The solenoid is where most of your clues will be found and tests will be made. Luckily it is easily accessible near the battery. Ford systems are plagued with more loose connections than other manufacturer's systems because there are more connections in the high-current circuit. The copper studs and nuts are soft and tend to loosen over time. Loose nuts on the large solenoid studs typically cause a no-crank condition and you will hear the single loud clunk from the solenoid each time the ignition switch is turned to "crank". Start with a visual inspection and tug gently on the battery cables at the battery posts and the large solenoid terminals. Tighten any that can be moved.
Use caution when tightening the nuts on the solenoid or the positive battery cable. The wrench must not contact any body sheet metal part or the battery hold-down bracket while it is in contact with anything connected to the positive circuit.
When connecting temporary jumper wires, whenever possible, make the final connection away from the battery. Batteries give off hydrogen gas. Sparks near the battery could cause an explosion. A spark will occur at the location of the final connection of the jumper cable.
In the first scenario, no sound is heard from the solenoid when the ignition switch is turned to "crank". The problem is in the low-current circuit. The circuit can quickly be split in half right at the solenoid.
Place the transmission in park, (automatic), or out of gear, (manual) because the goal of the following tests is to find out what can be done to make the starter crank the engine. It will be terribly inconvenient is the vehicle leaves you or runs over your foot!
Remove the red plug-in connector from the top of the solenoid. Connect a jumper wire to the battery positive post or the solenoid stud the battery cable is connected to. Touch the other end to the small solenoid terminal as shown in Figure 1.
Figure 1. Bypassing the ignition switch circuit.
In this test, you are bypassing the ignition and neutral safety switches. If the solenoid clicks and the starter cranks the engine, you'll need to diagnose the problem in this circuit.
Figure 2. Measuring voltage in the high-current circuit.
Another no-crank condition is caused by a break in the high current circuit to the starter. You WILL easily hear the solenoid click but the starter will not spin. You could have a helper hold the ignition switch in the "crank" position while you measure voltages, but it's just as easy to use the jumper wire as in the previous problem.
Be aware the coils in the solenoid will overheat if the jumper wire remains connected continuously for more than 30 - 60 seconds. The solenoid could be permanently damaged creating a second problem in the circuit.
In Figure 2 the solenoid has been switched on with the jumper wire used previously, then voltage is measured on the switched side. No voltage is present where there should be full battery voltage. The break in the circuit is due to one of the high-current contacts inside the solenoid, but the same symptoms would exist when there is a break anywhere in that circuit.
Figure 3. Testing for a bad connection
in the high-current circuit.
Worn contacts inside the solenoid will cause a no-crank condition and you'll hear the loud click when it engages, but loose cable connections are much more common. Usually you'll find a loose cable by pulling on it and trying to turn it, but sometimes arcing takes place between the cable end and nut where they make contact. The burned area can block current flow even though the cable is tight.
Typically the resistance is way too small to measure with an ohm meter but it is possible to find that resistance with voltage measurements. The four measurements can be taken very quickly before the coils in the solenoid have time to overheat. Activate the solenoid with the jumper wire, then take the following readings until you find the one where there is no voltage.
These tests are done right at the large studs on the solenoid. The first measurement is taken on the terminal on the end of the cable coming from the battery. If voltage is missing there, check the two cable connections at the battery posts.
For the second measurement, move the voltmeter probe from the terminal to the stud it is mounted to. If this is the first place voltage is missing, that cable needs to be removed and cleaned. Remember it is still connected to the battery so be careful that no metal tools touch the cable and any metal part of the car at the same time. Also prevent the cable from touching anything metal on the car when it is unbolted from the stud.
The third voltage reading is taken on the other stud. If voltage is missing, there's a worn or pitted contact inside the solenoid and it must be replaced. That's what is shown in Figure 2.
Finally, move the probe to the terminal on the end of the cable and take the fourth measurement. Voltage missing here would be caused by a loose or pitted connection between the terminal and stud.
Figure 4. Measuring voltage at the starter cable connection.
If you still haven't found the bad connection, the last three places to measure are right at the starter motor. Point 5 is right on the terminal on the end of the starter cable. This is a common place to find 0 volts and is caused by corroded wire strands under the insulation. Replace the cable to solve this problem. An additional clue is that wiggling the cable will usually get it to work a few more times but as more strands corrode away, the high current causes the few remaining strands to burn open so the problem gets worse faster. As the number of good strands of wire decreases, the starter will crank slower and slower.
The next place to check is on the tab the cable is bolted to. As you can probably guess, if voltage is missing here, unbolt the cable and clean and tighten the terminal. There won't be any voltage anywhere on this cable when the solenoid is turned off so you don't have to worry about arcing between the wrench and other metal parts.
The final point to test is right on the case of the starter. If you find voltage all the way to the tab on the starter, point 6, there is a problem inside the starter, but to double-check, measure for voltage on the starter housing. If you find voltage there too, the ground cable between the engine and the battery negative post has a bad connection.