i have 1999 3500 automatic.the transmission will go from high down a gear at about 45 mph. somebody told me it was the ground wires,so i cleaned them and that helped for a while.It keeps doing this ever 3 days i can take the ground loose from the passenger side and clean it and it will help here again for 3 days.i have even run a ground from each battery to each other with no results. does anybody else have this problem can you please help me Thanks

 

Which grounds are you cleaning ?
There are a few of them , with this issue , if it is ground related , you need to know where the computer grounds are , the computer controls the lockup & some other shifting , but the computer needs to see a number of things in order for it to make the correct decisions , things like the brake pedal switch .
You need to do some diagnostics 1st before replacing anything , but finding all grounds , cleaning & putting something like dielectric grease on to keep in good condition , is always a good thing to start with .

 

Take a Ignition condenser and wire from alternator output terminal to ground. Or you can buy the $75.00 kit from some of the tranny suppliers. You may have a alternator with the slip rings dirty or weak condenser inside alternator. Ask me how I figured it out...

 

There is a flash for the PCM that is a good starting point,then start cleaning electrical noise that may be induced from many different sources.Flakey batteries should not be overlooked.DTT has a noise filter that will cure 99% of the cases.
http://www.dieseltrans.com/phpBB/viewtopic.php?t=405

 

thanks alot guys you have been lots of help,Donnie in Shelby N.C.