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need help with car problem …..

Old 06-12-2014, 10:18 AM
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Default need help with car problem …..

Sitting at a traffic light at the exit for our local Walmart (no I wasn't shopping there!) waiting for it to turn green. Engine just stops running! Turn the key….engine turns over fine (relatively new battery), I can hear the fuel pump operating, but engine won't catch, just keeps turning. To me, it seems like the ignition coil. Similar thing happened to it several years ago. Had it towed to our mechanic. The next day it's repaired. He replaced both the ignition coil and the crankshaft position sensor at a cost of $450. I am confused because to me, both the sensor and the coil shouldn't be bad, it should be one or the other that would have caused the problem. I felt that they did the easy job first, replaced the coil, then found that it still didn't start so then checked/replaced the sensor. But the mechanic insists that a bad sensor could fry the coil. Telling me that he checked the sensor on the oscilloscope which indicated that is was bad.

Can a bad crankshaft position sensor fry the ignition coil?

thanks!
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Old 06-12-2014, 10:44 AM
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I'd like to know how it could; but I'd like to know.

If it fails in a way to send constant voltage to the coil, the car won't run.

It it fails in a way that it sends voltage to coil in a high frequency chop, the car won't run.

So what would be the failure mode? Again, just trying to learn.

It's easier to change the CPS than any of the coils on my car, btw.
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Old 06-12-2014, 10:55 AM
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I don't believe a CPS can make a coil fail. I think your assumptions are correct. Poor original diagnosis.
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Old 06-12-2014, 11:23 AM
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In short, no. It cannot fry the coil that I'm aware of. It's a very low voltage signal that tells the computer when to fire the coil. No direct connection between the two that I've ever seen in modern vehicles.

A bad CPS can kill your engine instantly and prevent restart, though. It's the ONLY thing that ever went out on my 7.3 diesel. Driving down the road at 55 and BAM! Truck just dies for no apparent reason. Ford had a batch of bad CPS in that year and issued a recall.

My guess, not knowing your mechanic, would be that he knew one OR the other was bad and did a two-fer on both.
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Old 06-12-2014, 11:31 AM
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Originally Posted by sea tonic View Post
Sitting at a traffic light at the exit for our local Walmart (no I wasn't shopping there!) waiting for it to turn green. Engine just stops running! Turn the key….engine turns over fine (relatively new battery), I can hear the fuel pump operating, but engine won't catch, just keeps turning. To me, it seems like the ignition coil. Similar thing happened to it several years ago. Had it towed to our mechanic. The next day it's repaired. He replaced both the ignition coil and the crankshaft position sensor at a cost of $450. I am confused because to me, both the sensor and the coil shouldn't be bad, it should be one or the other that would have caused the problem. I felt that they did the easy job first, replaced the coil, then found that it still didn't start so then checked/replaced the sensor. But the mechanic insists that a bad sensor could fry the coil. Telling me that he checked the sensor on the oscilloscope which indicated that is was bad.

Can a bad crankshaft position sensor fry the ignition coil?

thanks!
What kind of vehicle is this?

Does your key have the embedded chip?

I know that if the battery in the transponder key dies the car won't start since it can't send a signal to the vehicle's computer. Some transponder keys don't use batteries, but I think most do.
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Old 06-12-2014, 11:39 AM
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Sorry, it's a 96 jeep grand cherokee v8 and the CPS is a PIA to replace, that's why I think they went the easy route first, replacing the coil.
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Old 06-12-2014, 01:38 PM
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No it can not. Your crank sensor has 3 wires ( signal reference, 5 volt reference, and sensor ground. It does not even share the 5 volt reference circuit with the coil. Your coil is a 2 wire set up, The driver wire and the return. They don't share the same circuitry. Impossible for the crank sensor to burn the coil out. Make sure they replaced the parts with factory Chrysler parts otherwise you'll be in the same boat in six months. Aftermarket parts don't last and Chrysler's.
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Old 06-12-2014, 01:54 PM
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Originally Posted by U561 View Post
No it can not. Your crank sensor has 3 wires ( signal reference, 5 volt reference, and sensor ground. It does not even share the 5 volt reference circuit with the coil. Your coil is a 2 wire set up, The driver wire and the return. They don't share the same circuitry. Impossible for the crank sensor to burn the coil out. Make sure they replaced the parts with factory Chrysler parts otherwise you'll be in the same boat in six months. Aftermarket parts don't last and Chrysler's.
Why would they have to share circuitry for the sensor to burn out the coil? When the stuxnet virus started destroying centrifuges needed for the enrichment of nuclear material, it did so by sending faulty information and causing the motors operate above and below their operating range. It didn't zap them.

I'm curious if bad information from a faulty sensor could shorten the life of a coil as well, but it seems like the ecu would throw some sort of fault codes if this were the case.
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Old 06-12-2014, 02:17 PM
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Originally Posted by jobowker View Post
I'm curious if bad information from a faulty sensor could shorten the life of a coil as well, but it seems like the ecu would throw some sort of fault codes if this were the case.
Bingo! I would suspect exactly the same. Not so much because it makes for a sloppy running engine, but because THEY HAVE TO monitor all this to ensure the vehicle isn't pushing too many emissions out the tailpipe! Stoichometric ratio is a wonderful thing and you ain't never gonna get it if the coil is misfiring.

Gotta love the EPA.
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Old 06-12-2014, 02:19 PM
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Flux capacitor! Get your check book out.
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Old 06-12-2014, 02:23 PM
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^ ^ ^ ^ ^ ^

I was sure you were gonna say: "Check for a dead cat under the hood!"
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Old 06-12-2014, 02:25 PM
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Their tails do like to get sucked into the serpentine belt, makes for a stinky mess.
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Old 06-12-2014, 03:07 PM
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had 94, 98 ,02 jgc crank sensor is 15 min job
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Old 06-12-2014, 03:21 PM
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Here, let me have a look at that......
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Old 06-12-2014, 05:50 PM
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A faulty CPS could tell the pcm that the coil needs to energize 100 percent and make it fry it fry from too much duty cycle. That's just in theory anyway. My team leader told me a story on a car he thought was hydrolocked or just locked up. Turns out either the CKP or CMP was shorted internally making an injector fire 100 percent duty cycle and filled the cylinder with fuel to the point it wouldn't turn over.

I take it you have the old 5.2/5.9 magnum?
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Old 06-12-2014, 11:00 PM
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Timing belt or chain?
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Old 06-13-2014, 03:11 AM
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The CPS is notorious for going out on Cherokees and not throwing a code.
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Old 06-13-2014, 03:59 AM
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If you have an "ignition coil primary circuit" fault code, it's the pcm. It's killing the driver on the coil. Just to rule it out, you should have a fuel pressure gauge on the fuel rail (there's a port to tap into) and make sure you have good fuel pressure when it happens. The pump can still be making noise, but not building pressure. However, I do suspect this is an ignition issue.
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Old 06-13-2014, 04:01 AM
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Originally Posted by hatterasman View Post
had 94, 98 ,02 jgc crank sensor is 15 min job
Not on a 5.2 V8, especially the ones with the egr tube back there.
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Old 06-13-2014, 06:37 AM
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I think the original work was a mechanic changing the possibly likely older or failed parts in the hopes that it fixes the problems. That being said, these intermittant shut-offs that can not easily be replicated can be tricky to track down.
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