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WHY does a Crank sensor fail? Need help.

bulfo4

NAXJA Forum User
Location
New mexico
1996 XJ with Golen 4.6L stroker, Auto, don't recall on transfer case

Hi, we've been having trouble getting my XJ back to running for a while, but it's been in "crank, no start" for a while.

We've done a LOT of replacements to the stuff with lifetime warranties (sensors, distributor, plug wires, ignition coil, etc). We've also changed out the fuel pump, and the fuel pressure regulator. It gets fuel to the rail.

We've replaced the PCM as well.

Using a spark plug light, no spark from ignition coil to distributor.

Seems like that means the Crank Position Sensor isn't telling the PCM when to send sparks, which over and over seems to be the culprit.

From the plug that goes TO the CPS, it reads 13v ground, and 5.9v for the send signal.

We had two extra CPS's sitting around, and tested those and the one that was in there, all of which showed some resistance on the two pins that are supposed to have resistance (can't recall how much though).

On a hunch, I took one of them back and warranty replaced it from Autozone, and IT STARTED FINALLY!

Drove it around for a day and a half trying to figure out (***other problem below***).

Then, all of a sudden, cuts out completely, gives codes P0201, P0202 for Open circuit on injectors 1 & 2. I cleared the codes and they never came back, but it was back to "crank and no start".

I've heard, BTW, that the Autozone ones can be bad compared to MOPAR and I'm 100% up for getting a MOPAR one, but before I do that...
What makes a CPS fail? I've heard physical damage to the unit by the wheel, as well as I'm sure it can get electrically fried somehow. Anybody have any insights or suggestions? What should*we check or look for?

Thank you!
Tom

***The Other Problem***
For about the last 3 years the XJ has been unreliably dying while driving. Usually it'll sputter and falter a little bit, and if I give it some gas it comes back, but sometimes it doesn't and I'm on the side of the road. Other times it quits all together. Every time though, after a few minutes, maybe 20 or so, it'll start back up and keep running. The weird thing is it does this sometimes after 3-4 minutes, and sometimes an hour and a half. Sometimes while driving, and sometimes while sitting in the driveway. We've moved every wire we can think of, flushed the gas, jumped up and down on it and torqued it around like going over bumps, had it tilted uphill and downhill and level.*

Plus we've replaced:
Whole Distributor
Plug wires
Spark Plugs
CPS (multiple times)
Ignition Coil
MAP sensor
Idle air sensor
Fuel Pump
Fuel Filter
Fuel Pressure Regulator
PCM
Ignition Switch
Every relay we could find
and probably other stuff I don't recall off hand.

But we have to have it running first, so I need to fix "crank, no start" first.*
Thanks on this one too.
 
Heat is the CPS nemesis.

What you should do is buy a proven to run 150,000+ miles genuine Jeep CPS. Cheap parts are cheap for a reason.

Most auto parts stores sell cheap crappy Chinese made parts, some that even come with a "Lifetime Warranty". These parts are poorly manufactured and/or made from inferior materials. They are often out of specification, or even failed, right out of the box. The ones that are not faulty many times will have a short service life before they fail. Always buy top quality replacement parts and genuine Jeep engine sensors. Numerous threads detail long and frustrating searches for a "problem" that ended up being cured simply with genuine Jeep repair parts.
 
Last edited:
Good parts, you bet!

So, if I get a good quality new CPS - will do!

So, heat kills them. Is the heat tolerance about the normal operating temp for the Jeep? (engine wasn't hotter than normal)

Any other ideas or things to look out for?

(and you have no idea how happy I'd be if it was just buying the better CPS and being done with it.)
 
Most of these systems are basic electronics, in this case a CPS is basically just a coil that builds a small charge whenever a magnet passes over it. Heat destroys the coil insulation, and the coil shorts out, or develops a weak field. Its just simple failure of simple electronics.
 
Ok, that makes sense. I think I need to replace it but check the grounds and the wires leading to the CPS for shorts (so I don’t fry another one). Does that sound about right?

Thank you. You guys are the best.


Sent from my iPhone using Tapatalk
 
The CPS also has a magnetic core that is often undersized on the cheap parts. Over time it can lose it magnetism due to heat. x2 on checking the wiring, particularly the harness from along the injectors to the firewall if you had an intermittent code for the injectors. I've seen a few where the harness wore through and shorted or the wires started to break from flexing (which also leads to me checking the engine mounts).
 
