1998 Infiniti Infinity QX4 Solenoid

You gotta be a lot more specific or detailed than that. If you're going to make me guess what you're doing, you know I'm going to guess wrong.

Why did you remove the starter from the engine? That is the most inaccurate way to test it. The starter needs to be under load from trying to turn the engine, otherwise it is real easy for a bad starter to appear to be okay.

Kindly list exactly where you connected cables, what you used for the 12 volt supply, and what happened at each step. Also, what was the symptom that started all this?

From the battery, the negative cable clamps to the starter's housing. The positive cable clamps to the large terminal on the solenoid that has nothing else attached to it. To make it engage, use a smaller wire to connect 12 volts to the smaller solenoid terminal that's inside the black plastic cover. On some older solenoids there can be two small terminals. Nothing bad will happen if you touch 12 volts to the wrong one. When you get the right one, the plunger will engage with a real loud, single clunk. THAT is what turns on the internal contacts to send current to the starter motor to make it spin. If you're getting that nice loud clunk, but the motor doesn't spin, the internal solenoid contacts are the main suspect, but we have to rule out a bad motor first.

While you have the solenoid engaged, if the motor doesn't spin, use a test light with its ground clip attached to the motor's housing or to the battery's negative post, then touch the probe to the second large terminal on the solenoid with the braided wire attached to it. That one is supposed to have 12 volts switched onto it when the solenoid is engaged. If you find 12 volts there, and the motor isn't spinning, it's defective. If you find 0 volts or very low voltage, suspect the solenoid.

Depends on the symptoms. Defective starter relay if one is used in your car. Defective neutral safety switch, corroded battery cable. Loose smaller positive battery wire where it's bolted to the under-hood fuse box. Rusty connection where the smaller negative battery wire is bolted to the body.

Do you have a test light and / or digital voltmeter and know how to use 'em?

I'm not familiar with your model but there;s usually some indication if the anti-theft system has kicked in, like a small flashing light on the dash, honking horn, or something like that. Most anti-theft systems stop the fuel pump from turning on. There are some that prevent the starter from working, but that isn't as common, especially on a '98 model.

Pop the starter back in, then see exactly what the symptoms are. I'm going to try to find a service manual with a wiring diagram, then I'll be back tomorrow night.

If your car has a starter relay in the under-hood fuse box, remove it, pop its cover off, reinstall it that way, then see if the starter cranks when you squeeze the contact. That is a quick test to see if two of the four parts of the starter system are working. That can cut the diagnostic steps we have to do in half.

Dandy. If the solenoid is activating, we're almost done. Grab your test light and put the ground clip on a paint-free point on the engine or transmission. Put the probe on the large terminal on the starter solenoid. The one with the battery cable attached, not the lower one with the braided cable. The test light will light up. Hold it there while a helper turns the ignition switch to "crank". If the test light goes out, you have a problem with one of the battery cables. If it stays bright, most likely the solenoid has bad contacts inside. (Those may not show up and act like that off the engine). If the light gets dim, either condition could be the cause or the engine could be locked up. Try turning the crankshaft pulley or the belts by hand. The engine will be hard to turn but not impossible unless it is locked up.

Forget any testing off the engine. I've never seen a starter test bench that puts a starter under load. While trying to turn the engine, that starter is going to draw perhaps 150 amps that has to get through the contacts in the solenoid. When it's just spinning freely, it is likely to draw less than 30 - 40 amps. 40 amps might get through the contacts while 150 amps won't. Also, you have to consider a bad connection on a battery cable, and those aren't getting tested with the starter off the engine.

Rather than rereading everything and trying to figure out where we are, grab that solenoid with the starter on the engine, then have a helper try to crank the engine. We want to be absolutely certain that solenoid is engaging. You'll feel and hear it. If it is engaging, grab a test light for the next tests.

Ground the clip lead to a paint-free point on the engine or transmission, not at the battery. Put the probe on the upper fat solenoid terminal with the battery positive cable attached to it. The light should light up brightly. Now have your helper try to crank the engine. If the light goes off or gets dim, there is a bad connection in one of the two battery cables. If the light stays bright, move the probe to the other larger terminal on the solenoid. It should not light up at first. It should light up when your helper tries to crank the engine. If it doesn't, the contacts are burned inside the solenoid. Those can be easily replaced on most Chryslers and Toyotas, but for yours you would replace the solenoid. If the light turns on there when the solenoid engages but the starter doesn't spin, it has worn brushes. Replace the starter for that. It will come with a new solenoid.