How do you reset a computer on 1991 ford econoline van?

A 1991 Ford Econoline van doesn't have a "computer" in the same way a modern personal computer does. It has an Engine Control Module (ECM), also sometimes called a Powertrain Control Module (PCM), which is a type of computer that controls various engine functions. There's no way to "reset" it like you would a PC by restarting it.

What you might be experiencing, and what people often refer to as "resetting the computer," is actually one of these things:

* Disconnecting the battery: This is the most common method. Disconnecting the negative (-) battery terminal for about 30-60 seconds will clear the ECM's short-term memory. This can help resolve minor issues like erratic idling or trouble codes that are temporary glitches. Always disconnect the negative terminal first. Reconnecting the battery may cause some initial delays as the various systems re-initialize.

* Clearing trouble codes: A more precise "reset" involves using a scan tool to clear any diagnostic trouble codes (DTCs) stored in the ECM's memory. This requires a code reader that's compatible with your van's year and model. Simply clearing codes doesn't fix the underlying problem, but it allows you to start fresh with diagnosing the issue.

* Replacing the ECM (rare): In extremely rare cases, a faulty ECM might require replacement. This is not a simple task and generally requires a professional mechanic.

If you're having problems with your van, specifying the problem will help determine the appropriate course of action. Simply disconnecting the battery is often the first step in troubleshooting, but a mechanic might be needed to diagnose and resolve more complex issues.