2001 Mustang GT P0340

Davisenvy

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Jul 14, 2016
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I have a 2001 throwing code P0340. The battery light was on so I replaced the alternator with a new one form American Muscle (the $100 one). The battery light is now off, but I am still getting the P0340 about every 2 weeks. I just had the alternator tested at Autozone and they said the alternator was fine, but the battery was low. The car is displaying symptoms of a bad cam shaft position sensor with a stumbling idle, runs very rough below 2k rpm (sometimes, especially when it rains), and every so often it is hard to start unless i press the gas pedal while starting. Could this be a true sensor problem? Or could this be from a bad battery? Are the $100 alternators from American Muscle junk? Any help would be appreciated.
 
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wmburns

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Aug 14, 2009
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Anytime the term "Circuit Malfunction" is used it means there's something so wrong at a basic circuit level that the PCM is not able to perform any meaningful diagnosis.

So if your sure that the alternator and charging system is in tip top shape, then the next logical item to check is the sensor itself or the wring to/from the sensor. A through visual inspection of the wiring harness and the connectors to/from all sensors.

>>>
P0340 - Camshaft Position (CMP) Sensor Circuit Malfunction

The test fails when the PCM can no longer detect the signal from the CMP sensor.
  • CMP circuit open
  • CMP circuit short to GND
  • CMP circuit short to PWR
  • SIG RTN open (VR sensor)
  • CMP GND open (Hall effect sensor)
  • CMP misinstalled (Hall effect sensor)
  • Damaged CMP sensor shielding
  • Damaged CMP sensor
  • Damaged PCM
Harness routing, harness alterations, improper shielding, or electrical interference from other improperly functioning systems may have intermittent impact on the CMP signal.
 

Davisenvy

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Jul 14, 2016
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Thanks for your response. I am going to get a second opinion tomorrow just in case AutoZone was mistaken. It just seems like every P0340 was an alternator problem and I haven't seen any threads where the actual sensor was at fault. Didn't know how often the sensors failed. The car has 125k miles on it. Most likely I'll be picking up a new battery and sensor tomorrow.
Thanks again for your help.
 

wmburns

SN Certified Technician
Aug 14, 2009
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Houston Texas
Here's some more charging system information that may help.

Howto perform charging system voltage drop test
https://www.stangnet.com/mustang-forums/resources/howto-perform-charging-system-voltage-drop-test.56/

I also remember helping someone with a cam problem after installing a ton of upgrades on their car. The problem turned out to be there were extra metal parts on the cam that were under the same circle as the cam sensor. The cam sensor was being "fooled" by the extra screws into sending extra cam pulses.

Obviously this sort of problem isn't likely to be solved without a lab O scope (or luck combined with experience).

I'm also a big believer in Occam's Razor. This tells us to look first for the simplest answer. That is why experience has taught us to rule out an alternator problem first. The next most logical thing on the list should be to perform a through visual inspection of the wiring and connectors.

And yes the sensors can and do fail even though it's rare. One way to test is to confirm IF the sensor is magnetic. However for the home DIY mechanic under some conditions the sensor is cheap enough and easy enough to change for pure diagnostic purposes.
 

Davisenvy

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KISS, Keep It Simple Stupid. I went to the auto parts store, picked up a new battery and new cam sensor. Got home and went to find the cam sensor to replace and trace the wires for any broken parts. The latch on the sensor connector was broken and the sensor was totally unplugged. I feel like an idiot for not finding this first, but the car ran pretty good without it plugged in. Anyways now the car idles so smooth and seems smoother through the rpm's. Thanks for the help and Merry Christmas.
 

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