The Series Parallel switch on your truck provides 24 volts to the batteries when you are cranking the engine to start (very rare these days). Because of the "SP" switch you CAN'T "jump" the solenoid. There is no power at the starter on this system until the SP switch is activated by the key switch and then it's 24v (only while cranking).
These systems are very temperamental and require all components to be in good condition. The first thing you need to do is be certain your batteries are good. You need to have them "load tested". Charge them for about 8hrs. Then take them out and have your local parts store or battery dealer "Load Test" them. (if you don't charge them first you will almost always be told they are junk). They will do it for free. You need all 4 to be in working order. If not your 12v may work fine but when you need 24v to start it won't work.
Next, if you have any cables with corrosion, replace them. Corrosion will travel up the cable inside of the sheathing. If you can't afford to replace them hopefully they are long enough to cut (far enough back to get to clean wire) and then SOLDER on new ends. If you are trying to use "temporary" type bolt-on cable ends, they won't work very long if at all. Remeber when you buy those, they say "temporary"!
At this point you have new batteries (or at least verified yours are good) and good cables. The SP switch almost never fails so ignore it. If the starter is still dragging (and you didn't try to cheat on the cable repairs) then it's starter time. There is no real guideline for starter amperage draw for trucks because there are too many variables as opposed to automotive engines so you just have to be sure everything else is in working order before condemning the starter. The SP switch pretty much either works or it doesn't (12v to the cab at all times and supplies 24v to the starter when cranking) I won't get into the workings of the SP switch. Just don't mix up which battery cables go where otherwise you won't get your 24v when you need it.
Starter efficiancy has improved about 500% since '79. You might think about replacing your 6v batteries with 12v. Replace the 24v starter with a 12v. Then you only need 1 good ground and 1 good power cable going to the starter. And no SP switch. The key-activated "start" wire at the SP switch will hook to the new starter solenoid. A little more expensive right now but much more reliable.
Good luck.
for anyone confused by this.. of his four 6v batteries, there are 2 banks of 2. each bank is hooked in series to produce 12v. The SP switch keeps them hooked in Parralell for a constant 12v. When cranking, the SP switch hooks both banks of batteries (each producing 12v) in Series AGAIN, now producing 24v. A very complicated system to understand for those that don't enjoy doing electrical work!
Good Luck.
