Your transmission shifts are controlled by the PCM, Powertrain Control Module. It can be scanned for transmission fault codes which can guide your diagnostics. Autozone can do this for free. You would have to get the car there or find a technician who will bring their hand held scanner to our car.
But, try this first. Disconnect the battery for 1/2 hour. This will clear all the shift tables and the Keep Alive Memory (KAM). Reconnect battery and see if you now have forward gears.
If no luck with fault codes, this could be a possibility that requires a trans teardown.
Some vehicles may exhibit no forward gear ranges due to the misalignment of the forward/coast clutch cylinder snap ring with respect to the legs of the forward clutch piston.
During assembly of the forward/coast clutch cylinder, the gap in the select fit retaining ring should be located midway (i.e., 45 degrees) between adjacent legs of the forward clutch piston. Refer to the following Service Procedure for details.
- Prior to installing the forward and coast clutch plates into the cylinder, note the location of the four (4) forward piston legs ( Figure 1 ).
- Mark the top of the cylinder with a felt tip marker midway between any two adjacent legs (i.e., 45 degrees or 4 external spline teeth from the center of any leg) ( Figure 2 ).
THE FORWARD PISTON LEGS WILL NO LONGER BE VISIBLE WHEN CLUTCH PACK AND RETAINER IS INSTALLED.
- Following the Service or Workshop Manual procedure, install the clutch plates.
- Position the ends of the snap ring such that the gap is aligned with the mark on the top of the cylinder (the 45 degree mark) ( Figure 3 ).
- Perform end clearance check as required.