Starter will not engage. Have checked for voltage at the starter on the small wire. Nothing. I can engage starter and start vehicle by jumping battery cable on starter to small terminal. What would prevent voltage from getting to starter? Vehicle has been sitting for over a year without starting. Battery is fully charged.

This sounds like the starter solenoid isn't getting enough power. Jumping the solenoid is you simply bypassing it. Go back to basics. I may suggest things here you have already done but it helps to treat this as a simple check list.
- Check the fuse, sometimes the fix is that easy.
- Check the relay for the starter motor. It should click when the key is turned sending power to the solenoid.
- Check the voltage supply at the small wires at the solenoid. Using a multi meter set to DC Volts you should get around 12 volts when you turn the key. You might need a helper to do this.

When it won't start testing the electrical systems is the proper way to find a fix . Using a DVOM - digital volt ohm meter and power flow diagrams . Using voltage drop testing !
How to voltage drop test starter motor circuit
How to test starter engine doesn crank GM 3800 engine
Electric Testing Techniques You Need to Know
Do you know what a wiring diagram is ? Free wiring diagrams here http://www.bbbind.com/free_tsb.html Enter vehicle info. year , make , model an engine size . Click on engine under system then under subsystem click on starting .Then click on the search button , then on the blue link . This is a wiring diagram of the starter electrical circuit. Do you know what ignition switch is ? the electrical part. , how about the starter relay , transmission range sensor ! There is one thing you can try , move gear selector to neutral an see if it starts then . If that does nothing ,check for B+ voltage going at the starter relay . Jumping pins 30 & 87 at the relay socket , where the relay plugs into . will cause the starter to engage an spin the engine . If you look on the bottom of the relay you will see four sets of numbers 30 & 87 - 30 should have B+ voltage , 87 goes to the S terminal on the starter solenoid . An 85 & 86 are the control side .86 at the relay socket comes from ignition switch through the trans range sensor to the starter relay coil ( electro magnet ) . Watch these videos too .
VEHICLE RELAYS Testing from the relay base

VEHICLE RELAYS Operation Diagnosis

You are saying that the starter motor is not working, that the engine is not cranking over, even with a boost?

If so, then either there is a problem in the starting circuit between the ignition switch and the starter, or the starter is bad. There is a way to check the starter on the vehicle: the starter will have 2 electrical connections. One is the positive battery cable, and the other is a small wire connection to the starter solenoid, mounted on the starter. The key should be off, the brake set, and the transmission in park or neutral. Use a small jumper wire or a small screwdriver, and jump the battery cable connection to the solenoid connector. Take off the solenoid electrical connector if you need room to do this. With key off, the engine won't fire and run, but if the starter is good, then jumping those two connections will make the starter engage and spin the engine over. (Of course, if key is in ON, the engine should start and run.) But you are just checking starter operation, so keep the key off. If starter is good it will crank the engine over. If jumping the two connections causes nothing, then it is pretty certain the starter motor or the solenoid is bad, and you need to replace it. (Take off the negative battery cable from the battery first to remove the starter from the vehicle.)

If jumping the starter caused it to operate, the starter and solenoid are good, and there is, again, a problem in the start circuit from the ignition switch to the starter solenoid. It could be simple like an electrical connection (like the one right there on the solenoid), or a component in the circuit between the ignition switch and the starter solenoid, like the neutral safety switch for example. You will need a wiring diagram and a multi-meter to diagnose further. Good luck, and hope you find the problem. Are you sure your battery is still charged up good?

Check Battery, if you try to charge or jump starter you may hear a click. If you're sure its a good charged battery then consider starter. Try to jump starter by passing switch, cross small wire to larger wire on starter (you may get sparks) (please be careful if you try this) this should make the starter spin & kick in turning the engine. If it only spins then its probably the starter solenoid. If this checks out then check ignition switch & harness connections. That's it..

do you hear a click if you do its probably your starter cylinoyed if it works when you jump the wire you are bypassing it hope this helps pat

9 times out of 10 it's the realy but check if stater is getting power to crank wire

IF you can start the engine by jumping a wire from the battery cable to the solenoid input, you've probably wired the starter wrong.
Some GM starters have two small connectors on the solenoid; one is an input to activate the solenoid, the other is an output to the coil. If you connect those wires wrong, the starter won't engage.
If you have the two wires that go to the solenoid for your vehicle, find a volt meter, disconnect the small wires on the starter then have a friend turn the keyswitch as though trying to start the engine. Use the volt meter to see which wire becomes activated ONLY when the keyswitch is in the "start" position. The other wire may have some voltage when the keyswitch is in the "on" position, or it may not have voltage at all.
The wire that is activated in the "start" position should go to the post that activates the solenoid when you jumper it.
Hope that helped.

Look under the car at the Starter. If you just have the main heavy gauge cable going to the Starter, the answer is no. The Starter is spinning and the Bendix drive is bad.

If you have Solenoid wires in addition to the heavy cable at the Starter, you can try to jump the Solenoid under the car at the Starter. If the secondary wires (small) are not providing power to the magnet that throws out the Bendix, jumping power to the magnet will make the Bendix work. You do not want the Bendix to stay engaged or you will damage the Flywheel or burn up the Starter. You might as well check the Secondary (small) wires for corrosion because the problem is outside the Starter.

If the contacts for the magnet inside the Starter are burnt, jumping the Terminals will do nothing.

The problem is either the starter solinoid (located on fender near battery) or you are not getting power to the "s" terminal on the starter solinoid from the ignition switch. If you follow the positive battery cable from the battery you'll follow it to the solinoid. Remove the small wire from the terminal marked "s" on the solinoid and using a screwdriver jump from "s" terminal (not the wire) to where positive battery cable connects to solinoid. (with battery connected). If starter doesn't do anything the solinoid is bad. If starter cranks then you are not getting voltage from ignition switch through "s" terminal wire.

resistor.............tell the mehanic