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Engine died, and won't start. Faulty coil?

br1anstorm

NAXJA Forum User
Location
United Kingdom
I have a 1993 XJ, 150,000 miles since new, and well-looked after but now showing its age. It usually starts first time and runs smoothly.

Today while driving along it suddenly "died". No hiccup or misfire, it just stopped. Then it wouldn't start. It cranks well, but won't fire.

I have done the basic checks (see below) but don't really have the expertise or equipment to run more detailed tests. Have looked through the forum and the most likely suspects seem to be the coil, or the CPS, or perhaps some connection that has just come apart. Any clues or advice would be helpful.

So far I have checked

- battery terminal connections: both are good and clean
- distributor cap: all six contacts inside seem clean, but I have sanded them to get rid of any crap
- plug leads/connections: all seem good
- fuses: all intact
- fuel supply: pump audibly comes on with ignition, and there's fuel at the Schrader valve in the rail

The battery is an Optima and seems to have plenty of power - it cranks the starter motor happily and repeatedly.

Suspecting that repeated attempts may have flooded it, I have tried the standard technique of trying to start it with throttle floored. No joy.

Two clues. I have tried to see if there is a spark from the centre-lead from coil to distributor cap by holding the end (with the help of a screwdriver-blade) close to an earth on the engine block, and getting someone to crank the starter. No spark, as far as I can see. Does that suggest the coil has failed?

The other possible clue is weird. When the ignition is switched on, the charging gauge (voltmeter) shows the state of the battery. Normally this is at or around the middle of the dial (12/13v). Now, with ignition on, the needle only just gets into the lower red zone (less than 9/10v?) and - predictably - it drops slightly when cranking. Yet the battery seems to have ample power to turn the starter motor (and even when cranking the interior light barely dims - as it would if the battery was weak).

I'm puzzled. I've left the Jeep at the roadside. But I'd like to try to get it going if I can, or at least identify the fault, rather than have to call out my garage to trailer it in for repair.

Any thoughts?
 
Sounds like the CPC(Crank Positioning Sensor). That happened to me a few months ago..running smoothing as usual and next thing you know it just died. Coasted to the side of the highway, checked everything. I thought it was the coils, purchased a new one, installed..nothing.

What you described is exactly what happened to me. Good thing I have a smart phone and was able to diagnose the problem and did everything on the side of the road. The whole job took 25mins to do just coz it was raining and getting dark but 4 hours total having to wait for the wife to buy the CPS and get my tool box (didn't have extension with me..lol)

this would help:
http://www.cherokeeforum.com/f51/jeep-cherokee-2000-4-0l-replace-crankshaft-position-sensor-17310/

G'luck!
 
The crank sensor is a very common failure on the Jeep 4.0. Without that input signal to the computer, you will not have spark and you will not have fuel going TO the fuel injectors.

The crank sensor can be tested, but testing is not always conclusive. If you buy one, please do yourself a favor and buy it directly from a Jeep dealer. Aftermarket crank sensors are "hit and miss" with a lot of miss. I buy plenty of aftermarket parts, but I will NOT buy any aftermarket crank sensor.
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Spark: 12 volts from ignition switch to the ASD relay; when the key is first turned on the PCM provides a ground to the ASD relay to energize it, then the ASD relay passes 12 volts to the coil primary side. If no crank sensor signal is generated after a couple of seconds the PCM cuts the ground to the ASD relay (and fuel pump relay too) and that cuts power to the coil (and fuel pump). Key to START and once again the PCM energizes the ASD relay. Now, a critical element is the crank sensor--no signal from the crank sensor and the PCM won't trigger the coil to provide spark.
 
the pick up coil can show identical symptoms, and carries less expense than the crankshaft position sensor. It is in the distributor under the rotor. This functions in the same way as the CPS but detects rotation of the camshaft via rotation of the distributor. Either this or the crank sensor will disable the ignition if malfunctioning. I replaced this pick up coil (also called camshaft position sensor) for 30 dollars and my jeep started right back up. I then replaced the CPS and purchased another pick up coil and placed my old CPS and new coil in my Jeep knowing that these sensor, even with a fully capable Jeep will strand me in the back country. My sensor failed while my jeep was parked for 6 months. Drove into the garage, and 6 months later would not start to to this sensor, they fail seemingly at random and without any predictable set of circumstances.
 
