Can you reset the computer on an 89 Ford 150 4.9 liter by disconnecting the positive battery cable and touching it to the negative cable?

You can, but it's not a good idea. You risk damaging the ECM that way. The best and quickest way to reset the ECM (without a scan tool) is to: 1) Disconnect the negative battery cable. 2) Turn on the headlight switch. 3) Wait 2 minutes, and turn the headlight switch off. 4) Reconnect the negative battery cable.

Doing it this way ensures that all of the capacitors are drained. The ECM will need to "relearn" your driving habits afterwards, which may take 30-60 minutes of driving.