Modern starter solenoids are an integral part of the starter. These solenoids serve two functions. They act as a relay, allowing low-amperage device like the switch, to control the high-amperage starter. The solenoid also moves the drive gear of the starter into a meshed position with the flywheel gear. This movement allows the gears to engage during cranking and release when the engine starts.
Older Ford and other vehicles used a solenoid that was external to the starter. This design acted as a relay and engagement of the starter gear was handled by another electromagnetic device, within the starter. Both solenoids act in a similar way, with regard to current flow.
Battery voltage is connected to one of the large terminals of this solenoid. When current is applied to the smaller switch-terminal, magnetic force closes an internal switch and connects the battery terminal to the starter output on the other side. This supplies voltage to the starter to crank the engine. The other small terminal is not used for cranking and was often used as an ignition bypass during cranking.
The starter mounted solenoid acts much like the external model. Designs vary from one vehicle to another and sometimes, there will be more than one small terminal. In this case, you will need to determine, which connects to the starter switch and operates the solenoid.
If there are more than one small terminal, a check should be made for current between each and ground. Current between the terminal and ground, without the switch turned to crank, means this is not the switch terminal. No voltage should be present on the small terminal until the key is turned to crank. The proper terminal energizes the starter solenoid when voltage is provided in crank position. As with the external model, battery voltage is present on one large terminal all of the time and on the other when cranking the engine.
Energizing the small terminal causes the solenoid plunger to move forward. The solenoid also moves a copper washer and connects battery voltage to the starter motor. The other end of the plunger moves the drive gear into a mesh position with the gear on the flexplate, which turns the engine. Full voltage at the solenoid terminal and at the large terminal and no cranking, shows the starter or solenoid are bad. A click when the key is turned to crank, and no voltage on the lower-large terminal, usually indicates a bad starter solenoid. Voltage on the lower terminal, and no cranking means the starter motor is bad.
No signal to the starter when cranking
No voltage on the small terminal when turning to start, means the starter signal is being interrupted. Try holding the key in the crank position and shifting the gear selector from park position into neutral. An engine that immediately starts to crank suggests the neutral-safety switch is bad. This switch keeps the vehicle from cranking when not in park or neutral. The park position can wear out, and the neutral contact may still work.
A vehicle that still has no current to the starter, will usually have a bad ignition switch, an open circuit or a security system that is blocking the starter. It may be less expensive to have a professional diagnose these problems.