What's happening with XJ CPS is B.S. For clarification, a magnetic pickup is a coil wrapped around a permanent magnetic. I've used them in the past and they're typically "bullet-proof". I agree completely. with this statement," Heat destroys the coil insulation, and the coil shorts out, or develops a weak field. Its just simple failure of simple electronics."

In my view, the quality of the permanent magnets, coil insulation, wire gauge, wire material, and encapsulating plastic used in the foreign-made CPS is garbage. This technology is well developed in the USA, reliable and most likely USA proprietary. But when made overseas the foreign companies substitute crap. Likewise, poor magnetic materials can lose their permanent magnetic properties quickly for a number of reasons.

Maybe we need to fab some CPS prototypes, using readily available US magnetic pickups, and configure them to work in XJs. Or maybe we need to pull together a large order of XJ CPS and approach a US magnetic pick-up manufacturer to produce a US-made CPS. I'd be interested in buying a bunch of USA-made CPS for XJs, We are not talking "rocket science" here.

Best regards,

CJR
 
Thank you, lawsonc. I'll look there for the grounding and wiring issues. We've suspected because for a while some of the issues were intermittent that it might be in the wiring itself.

CJR, thank you for the great explanation of how they can be manufactured to fail. Really disappointing to get something new that's junk, but this really explains WHY you get what you pay for in a way I can understand. I get some of the shortcuts in manufacturing now.

For getting a reliable, US-made CPS, what are some good options? NOT the Autozone ones. Won't make that mistake again. Is the Echlin Ignition one from NAPA any good?

Thanks!
 
Very often, the leak prone 4.0L exhaust manifold is blowing hot gasses in the direction of the CPS.


I have a spare Mopar/Jeep CPS in all three of my XJ's.
 
Thanks Tim_MN, that might be a factor too. I'd thought it sounded a little more exhaust-y than I remembered. Your comment spurred on a new thought.

So, I totally agree with you guys, not arguing, but hoping for a new solution to the old problem...if everyone carries an extra CPS with them, and that's even if it's the good quality Mopar/Jeep one, and if there are so many threads about how to replace them and people cussing and swearing trying to get to the darn things, it sounds like a pretty bad design (from Jeep) and a pretty weak link in the chain of important parts that can go wrong. No one carries an extra PCM with them, and it's important too - that would be much easier to get to on the trail.

So, two questions - Is there a CPS for a different vehicle that works and is maybe built better (like when people upgrade alternators from Durango's I think)? Or is there some mod that can be done to the CPS itself or the Jeep that makes it more bulletproof?
 
There is the mod of drilling out the mounting holes one size larger to move it closer to improve the signal. It's bolted to the bellhousing, which should be a reasonably heat sink.


The earlier Renix CPS sensors were 2-wire, passive variable reluctance (VR) sensor - your basic coil around a magnet design. They generate a voltage spike as the teeth in the flywheel/flexplate pass. The voltage was roughly proportional to engine speed, which made them sensitive to wiring issues. That's why there was a kit to bypass the C101 connector and go straight the pcm. That signal isn't a sine wave, but you could get an idea of the health by measuring with a voltmeter at idle. Side note, the speed sensors in the AW4 started out as magnetic reed switches and changed to VR in the later years.



Newer CPS sensor are 3-wire with ground, +5v, and an output. I haven't scoped the signal on a newer CPS, I think they are still VR types with a conditioning circuit to generate a 5-volt square wave.


If we're going to design a better sensor, I'd use a hall effect sensor instead of VR.
 
It is my understanding, that the RENIX CPS are magnetic pickup types and the newer CPS are Hall sensors. In my view, the best starting point is to use Cruiser54's tips to get the sensor closer to the flywheel, i.e. drill one larger mounting hole in the CPS. The next tip, already cited on this form, is to slot both mounting holes in the CPS. So once you remove the CPS mounting bolts (a PITA!), you re-insert the CPS now modified with slotted mounting holes. So then, the next time you need to change the CPS, all you do is loosen the CPS mounting bolts, slip out the bad CPS and then slip in a new slotted CPS. That is easy and fast to do. No more B.S. removing CPS mounting bolts and then putting a standard CPS in, with the typical two (2) mounting bolts, and re-inserting the mounting bolts..

