This has been a real common symptom on Fords for decades, but the cause has changed. In the past it was connections on the solenoid on the inner fender that worked loose. Tightening those nuts took the better part of fifteen seconds. On the newer vehicles, they still use that same part on the inner fender, but they call it a relay now because it does not have to switch very high current like it did before.
Your starter should have the solenoid built onto the top of it like GM did for many decades. The most common cause of a failure to crank now is the battery cable is corroded away under the insulation where you cannot see it, right at the starter. To verify that, use a test light to measure the voltage right on the stud on the starter solenoid that the cable is bolted to. You will find twelve volts there and the test light will be full brightness. The issue now is to catch it while the problem is occurring. Under normal operation, that twelve volts will get drawn down to as low as ten volts during cranking. You will see the test light get a little dimmer when a helper cranks the engine. If the problem occurs, and it is due to that corroded cable, the voltage will drop to zero volts during the attempted cranking, and the test light will go out.
It is also possible, but much less common, to have a bad pair of contacts inside the solenoid. If that is the case, the test light will remain full brightness when the single clunk/no-crank occurs. That full twelve volts proves all the rest of the circuitry is okay.
Saturday, June 10th, 2017 AT 8:16 PM