2002 Altima no crank , start . I have already taken apart & cleaned all connections grounds etc . power @ relay , no power @ solenoid terminal while try to start ,12 volts @ battery

Hey! If your 2002 Nissan Xterra attempted to start without the keys in the ignition after replacing the starter, there may be an issue with the electrical system or the starter installation. Here are a few things you can check:

1. Wiring connections: Double-check the wiring connections on the starter. Ensure that the positive and negative cables are securely attached to the appropriate terminals on the starter. Make sure there are no loose or exposed wires.
2. Starter solenoid: The starter solenoid is responsible for engaging the starter motor when the ignition key is turned. It's possible that the solenoid is malfunctioning or wired incorrectly. Check the solenoid connections and wiring to ensure they are properly connected and in the correct configuration.
3. Ignition switch: It's unusual for a vehicle to attempt to start without the keys in the ignition. There could be an issue with the ignition switch or the wiring related to the ignition system. Inspect the ignition switch and its wiring for any signs of damage or loose connections.
4. Security system: Some vehicles have a security system that can prevent unauthorized starting. If your Xterra is equipped with a security system, ensure that it is not causing the issue. Refer to the vehicle's manual or contact a Nissan dealership for instructions on troubleshooting the security system.
5. Starter replacement: If all the wiring connections and components appear to be in order, it's possible that the replacement starter is faulty or incompatible with your vehicle. Consider double-checking the specifications of the new starter to ensure it is the correct model for your Xterra.

If you are unsure or uncomfortable with performing these checks yourself, it is recommended to consult a professional mechanic or contact a Nissan dealership for assistance. They will have the expertise and tools necessary to diagnose and resolve the issue with your Xterra's starting system.

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.

I assume you mean cranks? There isn't much to the starting system, you have the ignition switch, the starter relay, and the starter then of course the wires and connections. If you have made sure that there are no corroded wires and all connections are clean and tight, I would take a closer look at the ignition switch. make sure the signal is going out to the relay.

Does your key switch work good? Do the warning lights and gauges on the dash all turn on with key in on? That verifies the ignition switch has power. If not, check fuse #9 in the Power Distribution Center (under the hood). It is a 30 amp fuse, providing power to the switch.
Are the battery cables clean and tightly connected at the battery and starter and the battery ground connection?
There is an Engine Starter Motor relay in the Power Distribution Center, the only thing between the ignition switch and the starter. With key in start, power is sent from the switch to this relay, and if relay is activated by the PCM (engine computer), then power goes on to the starter solenoid on the starter, and starter should then operate and crank the engine. If you have a voltmeter, you could do some tests for power at the terminals the relay plugs into. There are only 4 terminals. Post back with comments, or if you need help with testing those terminals.

Let's see if I can explain it. Pretty simple start circuit, but starting issues are very common to Hondas and Toyotas of the '80's and early'90's. Lots of guys used to rewire their start circuits with new wiring and relays from ignition switch to the starter solenoid just to stop those no starts.
From the ignition switch is the black/white start wire, it is hot only in start, goes direct to the "starter cut relay"- a 4 pole starter relay, the black/ white wire (black with a white stripe) is both the power feed and the coil power to the relay. Before the wire enters the relay it is spliced. one wire goes to power feed at relay, other wire goes to coil side of relay. Here's where the clutch switch comes in: the clutch switch acts as the ground for the relay's coil side. Okay, you turn key to start, power goes to the relay on both the power side and the coil side. When clutch is depressed, it grounds the coil side of relay, this energizes the relay, so power goes out of relay on the power side, as a black/red wire...and goes direct to the starter solenoid. Somewhere before it reaches the solenoid, it turns back into a black/white wire. But nothing else is in a wiring diagram for your start circuit. Buy a can of CRC electronic cleaner. Spray all connectors in the start circuit. Pull off the solenoid wire at starter-that connector. Also the starter relay , pull it out and clean the relay terminals where it plugs in. Get all clean connections in the circuit. If no help, swap in a different relay. If still no help, get a voltmeter (or a test light), when the starter doesn't want to work, check for power at the starter solenoid with key held in start, and clutch depressed. If power there, the solenoid is the problem. If no power, work back to the relay. Pull relay out. With key held in start, two terminals for the relay should have power-the coil plus side, and the power feed from the ignition switch. If both have power, may be the clutch switch not grounding the relays coil side. If no power to relay, check the start wire out of the ignition switch.
Need a wiring diagram? This site is awesome-free wir. diagrams and TSB's for every make and model:

Change the actual ignition switch? Test light at starter signal wire while someone turns key to see if you have power at signal wire. Check all your grounds after that. Still nothing take steering column apart and change ignition switch .

Here are some possible reasons:

Park/neutral switch is open.
Poor ground connection to starter.

You can test the park/neutral switch by either jumping the solenoid (connect the hot terminal to the start terminal to see if the starter cranks--if it does, the ground is good) or by interrogating the starter relay socket. To interrogate the relay socket, use a 12 volt test light connection the clip to battery positive. Probe the socket terminals with the test light. If only one terminal lights the light, your park/neutral switch is bad or the start wire to the solenoid is bad.

I had that problem on the exact same vehicle but before I completed diagnosis it got hit in the rear and totaled.
Ill give you what I had and my additional thoughts so you can take it from there..... The two terminals on the safety switch that are connected when starting, do not supply power to the relay but a ground. If you take the power distribution center apart, you will find one connector on the starter relay has two small black wires going to it. that is ground. What I did, since I did not have time to find out why the ground wasn't being completed was to run a momentary switch (push button) to that connector. That completes the ground so you can get it started. If you use a permanent connection and just ground it, the check engine light will come on so don't do that. So, you could either do that, or find which lead on the neutral switch supplies the ground and just make a new ground there or find which original ground point isn't connected well. Terminals B and C on the switch connector are the two you will be looking at. one goes to the relay the other to ground.

Assuming you have a solid ground connection from the battery to the engine, try shorting the hot red terminal on the solenoid to the smaller terminal (not the ground terminal!)--the motor should crank. If not, you know the starter/solenoid is bad. If it cranks better than from the ignition switch, back up and check for bad starter relay, etc. Let us know what you find out, and we can continue troubleshooting.

Check the transmission neutral start inhibit switch is not the cause of your problem. (try both 'P' to 'N' positions).
If the headlights work ok there should be enough power in the battery to at least make a solenoid type noise when start is attempted- so it may not be caused by a faulty alternator.
Check the battery terminals are clean and firmly attached to the battery,
Check battery is properly grounded to the chassis and motor,
Check that +12v is available at the starter motor engage solenoid when the ignition switch is held in the 'start' position - check any start slave relay operation.
If all above is OK, you may have a faulty starter motor.