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  #1  
Old 05-07-2007, 07:30 PM
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Cam Position Sensor - Multiple Failures - 4.6L

Was hoping to get some advice on possible cause of multiple CMP failures. I have replaced the sensor twice because of a bad cps code (checked at Autozone). Seems that a new sensor doesn't even last 5 minutes and fails again (check engine light comes on again, same cps code). Can anyone suggest what may be causing the Cam Position Sensor to fail so quickly?

Truck: 1998 F150 4.6L

Truck does run a bit rough (miss) between 1500-2300 RPM's.

Any input is greatly appreciated!

Last edited by Snook212; 05-08-2007 at 05:57 PM.
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  #2  
Old 05-07-2007, 07:44 PM
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Here's a thought - maybe your cam position sensor isn't the problem.

It could be a problem in the harness between the PCM and the CMP, could be RF interference from an outside source (usually the alternator), could be a base engine concern causing a noisy/erratic CMP signal, take your pick. You need to diagnose your concern instead of hanging parts on it at random.
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  #3  
Old 05-07-2007, 07:46 PM
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Its not the sensor. Post the code that you are getting and we can help. Remember when you go to Azone they sell parts and know nothing about diagnosing problems.
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  #4  
Old 05-07-2007, 08:16 PM
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Thanks for the reply!

I'm getting good voltage to the sensor (checked that). I test the sensor before I install and it passes an AC voltmeter fluctuation test. I put it in and after just a bit it spits the code. I pull the "new" sensor out and test it and it doesn't pass an AC fluctuation test. Something is killing the sensor. I'm not knowledgeable enough to know if RF interference and/or a base engine concern that's causing a noisy or erratic signal will cause a cam position sensor to go bad.

I didn't feel like I was hanging parts on it at random.
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  #5  
Old 05-07-2007, 09:02 PM
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P0340
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  #6  
Old 05-07-2007, 09:57 PM
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Question? Is the sensor showing any signs of rubbing on the cam gear?
I find it hard to believe the sensor dosn't respond to a resistance test and passing metal test 5 min after being run on the motor.
The cable harness has a grounded shield around it's two main leads.
This shield is the attempt to keep noise signals from being carried to the PCM over riding and affecting the timing of the injection.
The sensor has a winding on a magnetic core that makes it sensitive to passing metal on it's end. You should still feel the sleight magnetic affect and still read resistance.
The magnetics should never dissapear in that application but the winding can alway go open if subject to enough heat cycling to break the wire somewhere on the bobbon.
The cam gear has a slot that causes the cmp to sense as it passes and pulses a signal to the PCM as a reference for injection timing..
I think you have something else going on but still involves the CMP.
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Old 05-07-2007, 10:39 PM
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The sensor I have here on my desk (old one - was in truck for 6,000 miles approx) shows no sign of rubbing (on outer half/end of the magnetic cylinder) which I would assume is were the cam might rub it. It does have some wear on the o-ring (a 90 degree area) a small amount at the very base of the magnetic cylinder (right up next to o-ring) and also on the bottom side of the "arm" (for lack of a better word). All three areas of wear are within the same orientation or in-line. The other sensor is still in the truck - I'll pull it in the next day or so when I can. Unfortunately the third sensor is in a landfill somewhere. I did not notice this wear until you asked I only found it on closer examination.

They have all still been magnetic.

I appreciate the info and explanation on the shield and workings of the sensor!
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  #8  
Old 05-07-2007, 11:51 PM
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make sure there are no metal shavings on the gear inside.
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  #9  
Old 05-07-2007, 11:55 PM
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I posted earlier about my '07 5.4 with the same issue. It is funny that you say you have replaced the sensor because of the fault code. The dealer replaced mine earlier today. Told me all was good. I got it home and it died in the driveway.. took it for about a 10 mile drive and when I pulled in the driveway the symptoms returned. Rough and noisy idle, check engine light on again. I will let you know what the dealer finds.. it is going back first thing in the morning.
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  #10  
Old 05-08-2007, 05:40 PM
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Update - Pulled sensor again that is in there now (has been for about 200 miles). I put this sensor in a week or so ago (tested before installed it was ok, shortly after install light came back on, check code P0340, pulled sensor would not pass ac fluctuation test). This sensor shows no sign of wear anywhere- looks brand new. I see no indication of metal shavings.

Now for a new bit of info......I am getting no battery voltage to the harness - when I checked this a week or so ago I confirmed it had power, now it's getting nothing.

Last edited by Snook212; 05-08-2007 at 05:56 PM.
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  #11  
Old 05-08-2007, 08:39 PM
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You just found that there is an intermittant.
The code says error, not open or short.
The feed to it should be a Lt Blu.
This cmp lead is common to a number of other sensors and to the trans selector switch.
The dark green goes to the PCM as the signal input.
With only one code, I would suspect a splice failure at some point.
One way to see where it might be is to jumper from the cmp Lt Blu to the IAT sensor grey/ red lead and see if the fault goes away.
If so, try to find the problem or leave the jumper in place and dressed unless it picks up noise.
Otherwise cmp lead dark green goes to the PCM as the same color and may be going open somewhere in the harness or the plug.
Put you meter on and move the harnesses around to see if you get a reaction.
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  #12  
Old 05-09-2007, 12:33 AM
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Thank you very much for the help! I will try to figure out where the problem is this weekend. I will let you know result. Again, THANK YOU!
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Old 05-09-2007, 12:33 AM


 
 
 
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