The starter engaged while driving my 1996 Toyota Tacoma. The starter burned up. What would make this happen?
It could be a couple of things. The starter system on your truck is fairly simple. Turn the key and the ignition switch sends 12-volts to the starter relay. They starter relay then completes a higher amperage circuit to the starter itself.
Inside the starter is a solenoid which does two things. One: it physically engages the starter gear with the ring gear on the engine. Two: it completes the big-time amperage circuit (battery cables) from the battery to the starter motor itself.
What could cause the starter to take off on its own? A common cause would be the starter relay. The metallic electrical contacts inside the relay become corroded and stick together. More often than not, this happens when you try to start the vehicle -- turn the key and the starter engages, the engine starts, release the key but the starter fails to disengage.
The starter engaging without ever touching the key could also be caused by corroded relay contacts, along with a few other items. The ignition switch can be shorting out and applying power to the starter relay, or a shorted wire (somewhere) may be applying unwanted voltage to the relay or the starter itself. Either/or, the cause may be aggravated by bumps and vibrations while driving.
Your best bet is testing the circuits prior to hooking up a new starter. For instance, with the black & white wire which activates the starter disconnected, check the wire for voltage with a 12volt test light, with the ignition in the "Run" position. There should be no voltage. If there is, remove the starter relay (fuse block, left side of engine compartment).
If the voltage is no longer present at the starter, check for voltage at all the starter relay terminals. There should be only one "hot" terminal. If that's the case, replace the relay. If there are two "hot" terminals, it's either a shorted ignition switch or a shorted wire leading from the switch to the relay.