Our G20 van has been sitting from 13-19 months. The alternator was bad, but it would run and drive fine. It’s just the battery that wouldn’t charge. We took it to the repair shop to fix the alternator, but now it won't go into drive. It only works in first or second gear. The speedometer doesn't work and the ABS light is on part of the time. We did some testing before now and the Vehicle Speed Sensor on the transmission tail shaft measures 1150 Ohms in a cool 60 degrees Fahrenheit shop. I counted 36 codes and 36 flashes. I disconnected the battery a few times. We think maybe it’s the VSS buffer and DRAC or the wiring going from the buffer to the VSS. The brake lights works so we don't think it’s a fuse problem. I don't think it’s an internal mechanical problem because it worked OK the last time it was parked. There are no signs of the wires being chews by mice. Do you know where the buffer is located? What are your thoughts and ideas on the other problems?
The VSS (vehicle speed sensor) buffer is located on the drivers side near the ECM. I have heard of instances where the connector between the VSS and the buffer has become disconnected. The wiring between the VSS and the buffer should be traced to see if this is the issue. With the bad VSS signal, gear shifts and speedometer operation will not be possible. You may want to enlist the help of a mechanic, such as one from YourMechanic, who will have the tools and expertise to diagnose the shifting issue.