No start, weird spark

My girlfriend drives a 1999 Cherokee (4.0L, manual trans) and it died when we were off-roading. We checked fuel and air - both fine. We narrowed it down to spark - there was none. No spark at the plugs and no spark from the coil (using spark tester).

Checked fuses - all good.

Swapped relays - all good.

Did a quick check of the wires - all seemed fine, nothing ripped, braised, or out of the ordinary.

Pulled off the distributor cap. There was a bit of moisture in there (and corrosion). We let it dry and reassembled it.

Turns out the Crankshaft Position Sensor was bad (really common). This one was actually destroyed (melted, torn up and ugly - really uncommon?). We replaced it. Still no spark from the coil.

Replaced the coil. Still no spark. Tested voltage to coil - good.

Towed the Jeep to Advance, Autozone, and a mechanic. They all narrowed it down to the ECM. Bought a new one, flashed it, and swapped it in. After that we got a weird, intermittent spark from the coil.

Swapped the old ECM back in just to see if that was actually the problem. We got the same weird, intermittent spark from the coil - must have been testing wrong before? Guess the old ECM is good, but still testing everything with the new one.

When trying to start, the car cranks fine, sounds good, but the coil sends a spark every 3-5 seconds. I'm no expert, but it seems like it should be much more often - like every second or so. I'm not sure about the strength of the spark, but it was strong enough to be extremely unpleasant going through my hand.

Thought maybe the new CPS was faulty, so replaced it again. Same intermittent spark.

Also replaced the coil again. Same intermittent spark. Thank goodness Advance has such a great return policy!

Replaced distributor cap and rotor.

Replaced pickup coil underneath the cap.

Another thought: I heard about a freak accident about another Cherokee that was not getting a spark, and it turned out to be an O2 sensor after testing or replacing just about everything else. It doesn't seem like this would make sense at all, but I guess the sensor was sending a faulty code (or short) that prevented ignition. On our Cherokee, part of the exhaust actually fell off RIGHT after the rear O2 sensor on our Cherokee, and I'm 99% sure the sensor got wet when we went through water. I unplugged both sensors and still got the same intermittent spark.

Any ideas? All help is very very very much appreciated. This beast has been out of commission for 3-4 weeks now!

What in the hell cause the original crank sensor to melt?

The crank "sensor" is a signal generator, but there isn't enough output there to melt it.

See you mention playing water commando, you could have damaged the harness between the crank sensor and the PCM--pin it out with an ohm meter.

Does the 99 have a Cam Position Sensor (CPS) in the distributor. Offen can give same simtoms as bad Crank Position Sensor (CKPS). doesn't fail as often as the crank sensor, but with water involved, that would be my guess.

93gc40 said:

Does the 99 have a Cam Position Sensor (CPS) in the distributor. Offen can give same simtoms as bad Crank Position Sensor (CKPS). doesn't fail as often as the crank sensor, but with water involved, that would be my guess.

Click to expand...

Interesting point, on the Renix XJs the synch sensor won't prevent starting, however on the later Chryco XJs it will, but that would be a fuel problem, not a spark issue I would think--however, without proper synch sensor signal perhaps the PCM could be messing with the spark? Need a more experienced late model guy to chime in here.

The Cam Position Sensor in the distributor is the same thing as pickup coil - and yes, I replaced that. And yes, there was a bit of moisture under the cap immediately after wheeling. Let it dry out and that didn't do anything, so eventually replaced the rotor, cap, and cam sensor.

Basically, the only thing I have NOT replaced is the entire distributor... But I believe it is good because the rotor spins correctly when cranking...

I just looked further into that "weird spark". Here's the deal:

The coil is shooting a spark every 5 seconds or so. Right off the bat I realized this was weird... The coil should be firing much more rapidly when cranking, correct? I traced the spark through the distributor cap and it is sending the spark to the third cylinder every single time.

I'm leaning heavily towards a timing issue, but there's always a chance it could be electrical. Any ideas on this?

Yeah, there is something screwed up on your flexplate/flywheel and the PCM is only getting a signal one out of six times.

Now, pull the transmission and find out what burnt up the crank sensor--which is the only "pickup" for the ignition as the synch sensor in the distributor is for the fuel injection.

joe_peters said:

Yeah, there is something screwed up on your flexplate/flywheel and the PCM is only getting a signal one out of six times.

Now, pull the transmission and find out what burnt up the crank sensor--which is the only "pickup" for the ignition as the synch sensor in the distributor is for the fuel injection.

Click to expand...

Thanks for the help. How sure are you that this is the problem? I mean it would make sense, but it doesn't seem like the flywheel could possibly be so messed up that it's only send a signal to one of six cylinders.. That just seems crazy. Then again, this is a Jeep we're talking about :banghead:

I talked to a jeep tech and he thinks that the Auto Zone crank sensor could be causing the issue... he recommended I take out the new crank sensor and get a direct OEM replacement from a Chrysler dealership. Anybody have experience with this kinda situation?

Thanks to all of the knowledgeable people!

