My father-in-law's 2000 F150, 4.6L has intermittent no start, no cranks issues. Now it won't start at all (cold Canadian winter right now!)... Just a single click happens. We rapped on the starter with a hammer, still just a single click.

1. The battery was old, so we replaced it with a new one with more cold cranking amps. The battery terminal clamps are a bit worn but with battery terminal shims they attach nice and tight to the battery terminals. Headlights were nice and bright but still a single click coming from the starter solenoid (bolted to the firewall, passenger side).

2. We went and purchased a new starter solenoid for $12 (with four terminals, 2 bigger copper terminals, 2 smaller terminals marked "S" and "I"). We put in the new starter solenoid and the instruction said to connect the small lead to the "S" terminal and leave the "I" terminal without a lead. Then we had no power at all to the entire truck.... Now what? Bad battery cable? Fuse? We haven't swapped the old starter solenoid back into the truck to see if power is restored or not...

 

Sounds like you now have the cabling wrong. Did you take a photo or make a diagram before removing the cables?

 

I did take a photo and I'm quite sure I hooked the cables up the same as they were... puzzling... maybe the new starter solenoid is dead... would that cause the truck to have no power at all?

 

The lead that goes to the main fuse panel is on the wrong terminal. It has to be on the same terminal as the feed FROM the battery.

 

I am puzzled on how I hooked the cabling up wrong... I will have to post a pic of the original cabling hook up...

 

If the truck has "no power at all", the power lead for the fuse box isn't connected to where it needs to be.

Don't ask me how I learned that many years ago...

You should have three terminals on the relay, two large, one small. One of the large terminals is connected to the starter motor. The other large terminal has the lead from the battery and at least one lead that feeds the rest of the vehicle. There may be several connectors on this post. If you put this lead (and the rest) on the wrong post, the truck will never have power.

Here's a typical relay wiring from another thread:

 

This is the hook up with the original solenoid we suspected to be faulty... at least we had power to the truck, and the headlights were nice and bright...

 

Okay, where's the "now" picture?

 

I just texted my father-in-law and asked him to email me a pic of the new installed starter solenoid... stay tuned!

 

One additional possibility is that the fusible links in the harness between the battery positive terminal and the starter relay bottom terminal have been blown. A quick voltmeter check between that terminal and ground will tell if that's the case or not.