Expert: Roland Finston - 11/5/2009

Question
Hi Roland.  I have 1999 Concorde with 2.7L 44000 original miles.  Mother-in-law had car since new & rarely drove over 50mph.  Problem is at speeds of 50-65mph -- it seems like the locking torque converter is hunting.  The RPM gauge fluctuates 600rpm's.  Once I pull over and turn off the car and then restart it, it will travel for 100 miles with no problems.  All city driving shifting patterns are fine. I dropped trans pan, put new synthetic ATF+4, & changed filter.  Took to trans shop-found code P1784-low/REV.pressure switch detected.  Could this possibly be a solenoid problem? If it is could you please tell me where it is located.  From your previous Q&A's it seems like I might need new solenoid. Thanks for your advise or help.  

Answer
Hi Mark,
Just one of my manuals list the details about that code. Given its title and the symptom you describe, my belief is that you would want to remove the valve body (with solenoid pack attached) and check out low/reverse switch valve. The code occurs when the TCM monitors the pressure switches and finds one to be in the wrong position relevant to the L-R element. When it is detected the t/c lock up is inhibited, as you suspect, based upon the 'hunting'.  You have to remove the valve body in this model in order to service the solenoid pack too so I believe that both are dealt with together. Earlier models had a separable solenoid pack that was mounted externally.  Once the body and pack are out, the L-R switch would be the item to focus upon. Without a manual that lists the possible causes though I am not in a good position to suggest other items to check out. Given the complexity, if you try this yourself I would recommend a manual to avoid the pitfalls.
Roland