To determine if their is an electrical/computer problem, you need to have the trouble codes pulled. Any Advance Auto or Auto Zone will do this for free.
If nothing comes up you probably have a clutch pack or converter issue and need a rebuild.
Take it to a transmission shop as opposed to a regular repair facility. AAMCO is overpriced, but you can use the search engine on this site to find a reputable tranmsission shop near you.
Monday, March 26th, 2012 AT 10:47 PM