white wire all the way till it went under the dash to the shorted relay.

10 dollar part. simple.

autozone has some diagrams that are decent, but not exact. i used these in conjunction with the chiltons manual and viola!!