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Old 04-05-2008, 11:08 AM   #1
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Engine swap wiring help needed 4.2 to XJ 4.0

Hey all, this is the second time I have done this swap (the first one was replacing a 2.5L with the XJ driveline). This time I did a couple things differently and I am not getting anywhere.

Engine, transmission, xfercase, wiring harness and ECU are out of a 87 XJ. I built a 4.5 stroker over 2 years ago and put it in my Cherokee. Cherokee ran fantastic before giving its guts to my YJ.
YJ had 4.2 that was hydrolocked when I purchased it. I pulled the driveline and installed all the XJ stuff from the front harness point of the firewall up. Now, 8 years ago when I did this swap I took the fuse box and interior wiring junction from the XJ and modified it to power the YJ. I figured this time it would be easier to just transfer over the appropriate wires on the engine side of the connector to match the YJ harness and keep the interior wiring intact. With my summit EFI inline fuel pump I get fuel at the rails. I have power at the ignition and from what I have tested everywhere under the hood that is supposed to. The engine just rotates but does not fire up. There is also no tach while cranking. I have tested for pulse from the injectors and nothing. I figured this was probably the typical Jeep CPS failure so I tested the ohms and it does not show up as open as I was told it should, it shows up at .188ohms and so does my new spare sensor so I figure this is close enough.
What am I missing guys? Nothing on the ECU harness has been messed with, with the exception of matching up the wiring at the firewall. It seems like I am lacking power somewhere or a trigger somewhere at the ECU to tell it to fire up.

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1988 YJ Islander
4.5 stroker, AW4 transmission, spring over axle with stock springs, 33x13.5 Super Swampers, new interior, looking for a supercharger system from a Supercoupe.
1988 YJ 4.2L 5 speed 33x12.5 kumho mud terrains. Sailcloth soft top, frest paint, disc brake rear conversion.

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Old 04-05-2008, 11:17 AM   #2
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tough one, no pulse usually means something w/ crank or cam sensors, i think rpm's come from cam sensor, but it needs crank position to know where #1 is to fire. keep us posted on this, i wanna know cuz i'm doin similar swap.

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Old 04-05-2008, 11:19 AM   #3
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ill bet its the crank wire on my 95 harness it was yellow goes to starter blue to one side of coil (forget witch) i had this problem one of these wires im guessing yellow needs to have power only when cranking good luck hope this helps
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Old 04-05-2008, 11:51 AM   #4
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This is the older fuel injection system from 87-90. Both wires for the CPS end up at the ECU.
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1988 YJ Islander
4.5 stroker, AW4 transmission, spring over axle with stock springs, 33x13.5 Super Swampers, new interior, looking for a supercharger system from a Supercoupe.
1988 YJ 4.2L 5 speed 33x12.5 kumho mud terrains. Sailcloth soft top, frest paint, disc brake rear conversion.

Proudly Maintained by Sahara4x4.
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Old 04-05-2008, 11:53 AM   #5
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theres a power supply wire same on all fi models just not sure what color yours is one wire needs power in on and start the other needs power in start only
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Old 04-05-2008, 12:12 PM   #6
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Originally Posted by spazinweasel View Post
theres a power supply wire same on all fi models just not sure what color yours is one wire needs power in on and start the other needs power in start only
At the ECU I have a purple/white that is pinA on the CPS plug and the other wire is white/black at the ECU and is pinB on the CPS plug. I assume the ECU directs power to it one of these wires. The Haynes manual does not show the cps on their electrical diagram for these years so I am not sure if the wires make a junction and go elsewhere as well as to the ECU. I have tested wires looking for a power wire like you talk about but have found no other places they run to.
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1988 YJ Islander
4.5 stroker, AW4 transmission, spring over axle with stock springs, 33x13.5 Super Swampers, new interior, looking for a supercharger system from a Supercoupe.
1988 YJ 4.2L 5 speed 33x12.5 kumho mud terrains. Sailcloth soft top, frest paint, disc brake rear conversion.

Proudly Maintained by Sahara4x4.
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Old 04-05-2008, 12:15 PM   #7
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yeah they go into the ecm like all other sensors but there is a couple of power wires that send power to the ecm then the ecm directs the power where its needed
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Old 04-05-2008, 12:24 PM   #8
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yeah they go into the ecm like all other sensors but there is a couple of power wires that send power to the ecm then the ecm directs the power where its needed
Since the manual I am using does not show much, do you know off the top of your head what wires should be hot at all times and what should be hot with ignition on/cranking on the ECU side? I think this is my issue, I need another power wire from somewhere but am unsure from where. I'll go back through my notes real quick and post what has power to ECU and if it is hot all times or with ignition on. It seems like the hot while cranking is not there but I want to say it is...I'll go check. It would be nice to finally fire this thing up today
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1988 YJ Islander
4.5 stroker, AW4 transmission, spring over axle with stock springs, 33x13.5 Super Swampers, new interior, looking for a supercharger system from a Supercoupe.
1988 YJ 4.2L 5 speed 33x12.5 kumho mud terrains. Sailcloth soft top, frest paint, disc brake rear conversion.

Proudly Maintained by Sahara4x4.
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Old 04-05-2008, 08:06 PM   #9
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in my 95 harness, yellow goes to starter (for hot it start, small terminal) blue goes to coil (for hot in start and run)and the big red one of course goes to batt

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