Thanks for replies. That seems to narrow it down a bit: the crankshaft position sensor, the camshaft position sensor (aka pickup coil) or the ignition coil.

My Jeep's now in a friend's workshop: he has some testing equipment. There is quite definitely no spark from the coil, but the fuel pump comes on briefly with the ignition as normal. We now have to test and trace the precise fault, because replacement parts are hard to source and expensive over here in UK. So I can't experiment by swapping out assorted parts just to see what works. We need to pin down exactly what has failed so that we can order the right spare(s).

Point taken about getting OEM if it's the crankshaft sensor, but UK Jeep dealers tend to think of a price and then quadruple it! So once we find the fault, next stop is probably the Rock Auto or Morris 4x4 websites.
 
Alot of the crank sensors suffer from corroded terminals, oxidized terminals, and splayed open female connectors. I would clean both the sensor and pigtail terminals and use a pick to close the female terminals to ensure perfect continuity.
 
An update.... and a warning.

The story so far: the Jeep cranks but won't start, so it gets trailered to a friend's workshop since he has all the right kind of test equipment. I need to find out which part has failed, rather than just swapping parts out (since parts are hard to find and expensive).

My immediate reaction was that the coil had failed. But the CPS became the prime suspect, as the forums are full of tales of failed sensors and it is described as the most common/likely problem.

So step 1 - check the CPS, which means finding out what the test readings should be on the CPS connections. Internet search takes me to this tech-spec website http://www.lunghd.com/Tech_Articles/Engine/Basic_Sensors_Diagnostics.htm which says that for my Jeep's (4.0 litre HO) engine, the resistance across pins B and C of the CPS should be "near zero ohms".

Step 2 - test CPS on my Jeep. Guess what - open circuit (infinite resistance), rather than near zero. CPS must be bad. This leads to....

Step 3 - nationwide hunt for replacement CPS. According to dealers, it is no longer listed on their spare-parts system, as it is "obsolete and unavailable". MIraculously, I find one dealer at the other end of the country with a genuine OEM CPS on the shelf. Cost, approx 150 dollars. I bite the bullet and pay up.

Step 4 - fit new CPS, and start Jeep. No joy. It cranks, but won't fire. Weird. Is the new OEM CPS faulty?

Step 5 - test new CPS connections. Resistance across B and C terminals is open circuit, just like the 'old' one. Weird.

Step 6 (my engineer friend is on his own now, as this is out of my league) - check out ECU connections - all OK. Check signal from CPS when cranking - loud and clear. Check input to coil (using something called a "noid light"?) - 100% OK. Check coil output - hardly a flicker of spark. Aha! So my instinctive initial suspicion (see thread title) was right.

Conclusions? 1) the coil is shot, so I now need to find a new one; 2) the old CPS, and for that matter the new one, are both fine, so now I'm out 150 dollars needlessly; and the painful lesson 3) don't always believe what you get from a Jeep tech expert website!

More research eventually unearths some different tech-spec test guidance which says - surprise surprise - the resistance between B and C on the CPS should be open circuit. I found that advice on the NAXJA forum at #8 in this thread here http://www.naxja.org/forum/showthread.php?t=1062496. So the advice on the site at at lunghd.com is - unfortunately - wrong....

Lesson learned: you need to double check and cross-check what you find on the internet, especially when it relates to technical spec, factory settings, and test procedures.

The search for a replacement coil is on. Tune in soon for the next episode!
 
