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WHEEEEEEEEEE!
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Discussion Starter #1
I have some trouble getting spark on the old 4.0.

I dont think it is the CPS, I actually have 2 of them on the motor right now (long story, harmonic balancer based with 2 different brackets and 2 different CPS's...) and I cant believe 2 CPS's are bad, but who knows.

The computer was good last I checked, It started a 4.0 not too long ago (troubleshooting someone else's junk)

I have checked the continuity of most of the wires in the harness, I dont believe them to be bad.

Both hall effect sensors get good 8 volt signals, that is one of actionpaintballs frequent suggestions.


I am leaning towards the coil.

The two wires to the coil are "coil driver" grey and dg/or from the auto shutdown relay.

The dark green/orange wire is ground and the grey wire has 12 volts.

What can I do to test the coil besides the ohm tests? How exactly does the coil trigger?

I was suprised to see 12 volts from the coil driver wire. If the coil driver has 12 volts, and the dark green.orange wire is ground from the auto ahutdown solenoid, how does the coil know to fire??

If there is 12 volts to the grey wire is there something wrong in the harness?

Can I test it with a test light to make sure the coil is getting a signal to fire? Is there any way to test fire the coil?
 

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WHEEEEEEEEEE!
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Discussion Starter #3
No spark at the plugs.

No spark at the coil.

Coil is obviously not firing.


ASD relay wire (grn/orange) does not provide any voltage, but it does not make a difference whether it is jumpered or not (testing purposes)

Coil driver wire provides 12 volts at 0 amps, will not light a test light, even a LED one.

No differences in voltage or amperage when slowly rotating the engine.

Quick reading on coils say they need 2 12 volt sources, a constant and a switched. Constant creates a magnetic field and switched collapses the field creating the voltage required for a spark.

Is there a way to test the coil driver? is the switched slow enough to show up on a ohmmeter? Do they switch voltage or amperage?

Or am I thinking about this all wrong?
 

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sir

can you please post how u fixed this problem.
I am having the same trouble with my rebuild yj

thanks in advance
 

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WHEEEEEEEEEE!
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Discussion Starter #6
The CPS was in the wrong position. I got a new CPS mounting bracket from somewhere and it put the CPS higher up on the motor thinking the computer could figure it out, but the 4.0 computer aint programmed for that :)

The flywheel, or in my case the harmonic balancer, it notched asymmetrically to tell the computer where the crank is in its revolution. I did not even think that moving the CPS would cause an issue, but it did. It thought the motor was about 270 degrees out of where it should have been, and when it got the cam reference signal, it just shut down and did not fire.

A 4.0 needs ONLY 2 references to fire the spark plugs - cam position and crank position. The cam sensor is in the distributor, and the crank reference is on the bell housing.

If you are not getting spark in a 4.0, you have 4 big things to test, in order of probability of things going wrong:

1. CPS. It is bad, wires to it have frayed, the connection in the wiring harness has crap in it and no longer has continuity, the mount has moved away from the flywheel, etc. If t is a fully stock setup, it has probably just failed.

2. Cam position sensor. Pretty well protected, rarely goes bad. Located in the distributor under the cap. Stranger things have happened, and it is a cheap sensor.

3. Coil. Usually fine, i would "borrow" one from a friend with a 4.0 to test against a working coil, I think there are values on the internet to test against.

4. Computer. It is possible to fry it, but it is very simple and potted in epoxy, there is not much to go wrong.


I hit a wall trouble shooting mine, convinced my buddy to let me wire his motor with my harness and computer, and it fired right up. Then we put 2 and 2 together about the moved CPS.


If it is a stock setup, again, 99/100 times it is the crank position sensor or something related to its operation or wiring.

Good luck!
 

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NERD
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The CPS was in the wrong position. I got a new CPS mounting bracket from somewhere and it put the CPS higher up on the motor thinking the computer could figure it out, but the 4.0 computer aint programmed for that :)

The flywheel, or in my case the harmonic balancer, it notched asymmetrically to tell the computer where the crank is in its revolution. I did not even think that moving the CPS would cause an issue, but it did. It thought the motor was about 270 degrees out of where it should have been, and when it got the cam reference signal, it just shut down and did not fire.

A 4.0 needs ONLY 2 references to fire the spark plugs - cam position and crank position. The cam sensor is in the distributor, and the crank reference is on the bell housing.

If you are not getting spark in a 4.0, you have 4 big things to test, in order of probability of things going wrong:

1. CPS. It is bad, wires to it have frayed, the connection in the wiring harness has crap in it and no longer has continuity, the mount has moved away from the flywheel, etc. If t is a fully stock setup, it has probably just failed.

2. Cam position sensor. Pretty well protected, rarely goes bad. Located in the distributor under the cap. Stranger things have happened, and it is a cheap sensor.

3. Coil. Usually fine, i would "borrow" one from a friend with a 4.0 to test against a working coil, I think there are values on the internet to test against.

4. Computer. It is possible to fry it, but it is very simple and potted in epoxy, there is not much to go wrong.


I hit a wall trouble shooting mine, convinced my buddy to let me wire his motor with my harness and computer, and it fired right up. Then we put 2 and 2 together about the moved CPS.


If it is a stock setup, again, 99/100 times it is the crank position sensor or something related to its operation or wiring.

Good luck!
Bump.

Having a hell of a time getting my CPS to line up on the crank. 4.0/np435 with CJ bellhousing, I cut out the section for the CPS and can see the crank. I believe I can see the cutouts that the sensor reads. I've read that the distance between the flywheel and the CPS needs to be about .045 or the thickness of a dime. I have moved it and spaced it and can't get the rig to start. I still suspect CPS.

I also connected ohm meter to it, on 2000k setting, got 1950 and cross referenced another CPS, brand new from AutoZone, at 1250. Haynes says 200+/- 75 is good so it should be a good CPS.
 

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Snooty Poser
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are you in the same location as stock? as one of the other posts mentioned, the crank signal is encoded, so where the CPS is located will make a difference.
 

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NERD
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Should be between .020 and .045 from what I've read. Called Novak but they were of no help. Maybe Advance would know.

In same location, I'll try a piece of paper or part of a business card under the CPS to see if that works. Hesco makes a CPS relocation kit but it's a little pricey at $320.
 

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I looked in the 2000 TJ service parts manual and it says to use PN05252229 as the spacer between the target wheel and sensor. Sounds like it's about $5 or it comes with a new crank sensor. It's a piece of cardboard that spaces the sensor and gets ground down on 1st startup.

Quick google-fu showed that people got lots of random thicknesses when they measure it.
 

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Some say it reads the lip with machined indents (after the teeth)
This. It reads the gaps. Gap creates voltage change.

The teeth are constant and unchanging, it wouldn't lend itself to positioning, not sure who told you that.

Coils can also be tested by grounding, negative switches are what cause it to fire.
 

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Sorry, when you said teeth, I didn't realize you were talking the flywheel teeth. I read "flat surface" and thought to myself, what would it read in the flat surface?

As The Phenx stated, you want to read the encoded portion, which when I heard teeth is what I was thinking.

Sorry for the confusion.
 

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my 4.0 failed to start on 2 occasions. May be unrelated but all I did the first time was disconnect the battery for a minute and hooked it back up and it started. Second time was on the trail at midnight, camshaft position sensor broke in half causing a no fire situation. Had to replace the distributer.
 
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