As you said the rotor is turning, and will spark on at least one cylinder--could have been more--then something is screwing up the flywheels "notches" and/or the CPS/CKP crank sensors ability to "read" them.

There are 3 trigger notches--1/6, 2/5, 3/4 located at 120 degrees around the flywheel. Between each trigger notch is 20 smaller notches. Each small notch triggers a small signal from the crank sensor to the ECU/PCM which allows it to "count" and know when the larger trigger notch is coming. The larger trigger notch causes the crank sensor to generate a bigger signal, so the ECU/PCM knows when to fire the spark.

Add that to a melted crank sensor, and spark only on one (more likely two) cylinders and that tells me you are going to have to pull the transmission and fix it.

Good luck.

Did you look over the harness for the crank sensor, make sure no damage to the wires. Since the cps melted, look carefully for any bare copper.

joe_peters said:

Add that to a melted crank sensor, and spark only on one (more likely two) cylinders and that tells me you are going to have to pull the transmission and fix it.

Good luck.

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Thanks again. There is definitely only spark at one cylinder - 3rd from the back. Is there any way to examine the flywheel for damage without pulling the transmission? I'm at college with no place to really work on this thing. During the week it sits in a backyard, but during nights and weekends I can tow it to a parking garage to work on it...

jeeperz said:

Did you look over the harness for the crank sensor, make sure no damage to the wires. Since the cps melted, look carefully for any bare copper.

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I examined the plug for the crank sensor and there does actually appear to be a tiny amount of exposed copper wiring on one or two of the wires (engine side). I'm pretty sure there's not enough exposed to cause any electrical issues, and the copper isn't touching at all, but now I'm curious. I'll have to mess with that a little bit.

Here's a video of the spark coming from the ignition coil: http://www.youtube.com/watch?v=HILeRjsHxvs

and here's a video of the spark coming from the distributor to the only firing cylinder (4th from the front of the engine): http://www.youtube.com/watch?v=O6upD1BGQzU

Any ideas?

Definetly insulate the exposed wires at the harness.
Also you stated that you replaced the cam positioning sensor, did you sync the sensor also.
Here is an article on it using a toothpick.http://motorage.search-autoparts.com...30/article.pdf

Just thoughts--

-Would it help to pull the inspection cover on bottom and have someone hand turn the engine while inspecting?

-Is it the right CPS for sure?

-Exhaust leak (melting) in area?

-Loose parts inside bellhousing?


Good Luck,
Orange

just a thought could the crank sensor be some how not in all that way causing weak signals. so it wont pick up the other notches on the flywheel?

90xj06 you might be on to something there!

The gap is small so even small changes could mess up the signal.

-Is there any sign of any damage to CPS mounting area?

-Is it properly mounted with correct hardware?
-----------------------
-Ohm test ok?

-Harness wiring runs test ok -no shorts, no grounds, ?

-Direct off the coil a sparkplug to ground-results?

No inspection plate on the manual trans XJ, least not on the early-mid year ones I've seen.

Thanks, Joe!
:cheers:

Regards,
Orange

jeeperz said:

Definetly insulate the exposed wires at the harness.
Also you stated that you replaced the cam positioning sensor, did you sync the sensor also.
Here is an article on it using a toothpick.http://motorage.search-autoparts.com...30/article.pdf

Click to expand...

Didn't know that syncing was necessary... Thanks for the tip, but that link doesn't work. :rof:

90xj06 said:

just a thought could the crank sensor be some how not in all that way causing weak signals. so it wont pick up the other notches on the flywheel?

Click to expand...

There is absolutely no room for crank sensor adjustment. There are two bolts that attach the sensor directly to the bell housing with no wiggle room or anything like that.

orange said:

90xj06 you might be on to something there!

The gap is small so even small changes could mess up the signal.

-Is there any sign of any damage to CPS mounting area?

-Is it properly mounted with correct hardware?
-----------------------
-Ohm test ok?

-Harness wiring runs test ok -no shorts, no grounds, ?

-Direct off the coil a sparkplug to ground-results?

Click to expand...

There is no sign of any damage to the CPS mounting area, though there is a plastic grommet that is supposed to fill the hole in the bell housing around the CPS. This plastic shield was melted along with the original CPS. As far as I can tell, nobody sells this replacement grommet so I haven't been using one. The only thing that I can think of is that the grommet slightly pushes the CPS in one direction or another for exact positioning, though I HIGHLY doubt it, as it is intended only as a protective shield.

I had a friend manually turn the crank pulley while I inspected the flywheel from the CPS gap in the bell housing. As far as I could tell, everything looked normal except for one thing - there is some sort of metal object wedged in one of the 12 gaps. I believe these gaps are what the CPS senses for timing, but it doesn't seem like one small imperfection would throw off the entire system.... But I could be totally wrong!

Here's a photo of a 99 cherokee manual flywheel:


Notice the 12 square notches around the outside edge (3 sets of 4 notches). One of these notches is blocked.

I took a video of the flywheel and I can upload it if that helps.

orange said:

-Is it the right CPS for sure?
Orange

Click to expand...

Does a 99 have a dual coil/pickup crank sensor like a 00 has?
Just thinking with the key board.