I've been using the NAPA Echlin-brand CPS and they seem to work OK for me.

Best regards,

CJR
 
If the newer ones are hall effect, then they should be much more tolerant of distance unlike the older coil-only ones. The older Renix sensors being just a coil, a resistance test would identify a short or open circuit, but I'm not sure how useful it is on the newer ones.
 
Good ideas. Especially drilling the holes out or slotting them to get it closer. Was talking to my Dad who was an automotive engineer for many years and he said bench testing how close something metal or magnetic needs to be to the sensor might give an idea of how close it needs to be to the wheel when it’s in there. A baseline or target to aim for. Then how to test that out of the vehicle quickly got over my head and now I have to go out of town, so I won’t have a chance to test that for a bit.

He also asked how many miles I’d put on the Jeep before it failed the first time, and I said about 200k, and he replied that if the design engineers thought they would fail And would need to be replaced they’d put them some place easier to get to (like spark plugs). That points to my old (and admittedly move too much wiring harness as well as me not installing it quite as close as it should be (a friend and I drilled through to be able to put a bolt on one side and a nut on the other thinking that would make it “easier” - probably caused the headaches in the first place). The closeness test might tell me whether my drill through solution made it too far away somehow.

I’m going to make some new wires to go straight from the pcm to the plug for the CPS to eliminate the wiring harness potential problems. And I’ll check the grounds for everything while I’m in there.

I’ll definitely do like you guys said and get a good quality one to go in there, and if I can get a spare, I’ll probably do that too.

Thanks for all of the suggestions!
 
The PCM very rarely if ever fails, genuine Jeep CPS last about 150,000 miles plus or minus. There are almost 20 4.0L Jeeps in my local Jeep club, and same story, plenty of CPS failures, no PCM failures.

I have owned 8 97-01 XJ's, 5 needed a new CPS in that mileage range. Once you replace one, it is a 15-20 minute job for the second one.


The drill and slot thing is not necessary or beneficial for 1996-2001 CPS.
 
Several years ago I tested the magnetic pull between
a genuine MoPar CPS and one of the cheaper aftermarkets.

The off-brand was considerably weaker, which surprised me
at the time. (I don't remember what the brand name was...)
 
What's happening with XJ CPS is B.S. For clarification, a magnetic pickup is a coil wrapped around a permanent magnetic. I've used them in the past and they're typically "bullet-proof". I agree completely. with this statement," Heat destroys the coil insulation, and the coil shorts out, or develops a weak field. Its just simple failure of simple electronics."

In my view, the quality of the permanent magnets, coil insulation, wire gauge, wire material, and encapsulating plastic used in the foreign-made CPS is garbage. This technology is well developed in the USA, reliable and most likely USA proprietary. But when made overseas the foreign companies substitute crap. Likewise, poor magnetic materials can lose their permanent magnetic properties quickly for a number of reasons.

Maybe we need to fab some CPS prototypes, using readily available US magnetic pickups, and configure them to work in XJs. Or maybe we need to pull together a large order of XJ CPS and approach a US magnetic pick-up manufacturer to produce a US-made CPS. I'd be interested in buying a bunch of USA-made CPS for XJs, We are not talking "rocket science" here.

Best regards,

CJR

Very interesting post, there. Really lays out what's going on with why some sensors work well, some work but poorly, and some aren't worth the box they're shipped in. I now understand the problem with the aftermarket sensors better than before, thank you.

Now, let me ask a couple of potentially stupid questions:

1. Is there anything unique about an XJ CPS in terms of output signals compared to current-production vehicles? Could an OEM CPS from, say, a JK or Tahoe (or even Toyota, since many XJs have the AW4) be a valid substitute if one could find a way to adapt the mounting/connector?

2. If heat is part of the problem killing sensors, is there room for a heat shield down in there that wouldn't make replacement of the sensor even more of a hassle than it already is?


Rob

P.S. Oh, and Eric? Very interesting that dropping the front driveshaft makes accessing the CPS easier. I must remember to jot that down on a sticky note or something and tuck it into my FSM so I don't forget it...
 
Very often, the leak prone 4.0L exhaust manifold is blowing hot gasses in the direction of the CPS.


I have a spare Mopar/Jeep CPS in all three of my XJ's.

Will some sort of heat shield work for the CPS similar to what many use on their injectors?
 
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