Rebuilt 4.0 little spark - JeepForum.com
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post #1 of 17 Old 11-10-2019, 05:55 PM Thread Starter
jcr805
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Rebuilt 4.0 little spark

Good afternoon,
So I am just finishing up completely rebuilding my 97 4.0, that has taken several years to finish. I had broken a piston skirt, so since then I took my block to a machine shop, got the hot tank, bored it .040 over, new pistons and rings, same connecting rods and crank. Since I was doing a rebuild I wanted to put in a new cam, ended up using a clegg engine kit with a Howard's cam, and slightly oversized (Chevy?) valves. Same machine shop also worked on my head. Basically it is a brand new motor, every nut and bolt got a thorough cleaning or was replaced. I am also using performance distributors firepower kit, with their sceamin demon ignition coil, and a duralast gold distributor. Since the motor was out, I also replaced the cps with the genuine mopar one.

The first time I tried to fire it, my fuel pump had 0 pressure, fixed that and now have gas to the rail. When I check for spark, it is there but faintly. All my grounds got refreshed, maybe they are still not getting enough flow? I'm not sure what else it could be, and I'm too frustrated to think clearly... And could really use the help. Anything electrical is my least favorite thing to diagnose. Thanks a bunch for even reading this through.

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post #2 of 17 Old 11-10-2019, 06:29 PM
CJ7-Tim
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Did you test the coil ?

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post #3 of 17 Old 11-10-2019, 07:18 PM Thread Starter
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I just took one of the spark plugs and stuck it on the end of the coil wire, got a little better spark but still pretty weak. I found an article that said to test voltage on the plug side, so there's a white/gray and a green one. The article said the white wire should have a constant 12v with ignition on, and the green one sends the pulses. I had some mixed results, the white one read .01v and then the green one spiked at 12v and then dropped to 0v with the ignition on. I have yet to test the actual resistance on the coil, and will do that first thing after work tomorrow.
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post #4 of 17 Old 11-11-2019, 06:32 PM Thread Starter
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The new coil tested at .8 ohm, I tested my old coil when it ran fine and it tested at 1 ohm. Out of curiosity I put the old coil and tested it with a stock plug and stock gap, and same thing if not even less of a spark 😕 could I have possibly gotten some grounds mixed up? Or any other wires? I also re re freshed the block grounds In case my clear paint on the block was not letting enough current through
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post #5 of 17 Old 11-11-2019, 06:36 PM Thread Starter
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Here's the spot where I believe those block grounds go
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1111191733_1573518907995.jpg  
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post #6 of 17 Old 11-11-2019, 06:59 PM Thread Starter
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Also did another multimeter test on the two wires that go into the coil... Same results as before, but I was able to crank it this time and see. The white one was .01v with ignition on, and jumped to .1 to .12v while cranking. The green peaked at 12v and dropped back to 0 with key on, and had a constant 10v while cranking.
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post #7 of 17 Old 11-12-2019, 05:56 AM
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Quote:
Originally Posted by jcr805 View Post
I just took one of the spark plugs and stuck it on the end of the coil wire, got a little better spark but still pretty weak. I found an article that said to test voltage on the plug side, so there's a white/gray and a green one. The article said the white wire should have a constant 12v with ignition on, and the green one sends the pulses. I had some mixed results, the white one read .01v and then the green one spiked at 12v and then dropped to 0v with the ignition on. I have yet to test the actual resistance on the coil, and will do that first thing after work tomorrow.
The green wire goes to ground to trigger the coil. The gray wire is the issue. You are missing the ignition twelve volts. Can't tell on a reworked 97 where it originates other than the ignition switch
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post #8 of 17 Old 11-12-2019, 06:44 AM
jtec
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I believe this is a 1997 XJ ? correct me if wrong.

With KOEO* -
DK/GN wire should be 12v, fuse #20 and thats gets its 12v from ASD.
The GR wire should not be grounded. this is driver, it goes to PCM.

*watch for 3 second time out if eng does not run. not sure without seeing wiring dia.

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #9 of 17 Old 11-12-2019, 03:12 PM Thread Starter
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Ive included a diagram and a picture of my pdc from my owners manual. Yes a 97 xj, with a mv3550. As far as I can tell the gray wire comes straight from the pcm. I traced it around the block and no signs of damage... What could cause the pcm to not send enough volts? I did change out the flywheel as well (it was actually cheaper than resurfacing the original) but they looked identical, and I would think even if they were slightly off it would change when it sparks not how much. The fuses looked ok as well, none appeared to be broken
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diagram32_1573592328298.jpg   1112191359_1573592432697.jpg  
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post #10 of 17 Old 11-12-2019, 03:24 PM
jtec
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the PCM does NOT send voltage to coil.
the ASD supplies 12v to coil -on DK/GN wire, the circuit continues to the PCM. the gray wire is the driver - the PCM controls ground on this wire to fire the coil,
SO with coil unplugged no 12V on gray wire. CORRECT?

The fuse MAY be omitted
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File Type: pdf XJ coil.pdf (58.2 KB, 3 views)

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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post #11 of 17 Old 11-12-2019, 06:15 PM Thread Starter
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Also read another thread with a simular issue, and he ended having his distributor off 180. So I will also be double checking my distributor position and making sure my TDC that i marked on the damper when the head was off, is still really TDC
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post #12 of 17 Old 11-12-2019, 06:43 PM Thread Starter
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Thank you so much for workin with me, and correct, I have like 1/10th of a volt on the grey. So my owners manual calls for a 25amp fuse for the asd in spot #18, and in the jeep is a 20amp fuse... They are cheap enough to go ahead put in a 25amp like it calls for. Sorry I couldnt pull up the image either, it kept coming up as a broken image but ill check again when I get back to better cell service
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post #13 of 17 Old 11-13-2019, 11:25 AM
JWELK
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Was the 20 amp fuse open? If not, that is not your problem. As you have fuel pressure that is NOT your problem. The DK/GN wire is the issue, it is broken somewhere.
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post #14 of 17 Old 11-13-2019, 01:55 PM Thread Starter
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Well I think I got too exited and saw something that wasn't there, all fuses were in the right spot, all tesed ok, but still replaced them anyway. Also checked the asd relay, and that's not it... I did gain a few hundredths of a volt but its still not where it needs to be. Triple checked tdc, and that also was good so I'm sure my distributor is in the right spot. Im attaching a pic of the diagram and I highlighted the drk/grn wire, ill follow it in the jeep and look for any damage
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diagram32_dk_grn_1573674932607.jpg  
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post #15 of 17 Old 11-13-2019, 03:15 PM
jtec
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IIRC you may need
2 people one to turn the key the other to test for voltage - you may have only 3 seconds of
voltage if engine does not start!
unplug the coil, what is the voltage on the DK/GN wire.

I will find time to check on the ASD timing out along with fuel pump relay.

!997 MAY have systems more common to a 1996, as is your diagram, the earlier type.

When I see the price of OEM I think aftermarket.
When I see the quality of aftermarket I think OEM.
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