Transmission not shifting automatically

The answer to this is pretty straightforward. Before, say, 1990 as digital electronics became more refined, a series of gears and other mechanical features chose a specific gear, once you chose the gear you wanted to access with your gearshift selector.

There was a problem with this. As transmissions were called on to become more exacting with regard to upshift points — due to emissions — mechanical means of choosing gears just couldn’t cut it. If you needed a certain engine speed/tranny speed combination to maintain emissions, the mechanical way of doing things just couldn’t cut it. Vehicles couldn’t meet the more exacting shifting standards that were needed to meet emissions requirements.

Enter the digital computer system. While microcomputers were being used as engine control modules (ECM) or transmission control modules (TCM), giving a greater measure of control, there were still areas where mechanical features were used.Taking advantage of the greater precision offered by solenoid (switch) technology, the automakers opted to use solenoids to control the gearing chosen.

Using solenoids allowed greater precision in gear choice with regards to engine speed and transmission matching so that emissions control could be more precise.

Usually, they opted for two solenoid packs, one on the inside of the valve body and one on the outside, to set gearing. Each solenoid controlled two of the gears. For example, let’s say solenoid pack 1 switch 1 and solenoid pack 2 switch 2, put you in first gear, while pack 1 switch 2 and pack 2 switch 1 represented second and so on. Each gear was represented by two switch settings.

When those solenoids fail, you can no longer shift, other than mechanically as you state.

So, you are either going to have to do it yourself or take the pickup into a service area where the two solenoids will be swapped out and replaced. I think this will solve your problem. You are lucky as this is one of the more modest repairs costing about $600 — give or take a little — for parts and labor and materials.