P0340 with electrical load

[3inarow2001]

Hi to all 2001 Explorer AWD owners,
Just threw a P0340 code (cam position sensor), so tried to fix it myself. Replaced the CMP, no problem but engine still stumbling, bucking. Googled problem, sounded too complex for me, so took it to the Ford dealers here. Mechanic checked CMP sensor alignment, wiring, etc. Basically, didn't fix it. He thought maybe a bad PCM. When I picked it up it ran perfectly, until I turned on the AC with the fan on high. It started bucking and stumbling again. Turned AC off, bucking stopped. It bucked (stumbled, or missed) with the fan on high speed, but not with the fan on speed 3. So I tried the headlights, bucking was worse. Now it bucked with headlights and fan in speed 3, heater or AC on.
Apparently the bucking is related to the electrical load. A few weeks ago my serpentine belt started squealing, only at startup. If a slipping belt/alternator/battery issue is causing the engine to buck and throwing a P0340 code, can someone explain why? And what should I check/replace next?
Thanks for helping. (I have had three 2001 Explorers in a row, but this is my first V8)

 

[1998Exp]

Seems that you have found what your mechanic could not! The problem with a CMP misbehaving with an overloaded alternator has been reported before. Without knowing the design details of this sensor I could only guess that its power supply is poorly filtered, and the increased ripple on the battery voltage shows up as a false signal out of the sensor. Alternatively, it may just hadn't been designed to handle less than 12V on its power input. Whatever, the exact mechanism, the way to solve the problem is to stabilize your alternator output. If the serpentine belt hasn't been replaced in 100,000 miles, it's almost certainly stretched beyond the tensioner's ability to compensate. You can confirm that by carefully looking at the tensioner; it has an indicator. Otherwise, you may just have a bad tensioner. There is also a chance that your alternator itself has a problem like worn brushes or a burned diode, but I would deal with the squeaking belt first.

Hi to all 2001 Explorer AWD owners,
Just threw a P0340 code (cam position sensor), so tried to fix it myself. Replaced the CMP, no problem but engine still stumbling, bucking. Googled problem, sounded too complex for me, so took it to the Ford dealers here. Mechanic checked CMP sensor alignment, wiring, etc. Basically, didn't fix it.
...
Apparently the bucking is related to the electrical load. A few weeks ago my serpentine belt started squealing, only at startup. If a slipping belt/alternator/battery issue is causing the engine to buck and throwing a P0340 code, can someone explain why? And what should I check/replace next?
...

 

[koda2000]

i'm thinking your alternator may be the problem.

 

[3inarow2001]

Big thanks to "1998Exp". You were absolutely correct. I went to Advance Auto to buy a belt and they volunteered to check my battery, starter and alternator. Found a bad diode! I replaced both the alternator and belt and everything started working, even the scanner! [My scanner would not link to the computer while this "dreaded P0340" code problem was going on.] I had a few problems with the reman. alternator-- its mounting holes were a little off, had to rattail file. Also the power stud was short and only grabbed a few threads of the mounting nut. I was not in a hurry, weather was great, game was on TV, but I got it done. Too bad there was no YouTube video of the Explorer V8 alternator replacement. It's a lot tighter than some other 5L V8 models. I had to make some creative tools to thread the serpentine belt thru and rotate the tensioner. If this alternator fails, I will make a YouTube video, now that I know all the traps. Thanks also to Koda2000, it certainly was the alternator.