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No spark

Old 05-09-2019, 07:39 AM
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Default No spark

Have a 95 Jeep XJ limited with no spark. Crank sensor seemed ok but replaced anyway. Getting 3 codes 22, 23, 24. TPS was reading 6v so replaced - and still reading the same. The crank sensor is reading 5v. Seems strange I have the 3 codes - think computer now but wonder if Iím missing any XJ tricks. Help needed thx
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Old 05-10-2019, 01:21 AM
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when you turn the key on does your fuel pump turn off after about 3 seconds?
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Old 05-10-2019, 02:08 AM
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Yep can hear it come one and off

Thinking I will clean up all the grounds, trace the wires back to check for shorts...
Also notices the pcm has a pretty good hum\buzz to it which might imply an power issue... I've had zero issues leading up to this and the sensors are getting power so just not sure.
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Old 05-10-2019, 09:47 AM
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You should use a test lamp or meter to check the ASD relay fuse while starting. If it is energized it means the computer wants to start - but likely the ignition coil is not working or not being commanded properly. If that fuse is not energized it means either the computer does not want to start (cam and crank sensors are likely candidates) or that the ignition switch is not providing power.

Basically, for no-spark, everything comes together at the ASD relay. So it provides an significant opportunity to find the correct direction to go in. ASD Relay:

1. Low Current Control - ASD Relay coil positive comes from ignition switch RUN/START position
2. Low Current Control - ASD Relay coil negative comes from PCM (when it is happy and is going to allow spark to happen - primarily this means happy with cam and crank sensor signals, and engine rpm while cranking)
3. High Current - ASD Relay load is always on and comes from PDC and battery. When this circuit is closed the power will go through the ASD relay fuse and on to the ignition coil (constant, always on)
4. PCM will control the ignition coil by adding and removing ground
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Old 05-13-2019, 01:40 AM
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Thanks for the info
Did some more testing and discovered the pcm is suppling power to the sensors but not a ground. If I manually ground the TPS for example, the TPS works correctly and even the codes disappear. The pcm also controls the ground which I didn't know hence why my sensors were not working as expected. Picking up a pcm tomorrow so fingers crossed. Will post when I confirm the solution.
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Old 05-13-2019, 06:27 AM
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Originally Posted by Andre Morton View Post
Yep can hear it come one and off

Thinking I will clean up all the grounds, trace the wires back to check for shorts...

Shorts blow fuses. You are looking for opens, or partial opens (corroded connections). Make sure you use a test light AND a meter.

See my sig, #1.

Last edited by BlueRidgeMark; 05-13-2019 at 06:41 AM.
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Old 05-13-2019, 08:34 PM
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All fuse's checked with a multimeter. No issues
Quickly installed a new pcm this morning and still no spark. I'll keep digging
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Old 05-14-2019, 07:10 PM
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Did you check with a test light? Meters can be fooled by corroded connections. Meters don't put any load on the circuit, so they will show the correct voltage even when there is corrosion upstream. Put a load on it, and the voltage drops.

Also, see my sig, #1.
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Old 05-16-2019, 08:42 PM
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Default Intermittent no spark

Hello, new to forum. Need help. I recently had 93 xj 4.0 hard crank and then run poor then lost spark. Cam sensor changed at mechanic. Ran fine to the house then Did same thing again, no spark just turned over. Intermittently would crank fine. I mean like no start then 10 min later crank fine. Anyone know of any obvious wiring issues that may cause this?? Iím lost. Have owned the Jeep for two yrs and this is new symptom. If this is in the wrong spot I apologize, will learn the ropes.
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Old 05-17-2019, 08:19 AM
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Originally Posted by Sleepers View Post
Hello, new to forum. Need help. I recently had 93 xj 4.0 hard crank and then run poor then lost spark. Cam sensor changed at mechanic. Ran fine to the house then Did same thing again, no spark just turned over. Intermittently would crank fine. I mean like no start then 10 min later crank fine. Anyone know of any obvious wiring issues that may cause this?? Iím lost. Have owned the Jeep for two yrs and this is new symptom. If this is in the wrong spot I apologize, will learn the ropes.
Welcome to the forum Sleepers.

You should start your own post (with a good title). That way people can focus on your question by itself.
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Old 05-17-2019, 10:25 AM
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Originally Posted by Andre Morton View Post
Thanks for the info
Did some more testing and discovered the pcm is suppling power to the sensors but not a ground. If I manually ground the TPS for example, the TPS works correctly and even the codes disappear. The pcm also controls the ground which I didn't know hence why my sensors were not working as expected. Picking up a pcm tomorrow so fingers crossed. Will post when I confirm the solution.

The PCM doesn't control the ground. Start checking the ground connections. In particular the ground connections at the dipstick and the grounding strap at the back of the block to the firewall. If needed, you can add your own ground at the TPS sensor to somewhere convenient like the intake manifold. Plenty of people have done that instead of digging into the wiring harness to find the broken connection.
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Old 05-19-2019, 07:10 PM
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Found the issue

There was some damage to the wiring loom near the pcm so the symptons were all a bit misleading.

The main issue is there is a ground wire that comes out of the pcm (K4 - black\blue wire) that feeds the sensors. On a 95 Jeep XJ the sensors do not have a direct ground (don't know about newer versions). Without this ground I was getting 5-6 volts readings on the ground wire at the sensors so the imput voltage was passing straight through the sensors. This is why I thought I have a short. Once I patched the loom all is well in the world again.

Thanks for the assist!!
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Old 05-19-2019, 08:15 PM
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Originally Posted by lawsoncl View Post
The PCM doesn't control the ground.

The PCM controls the injectors and the spark by providing ground when it's needed. 12v is supplied to the coil and injectors at all times.
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Old 05-19-2019, 08:45 PM
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Originally Posted by BlueRidgeMark View Post
The PCM controls the injectors and the spark by providing ground when it's needed. 12v is supplied to the coil and injectors at all times.
True. I was referring to the sensors where he thought the pcm wasn't supplying the ground. Don't forget about the ASD relay...
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