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Unread 10-20-2014, 10:36 AM   #1
dodgemahal
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Crank position sensor question?????

What happens when a crank sensor doesn't function? No more 12+ volts to coil? The reason I ask is because I back-probed the B and C(ground and signal) terminals on the cps connector and measured the signal volts to the pcm while cranking; with my analog volt-meter it show a constant 5 volts. Shouldn't the notches in the tone ring interrupt the 5 volts? 5-0-5-0.......

The 95 ZJ I have has a no-spark condition. The wire from the pcm that supplies power to the ASD relay shows voltage for 2 seconds and then stops. It should flicker consantly. I'm wondering if a faulty crank sensor is causing this.

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Unread 10-20-2014, 01:26 PM   #2
Uniblurb
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I tried to answer your questions back here in the thread where you originally posted the same thing. Kinda makes things confusing when double posting.

http://www.jeepforum.com/forum/f13/9...l#post26173898

I'll include the text from the above link if you want to keep this thread going...

Actually believe the PCM connector diagram is correct for the 93 4.0. If you look at the bottom left under the connector it says, "VIEWED FROM WIRE END". Also note the numbering is backwards going from the top right to the left. This would match correct numbering progression going from left to right on the PCM connector itself when looking at it straight on.

You're correct you can loose spark with a bad crank sensor. Good move using an analog meter for checking the crank sensor signal but the tone ring may be moving too fast, even if just cranking over, to notice any signal interruption on your meter.

Actually instead of connecting your meter to connector pins B (ground cavity) and C (power) you should be back-probing A (signal) while connecting the black probe to any ground. Then you can back-pin the black probe to the middle C wire, while having red probe back-pinned to A, to make sure the ground is functioning.

There's some crank sensor testing procedures as links in my signature. But this below one may be even better where you turn the engine over by hand using a ratchet/wrench to test the signal. I've tried to install this test in my signature but not enough room. Lol, but not all that funny.

http://troubleshootmyvehicle.com/jee...crank-sensor-1
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96 4.0 ZJ Laredo, 2004 4.7L WJ Limited, 93 4.0 XJ (spare), 96 4.0 XJ (son's)

-Stalling ZJ? 12 things to check before replacing a sensor; the Dirty Dozen
-Crankshaft position sensor multimeter test. & video of testing.
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Unread 10-20-2014, 06:54 PM   #3
ZeeJay1997
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Quote:
Originally Posted by dodgemahal View Post
The 95 ZJ I have has a no-spark condition. The wire from the pcm that supplies power to the ASD relay shows voltage for 2 seconds and then stops. It should flicker consantly. I'm wondering if a faulty crank sensor is causing this.
unless i'm caught in the 95 model confusion, the pcm doesnt supply power to the asd. it supplies a ground. power is furnished by the same fuse that powers up the pcm. at least that's the way it works on 96-98
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Unread 10-21-2014, 03:18 PM   #4
dodgemahal
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I'm very sorry to post twice......only because I realized the value of the knowledge you two guys possess, I was somehow worried you would miss my post in the other thread. I'm kinda new here. I'm a little green on the terminology and techniques used so I will have questions later. I am rather enjoying the troubleshooting that will eventually lead to my fixing the no-spark condition I am experiencing. Thanks for passing on your know-how.

Please tell me if I am confused, LOL: looking at the connector with the locking tab up, the A wire(left-most) is the signal/output wire. The B wire(middle) is the ground. The C wire(right most) is the power/reference wire.....is that correct? I think I had that wrong in my previous post.

What is back-pin? Same as back-probe? Thanks for helping the 'green guy'.
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Unread 10-21-2014, 06:26 PM   #5
Uniblurb
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No problem and we'll just carry on your thread from here.

Correct, "A" is the signal, "B" is ground cavity, and "C" is the power in the crank sensor connector as can be seen in the below post which is also the resistance test in my signature.

http://www.jeepforum.com/forum/f13/f...l#post15720747

To me back-pin is the same as back-probe and often you can't fit the probe in the back of the connector so you can use a pin to touch/connect the probe to it. This is typically for testing with the ignition on or running. If you don't have a set of alligator clips with red & black wires, which are almost a must for some testing, they're only $3 for 3M brand from Walmart.

So have your read through ZeeJay's thread on a no-start condition? It's a goodun'.

If you want to find out exactly what came on your ZJ from the factory fill out the below Jeep form and they'll email you your build sheet within about 2 days. Then plug this info into your profile which will help us to help you.

http://www.jeep.com/webselfservice/j...ull&category=U

Good luck.
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96 4.0 ZJ Laredo, 2004 4.7L WJ Limited, 93 4.0 XJ (spare), 96 4.0 XJ (son's)

-Stalling ZJ? 12 things to check before replacing a sensor; the Dirty Dozen
-Crankshaft position sensor multimeter test. & video of testing.
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Unread 10-21-2014, 09:24 PM   #6
dodgemahal
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I'd love to read ZeeJay's thread on the no-start. Do you have a link?

