Recently in our forums, user maryjean posted an issue regarding her vehicle's starting system. Most starting issues are caused by a defect in the starting circuit or starter motor. The first thing to check in maryjean's case is the starting circuit, starting at the battery terminals. Her vehicle's symptoms indicate that it's likely that her battery cables are not connected tightly enough to deliver enough juice to get her car started. Either the positive terminal connection is not tight enough or needs cleaning or the ground cable is loose.
The starter solenoid could also be the culprit. Since the solenoid is part of the starter on most modern vehicles, we'll start there and go through diagnostic procedures on a starter motor.
Starter "Clicks" When Key Is Turned
If the battery connections are good, then the most common result of a bad starter or solenoid will be a "clicking" that does not result in the starter turning over the engine. This can also be caused by the electrical problems above, but may also indicate a bad solenoid or a defective starter.
Using a multimeter, test the connections at the starter for continuity with the battery and for output when the starter is engaged (keyswitch is turned to "start"). This is best left to professionals, as it can be dangerous, but those who know automotive electrical and how to safely work with a live engine and starter can do this test.
The total number of volts and amps that should be going to the solenoid are dependent on the vehicle's makeup, but the "in" for the solenoid will be similar to the battery's output expectation. The connection from the solenoid to the starter (if they can be tested separately) will be at the starter's specification, which is much higher.
If the battery tests as okay and the coil tests as OK, then the problem is with the starter itself.
Grinding or Spinning Noise from Starter
if the starter works but isn't engaged, then a whining or grinding noise will be heard as the starter fails to engage the teeth of the transmission's flex plate or starting gear. Check for loose bolts or a broken starter shaft housing, which may be the reason engagement isn't happening.
If the problem isn't easily diagnosed, removing the starter and testing it on a bench is the next step. Pull the starter from the engine/transmission, taking care to carefully unplug all of the connections with the vehicle's power disconnected.
Place the starter on a bench and secure it in place. Then, using a 12V battery or power from the vehicle's battery, place a ground nearby and active power to the starter's input circuit. The starter should whir loudly. Check the engagement teeth on the shaft and the starter's body for damage. If any, replace the starter.