The lunghd article you used for a reference for testing your crank sensor is for RENIX XJs only (87-90). It is correct for those years. The article clearly states that his advice is for RENIX at the beginning of that article. You have a 93 (HO, not Renix) so the procedure for testing of your crank sensor requires a different procedure. That is not the fault of the author. The RENIX and HO sensors are not the same.

However, you did NOT waste your money. Eventually, you WILL need that crank sensor if you keep your XJ long enough. And with them being hard to find, it's a good thing you have one and should keep it in your XJ.

The ignition coil can be easily tested for primary and secondary resistances with a service manual and a meter. If the coil is shot, testing will tell the story. If you don't have a service manual or a multimeter, I suggest you buy them as they both are essential for working on your XJ. Don't assume it is the coil; it certainly could be but if you just buy one and install it, you are "throwing parts" at this problem instead of continuing with your testing based strategy.
 
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Thanks, Birchlake, for your comments.

Just a small point on the lunghd.com site. I know it is very clearly aimed at Renix engines. I looked at it initially mainly for the photos of the CPS location and access. But the text does include this specific guidance on testing the CPS (I paste directly from the site):

quote

Test#1 -
Get a volt/ohm meter and set it to read 0 - 500 ohms. Unplug the cps and measure across the CPS connector's A & B leads. Your meter should show a CPS resistance of between 125 - 275 ohms. If it's out of that range by much; replace it. (* - Note: HO engines are measured from the B & C connectors and should have near zero ohms.)

unquote

That final sentence in brackets, the Note, does refer explicitly to the HO engine. It is incorrect. That is what misled us. I rest my case.
 
Just a note. If you try for a long time to get a 4.0L to start, you can end up wet fouling the spark plugs. At a minimum I would pull them and clean them with carb cleaner and a toothbrush, then ideally bake them in an oven at about 300 degrees for an hour to dry them out. For some reason XJ plugs just don't like to fire after getting wet fouled without a bit of cleaning.

Personally, I buy a cheap set of Champions and put them in.
 
Useful tip. I remember reading about this somewhere a while back. I also recall something in the owner's handbook about the risk that repeated start attempts would flood the cylinders... where the advice was to crank and start with the gas pedal flat to the floor. Seemed odd to me, but I suppose it works.

In my current situation (ha!) the problem is that there's no current (ie spark) even reaching the plugs. The coil is getting juice in, but isn't delivering to the centre pin of the distributor. QED. New coil needed.
 
Just to bring the story in this thread to an end....with thanks to all who commented.

We tested the coil and it was indeed shot. Fitted new coil. Engine fired up first time the key was turned, and runs perfectly.

So now I just have a spare new CPS even though the original is still working fine after almost 20 years!

Lessons learned: the problem you have isn't always the most common one - it pays to test methodically and not make assumptions. And as mentioned before, double-check whatever tech advice you might find out in cyberspace!
 
Hi DJ

So far so good! It's a new radiator, of course, and the drain-plug arrangement is holding up fine. It was an s-o-b to get it all into place, as there is so little space in that area beside and behind the headlamp unit. But I managed to get it into position, with a cable-tie to stop the drain hose flapping around.

So.... in terms of major maintenance, in the past couple of years I have replaced the corroded fuel tank, fitted a new stainless steel exhaust, solved the death-wobble in the suspension/steering, put in a new radiator, and just replaced the ignition coil. Now I wonder - as we approach the 20 years/150,000 mile mark, what's the next thing likely to fail?
 
Hi DJ

So far so good! It's a new radiator, of course, and the drain-plug arrangement is holding up fine. It was an s-o-b to get it all into place, as there is so little space in that area beside and behind the headlamp unit. But I managed to get it into position, with a cable-tie to stop the drain hose flapping around.

So.... in terms of major maintenance, in the past couple of years I have replaced the corroded fuel tank, fitted a new stainless steel exhaust, solved the death-wobble in the suspension/steering, put in a new radiator, and just replaced the ignition coil. Now I wonder - as we approach the 20 years/150,000 mile mark, what's the next thing likely to fail?

You may be needing that spare crank sensor next.
 
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