To ZeeJay: I noticed that there was continuity(Dark green/black wire) from the PCM all the way to the power lead at the coil, and since, I think, the coil get's it's power from the ASD relay, then I figured the power to the ASD must come from the PCM. It was the same colored wire. Am I wrong? I'm probably not explaining this stuff very well, sorry. Anyway, I was testing the power wire to the coil when it lit up my test light for about two seconds and then goes out when I start cranking the engine. Same thing happened with the same wire at the other end tested back at the PCM.

Thanks for all the cool links. I'm very happy to glean all this info...wow.
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Unread 10-21-2014, 09:43 PM   #7
Uniblurb
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Guess I should have mentioned Zee's thread link is in my signature labeled the "Dirty Dozen". It's in big green link print in ZeeJay's signature above also.

You don't happen to have an alarm system do you? If so you could be having some malfunction where it cuts the spark off after a few seconds but the engine will keep cranking.
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96 4.0 ZJ Laredo, 2004 4.7L WJ Limited, 93 4.0 XJ (spare), 96 4.0 XJ (son's)

-Stalling ZJ? 12 things to check before replacing a sensor; the Dirty Dozen
-Crankshaft position sensor multimeter test. & video of testing.
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Unread 10-22-2014, 05:52 AM   #8
ZeeJay1997
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green/red (maybe green/orange?) is the 12v circuit from the battery through the ASD relay. It is the circuit that the relay controls. The green/red going to the PCM is a signal to the PCM that the relay is working.

The PCM furnishes a ground to the relay when it is satisfied that the engine can run. One of the parameters is crank sensor signal. The two seconds is the amount of time it takes the PCM to determine it doesnt want to take you where you need to go.

Did you ever get squared away with the crank signal test?
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Unread Yesterday, 11:19 AM   #9
dodgemahal
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I did not understand the explanation about the green/red or green/orange wire. I don't see a wire of those colors in my 95 wiring diagram. Maybe it will sink in my thick skull later. So I'm still wondering......... is the dark green wire with a black tracer that goes from the coil(power) all the way to the PCM, and it does have continuity as I mentioned earlier, is that correct?

Today I will finally have some time to check that crank signal using my new found knowledge. Life got in the way and kept me away for 2 days.

Is there a quick way to know if I have an alarm system? As I read this I don't really know. Maybe when I get my build sheet I will know then. Thanks for that!!
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Unread Yesterday, 11:42 AM   #10
ZeeJay1997
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Quote:
Originally Posted by dodgemahal View Post
is the dark green wire with a black tracer that goes from the coil(power) all the way to the PCM, and it does have continuity as I mentioned earlier, is that correct?
PCM does not furnish power to the coil. It furnishes a pulsed ground based on how it thinks the engine needs to run. The ASD relay powers the coil up and the pcm fires it. I dont have 95 diagrams, so sorry for any confusion I may have caused you.

Quote:
Originally Posted by dodgemahal View Post
Is there a quick way to know if I have an alarm system? As I read this I don't really know. Maybe when I get my build sheet I will know then. Thanks for that!!
yeah, you need the build sheet unless someone else knows.
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Unread Yesterday, 01:16 PM   #11
Uniblurb
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This 95 4.0 diagram may help with what Zee is saying. Power is supplied to the coil via the ASD relay on the black/green wire while the coil driver in the PCM tells the coil when to fire on the gray/white wire or just plain gray wire from PCM to coil.
zj-95-4.0-coil-iac-wiring-diagram-8w-30-11-650x800.jpg  
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96 4.0 ZJ Laredo, 2004 4.7L WJ Limited, 93 4.0 XJ (spare), 96 4.0 XJ (son's)

-Stalling ZJ? 12 things to check before replacing a sensor; the Dirty Dozen
-Crankshaft position sensor multimeter test. & video of testing.
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Unread Yesterday, 10:02 PM   #12
dodgemahal
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Quote:
Originally Posted by Uniblurb View Post
This 95 4.0 diagram may help with what Zee is saying. Power is supplied to the coil via the ASD relay on the black/green wire while the coil driver in the PCM tells the coil when to fire on the gray/white wire or just plain gray wire from PCM to coil.
Not to beat a dead horse here but what then is the PCM doing with that same black/green wire that shows continuity between PCM and coil?(It ends at the 4th pin from the left on the bottom row of pins as you look into the PCM.) That's a really good diagram.....thanks for teaching.
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Unread Yesterday, 11:11 PM   #13
dodgemahal
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I got to test my crank sensor finally-----with some strange results. The power wire showed 8 volts.....so far so good. The signal wire showed 5 volts, I presume at the cut-out notches, while I hand turned the crank. What seemed weird to me is that in-between the 5V spots were many smaller voltage spikes that would occur with small amounts of hand turns at the crank snout-----1 volt, 2 volt, 3 volt, and 4 volt fluctuations were all observed on an analog meter by my assistant('er wife) while I slowly turned. She was calling them out as they occured, usually with just a few degrees of turn. They were, in my assistant's words, "sharp spikes". Question: When the sensor gets to a notch in the flywheel and I stop turning the crank at that moment, should the 5 volts hang there or collapse to zero right away? I noticed that the meter would continue to show 5 volts if I happened to stop turning while the sensor was above a notch. Could all this point to a bad crank sensor?
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