I don't know much about electricity, so bear with me if this seems to be an elementary question:

The large ground wire from the back of the engine to the firewall is missing. What does this ground affect?

Also, my alternator is not charging; the needle on the AMMETER gauge is way over on the "D" side. Could that be a reason or just a coincidence?

Besides the junkyard, where can I buy a replacement? Does it have to be a wire, or can a braided strap be used?

 

A braided strap is the desired ground connection. A complete vehicle ground connection is optimal.The larger the strap, the better. As far as the alternator is concerned, the ground maybe a coincidence to a failing regulator or alternator. How old is the alternator/regulator ?

 

So, I can use a ground strap in lieu of the ground cable. Will any of the ones at NAPA work, or is there something particular I need? My intake manifold is made of aluminum, can I connect it to that? Or do I need to connect it to the iron block?

The alternator is only about 5 years ol, but you know how these "remanufactured" units are. The regulator is about 3 years old and it was an NOS Motorcraft unit.

 

Connect the ground strap to steel/iron. Exhaust manifold bolts work well. Or a trans to block bolt. The Intake bolts go into a cast block/heads so I'm thinking that would work.

 

I would trust the regulator over the alternator for the reason that you sighted...I would search out a 6 volt style braided negative battery cable for the ground if possible. They are thicker. If that proves to be a no-go, put two NAPA style braided straps together for "girth".
When it comes to wire gauge...."the bigger the better to fill up the sweater".

 

the strap from the engine to the firewall was nothing more than to ensure everything was grounded properly. The ground battery cable hooks to the block to give the best path for power to circle for the starter and then you have a strap to ground the block to the cab to ensure the cab is grounded since its mounted in rubber. Same goes for the core support

I generally go out of my way adding ground straps from cab to frame and from core support to fender just to ensure everything has a good ground.

 

Missing, loose, or corroded ground straps are a great way to cause all sorts of strange and expensive or hard to locate problems. The more interesting examples involve the engine starting current (around 250 amps) coursing through components never designed to carry electricity - throttle cables and brake lines glowing red hot, gearboxes and wheel bearings ruined, etc. Take the time to clean connections, use new cables, grind down to bright shiny metal and tighten securely. Chassis grease or NO-OX will help keep road grime and corrosion at bay.

 

If you are looking for an exact ground spec schematic, I am of no use. What I can tell you is...Alternating Electricity (AC) is polarized. One way in, one way out in full circle , regardless of amperage. In 12V systems, relays play a huge part given specific applications for voltage reduction purposes within their respective applications. What I will say is.....ensure that all of the grounds are constant (assuming they are connected to clean, bare metal surfaces with oxidation protection) to ensure that all positions of the vehicle are grounded given the rubber or urethane bushings used in cab and bed mounts. The same mounts that protect you from a lightning strike while you are in your vehicle, prevent vehicle grounding, hence the need for total grounding (without you in it).