Battery Discharge Problem

Yes I think you can reuse the old battery cable for the “chassis to engine” ground cable if the cable itself is in good enough shape. Don’t forget that this is a well used cable and you should replace it eventually, but for temporary use and for test purposes it should be fine.
The important thing is that you have a good connection between the chassis and engine. Since your battery negative is bolted to the chassis/frame, the purpose of this new cable/wire you are putting on is to make a complete circuit from the chassis/frame to the engine.
If you didn’t do this, then the engine, the starter, the alternator, and everything else electrical would be trying to ground through motor mounts, bolts, transmission mounts, and even “other” wireing on the car.
I am still thinking (75%) this may have been the original problem with your truck because you saw it arc/spark/crackle when you were changeing out the cable.

As far as the "unplugged wire with the green connector, don’t worry about it too much. From what your Dad said it is something from long ago and is not a root cause of this problem since it has been disconnected for a long time. If it goes to right above the oil filter, then someone may have had an oil pressure guage in it at one time because many manufactures have oil ports there for oil press. switches. Just put some tape over the end of it in case it’s a power wire, and secure it some place out of the way.

I looked at the pictures, and think you should run the negative cable to the engine block FIRST, then run a similar gauge jump wire from the block to the chassis. A good ground connection to the engine is a necessity for the alternator and starter to work correctly and the ignition system to run well. Currently, I have no idea how a high-amp starter circuit is connected on the ground side if the battery cable is not attached to the engine.

Well??? Did that fix it?
Since you havent posted for 3 days, I’m asumeing you’re out driveing the wheels off it. Keep us posted

Sorry…I have been juggling m schedule with one vehicle. When you drive 60 miles a day to school makes planning kinda tricky.
No, did not fix it…when I did the test headlights brightened.
I replaces the ground wire, and like previous suggestions I ran the Negative lead directly from the engine block…so the ground wire I replaces is running from engine block to chassie to body. Still discharging battery once system is off.
I am thinking of saving some money and taking it into Goodyear for repair…someone mentioned 30 year old truck possibilities endless. I wish their way a way to disconnect all essentials and work with just the basics.
Any more ideas?

Garth; If the lights are getting brighter as you rev the engine, then YOU HAVE FIXED THIS PART OF THE PROBLEM.—> (ground wire that you fixed)

In some of your previous posts you said the battery was going dead WHILE DRIVEING it (this was because alternator was not chargeing the battery) but you said the battery would recover if you let it sit overnight and then the engine would start.

If the battery is going dead while sitting, then there is a draw. Something is draining the battery at rest.
Before you spend any more money at Goodyear, take the negative cable loose “at the battery” and hook up a test light between the battery negative post and the cable. IT SHOULD NOT LIGHT UP. Make sure EVERYTHING IS TURNED OFF, ALL SWITCHES ETC. If it lights up, go to your fuse box and pull ONE FUSE AT A TIME untill the light goes out. Put each fuse back in before pulling the next one. This will isolate the circuit that is drawing the current from your battery. I can’t think of anything on your '79 that should be drawing current at rest unless there is a burglar alarm on it, or maybe a clock. When you find out which circuit it is post back.

From what Garth told us about the neg. cable, that is the way Toyota routed it originally from the factory, from bat. neg—> to chassis—> to engine
He replaced the cable from bat. negative to chassis, and then ran another cable from chassis to engine so it was grounded. It was the same routeing as Toyota apparently approved when they built it. I think one of the American manufacturers did it the same in the 70’s and earlier.

If you read the previous posts, Garth told that the mechanic used a hand held digital volt meter (with no amp meter)to do the test, so no amprege test for the starter. There was also a bad ground that was allowing the alternator to max out.

With everything electrical turned off, disconnect a battery cable from the battery post. Do you see a tiny spark, or a larger spark as you make the disconnection? If a larger spark, there is a parasitic battery drain. A good repair shop can chase this critter down and kill it.