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?

connect a timing light to the coil wire out put to the cap.

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?

Almost forgot, codes pull 12 35 55, no help there

sir

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

thanks in advance

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!