Expert: Roland Finston - 9/11/2011

Question
QUESTION: I will try to keep this brief but I have had many problems with my 2001 grand caravan .  3.3 engine over 150,000 miles.  
It started with my head lights the left side would turn on while the right side wouldn’t. if I turned on the high beams the right side would turn on and the left side wouldn’t work.  I put in new bulbs and for awhile it was fine. Then I noticed that the lighted box that surrounds the gears to let you know what gear you are in was surrounding all the gears at once. while I was driving on the highway the car shuttered and dropped gears   finally dying on me.  I put in a new ignition coil and speed sensor and was able to drive it home although it didn’t go above 3rd gear.  It sounded very rough when it was running.  I took it apart and found a cracked piston.  I had the cylinders machined as well as the cylinder heads I also took in the transmission to get checked out as well. after getting these back in and putting in new parts and sensors all along the way, I could only get the car to run for a minute before it died.  I did a continuity check on the connector for the ignition coil to the PCM it didn’t check out well and after finding a break in the wire and soldering this as well as a new connector  for  the ignition coil on I did another continuity check and it was fine.  When I tried to start the car I found I was not getting a consistent spark.  The plugs are new the cables are new and the ignition coil is new.  Could it be PCM or TCM?

ANSWER:  Hi Chris,
Try the ignition key to get fault codes:"On-off-on-off-on and leave on" doing that in 5 seconds or less elapsed time. Then watch the odometer window to see if the mileage is replaced by any 4-digit numbers preceded by a P. Let me know if you find some.
Roland

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QUESTION: i get code p 1684

Answer
Hi Chris,
That code means the power to the pcm was disconnected recently. That code will be set by the battery having been disconnected in the past 50-100 key cycles, or the direct battery connection to pin 46 of the gray/gray plug at the pcm that goes through fuse 17 in the power box is flaky (check that fuse for a crack in the wire, and check the orange/red wire from fuse 17 to pin 46). Usually it is set when the battery is purposely disconnected from a clamp, but if you haven't done that in the past 50-100 key cycles then the fuse and wire as described are worth a check.
That wire is the main power for the pcm, but there is also power when the ignition switch is in run or start on pin 20 of the other pcm plug (pink/gray wire) which comes from the ignition switch pin 3 which in turn gets the current from fuse #23. So I would verify that you also get solid power on that circuit at pin 20 of the pcm when the key is in the run position.
Roland