Where is the low side charging port for AC on a 1991 econoline?

The low-side AC charging port on a 1991 Ford Econoline van is typically located on the compressor itself. It's usually a smaller port than the high-side port. However, the exact location can vary slightly depending on the specific model and the aftermarket modifications that might have been done to the system.

Because it's on the compressor, you'll need to locate the compressor (a relatively large component with belts attached) which is usually near the front of the engine. The low-side port will be a fitting on the compressor, often with a blue cap or a blue-colored schrader valve.

Important Note: Working with refrigerant requires specialized equipment and knowledge. Incorrect handling can lead to serious injury and environmental damage. If you're not experienced in automotive AC repair, you should take your van to a qualified mechanic for service. Don't attempt to recharge or repair the system yourself unless you are properly trained and equipped.