Found a broken wire today. One end bolts to firewall. The other end to the head near #4 plug. Toyota's description is the No. 1 Bond Cable. Is this a ground wire? With the amount of sludge on it I've been driving with it broken for awhile.

 

It is a ground cable

 

I made a new one with what wire I had on hand. It's not the same gage as original. How important is the wire gage? Just want to know what priority I need to give this.

 

Should be fine so long as not spiderweb thin.
Anything bigger than 16 gauge should be fine.
Doesn't even have to be insulated.

 

Put another one somewhere else. I double ground everything now with the new electronics. You can go frame to cab down by the starter or even by the back of the cab

 

That ground grounds the head. That's where the spark plugs get their ground. The head(s) don't get a good ground through the block. They get one, yes, but it's not the best. That's why they have what's known as a bonding strap over to the sheet metal of the cab. That way it has a direct ground path to the battery. Presuming a ground from the sheet metal of the body to the battery negative is in place, and in good shape, as it should be.

That's why there's a bonding strap from the alternator up to the mounting for the power steering pump. That way the alternator has a good, direct path through the head to the battery negative.

I would make the new bonding strap at least the same gauge as the old one, or slightly larger. The Toyota engineers made the bonding strap the size it is for a reason. They've been known to do some stupid things, I grant you, but they weren't entirely useless. I'd wager the bonding straps were assigned to a junior member of their team, who did his very best to try and impress.

After all, they DID come up with the COR system. You gotta give them that....
Pat☺

 

Thanks All! I definitely plan to remake the ground with at least the same gauge wire. I just didn't want to drive it without one so went with what I had on hand. The truck is my DD. I don't remember seeing any wire that size at a local auto parts. The part # for the ground strap is discontinued otherwise if reasonably priced, I'd have ordered one.

I'm wondering if it had been broken free for awhile or when I pulled the valve cover I knocked it loose....

 

Most parts stores have generic ground straps in different lengths, search amazon for ground strap and you get a lot of options.
Just measure how long it needs to be. If it were electronics I would suggest going for quality, but for just braided metal or copper wire you should be fine.
You could also use a negative battery cable (black).