1988 4.0 No Spark - JeepForum.com

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post #1 of 7 Old 01-23-2012, 05:39 PM Thread Starter
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1988 J-Series Truck 
Join Date: Jan 2012
Location: Toano
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1988 4.0 No Spark

I bought this jeep and somebody put a motor in it. I just had to finish it up and now i can't get spark. I replaced computer, coil, ignition modular, pick and distributor, and crank tigger but i may not have the right bolts in the crank trigger because it wasn't even on there. The motor was in, but i just had to hook up the wiring which was like jigsaw puzzle. I have one plug hanging underneath that is from the same wiring harness as the oxygen sensor, i can't figure out where it goes. I think it may be a crank trigger problem because i don't know what the bolts are suppose to look like. Is there a way to set the gap or anything? Thank you in advance!!!

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post #2 of 7 Old 01-23-2012, 06:17 PM
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1990 MJ Comanche 
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Location: Guston
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Sounds like a CPS issue.
I would go over to comancheclub.com and quiz them over there.
All the information you could possibly want on these trucks over there.

Good Luck!

90 MJ 4.0 4X4 5sp LB Pioneer Big Ton.
Truck contains parts from 3 MJs, 2 XJs, 1 TJ and a dishwasher.
Check out comancheclub.com...
My build thread: http://comancheclub.com/forums/viewt...hp?f=7&t=27756
Wifes: 97 TJ 2.5 4X4 5sp 31's stock. . . Slow but; reliable.
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post #3 of 7 Old 01-23-2012, 07:08 PM Thread Starter
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1988 J-Series Truck 
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thank you! i did sign up over there, waiting for a response from the admin., hopefully that will be soon. =)
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post #4 of 7 Old 01-23-2012, 07:09 PM
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1989 MJ Comanche 
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may help to.... i know comancheclub can help too...

photos help too..
some common plugs/wiring..
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post #5 of 7 Old 01-23-2012, 08:08 PM Thread Starter
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1988 J-Series Truck 
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Location: Toano
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Photos are a huge help! Thanks for that link. I've got quite a few possibilities that i'm going to try out tomorrow. All suggestions are so appreciated and i'll be sure to update any progress as soon as i make any =)
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post #6 of 7 Old 01-23-2012, 09:10 PM
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1986 MJ Comanche 
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Originally Posted by Tricia View Post
thank you! i did sign up over there, waiting for a response from the admin., hopefully that will be soon. =)
Did you send one of us a PM? I had no idea you were waiting until I found your comment... Anyway, you are live now!


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post #7 of 7 Old 01-24-2012, 04:50 AM
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Test the CPS output first. Having the incorrect bolts holding it on can cause a weak signal as it may not be close enough to the flexplate/flywheel.

Renix CPS Testing and Adjusting

Renix CPSs have to put out a strong enough signal to the ECU so that it will provide spark.

Most tests for the CPS suggest checking it for an ohms value. This is unreliable and can cause some wasted time and aggravation in your diagnosis of a no-start issue as the CPS will test good when in fact it is bad.

The problem with the ohms test is you can have the correct amount of resistance through the CPS but it isn’t generating enough voltage to trigger the ECU to provide spark.

Unplug the harness connector from the CPS. Using your voltmeter set on AC volts and probing both wires in the connector going to the CPS, crank the engine over. It won’t start with the CPS disconnected.

You should get a reading of .5 AC volts.

If you are down in the .35 AC volts range or lower on your meter reading, you can have intermittent crank/no-start conditions from your Renix Jeep. Some NEW CPSs (from the big box parts stores) have registered only .2 AC volts while reading the proper resistance!! That’s a definite no-start condition. Best to buy your CPS from Napa or the dealer.

Sometimes on a manual transmission equipped Renix Jeep there is an accumulation of debris on the tip of the CPS. It’s worn off clutch material and since the CPS is a magnet, the metal sticks to the tip of the CPS causing a reduced voltage signal. You MAY get by with cleaning the tip of the CPS off.

A little trick for increasing the output of your CPS is to drill out it’s mounting holes with the first drill bit that just won’t fit through the original holes. Then, when mounting it, hold the CPS down as close to the flywheel as you can while tightening the bolts.

Revised 11-29-2011
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