I have a small block in my 70 Monte. Ever since I bought it 18 years ago, the starter needed to be given a jolt of juice once in a while. The previous owner had a jumper wire from the fuse block to the starter solenoid with a switch under the steering column to give it juice when needed. It has a good battery, starter doesn't make any funny noises, but sometimes it's just nothing at all (no click, grinding, nothing). I have replaced the battery cables and have tested the voltage at the battery and the starter end of it, both about the same. I think I tested it with the key on as if it was starting when I had the starter out last time to replace cables and it was real close there too. Do the solenoids go bad and if so, are they replaceable without replacing the entire starter? I know I could go with a new starter or mini one, but I was thinking maybe the solenoid may have an internal issue with the flow of power since it starts every time I hit the switch on the inside of the car. I'll have to admit, it is nice to have it not start with the key alone sometimes. It's like an anti theft device, but would like to see if I can fix it easily.

After a cup of coffee, I thought maybe it could be the ignition wire having an issue as well. I don't remember if I tested that when I had the starter out. I may test that next at the neutral safety switch in the console and again at the starter.

Number 1 issue is undersized wiring leading to low voltage causing slow or no crank.

But, if you get nothing, neutral switch or ignition switch are suspected. They could be worn or in need of adjustment.

You can jumper around both the neutral safety switch and ignition switch to eliminate them as the problem. However, since this appears to be an intermittent issue, that may not resolve anything during testing. Maybe next time it is acting up you can do the jumpering and see what changes..

Yes, solenoids can go bad.

Yes, they can be replaced.

However, it might be time to just up and buy a nice, shiny all new one and stop farting about with something that is old. Unless it's some sort of all original car that is and if it is rebuilding your old solenoid might be an idea to consider. No kits that I know of to do that, however, it can be done.

Well, dang, they do make a rebuild kit for your solenoid, assuming it is the original starter. Rock Auto has them. I;m used to old flathead mopars and no one makes kits for those. You have to hand wind your own, lol.

Yup, back in the day, a failed solenoid on a GM starter was not uncommon, and they are replaceable.
They were pretty chap back then, but we'd pull the starter to do that anyway.
Also damaged wiring, loose cable, etc.
If it's the original starter & solenoid, 54 years is a good long run.
But it's just as likely to be in the wiring to the solenoid.

What used to be, and what currently is may be completely different these days, but you used to be able to replace the solenoid on the old GM starters. You have to remove the starter from the car to replace the solenoid.
That said, I have to wonder if since someone ran a wire down to the starter, if the problem lays elsewhere (unless the starter is a royal pain to change). I'm betting the extra wire you spoke of bypasses everything between the battery and the starter. Someone didn't know how to chase down the problem, or couldn't find the problem, so they just added the extra wire. It is possible that simply replacing the extra wire with another will make it work "like it always has".

Personally, if it was mine, I'd replace the starter as a starting point, if its still the original, its spent a lot of time just sitting, or it could be completely wore out. You will have to pull it out to do just about anything with it, I probably would not put that old starter back in without a complete rebuild unless you know its full history. We have places in our town that actually rebuild the old starters and alternators, might be better then the new Chiniseum stuff on the market these days.

Number one problem when a GM starter just clicks and doesn't turn over is the solenoid. Number two problem when that happens is bad brushes. If you can rap the starter with a hammer and it cranks over, suspect it needs brushes. A jumper wire will cover up a lot of sins by delivering more voltage which just covers up the problem, but doesn't solve it. Either way, I'd rebuild the original starter if you want reliabilty, since many parts store rebuilds are carp. Another solution is replacing the old starter with a brand new super small PMGR starter. There are many later model cars that use them and older cars can be retrofitted, especially if the car or truck in question has a HEI distributor. The PMGR starters usually require different starter specific bolts, due to the starter having metric size mounting holes and the block having US spec threads. Some starters come with the needed bolts, some don't. Number of teeth on flywheel/flexplate determines whether a straight across or angled bolt pattern is used. Here's a PDF on Remy starters that can be interchanged.

Linky -->PMGR Remy Chevrolet starter info.

A 1970 Monte Carlo came with points, and a PMGR starter that has the "R" terminal would be needed. If the distributor has been replaced with an HEI, the "R" terminal on the starter is not needed.

If it works everytime with the bypass wire, I would say you have a switch or wiring issue.
GM's typically use a purple wire form the Ign. switch directly to the solenoid to energize it.
When the solenoid energizes it pulls a shorting contact forward that send battery power to the starter motor and on PRE HEI cars battery power to the coil bypassing the points resistance wire for starting. It also sends the Starter Bendix back to the flywheel engaging the teeth before the SM starts turning.

Energizing the solenoid is audibly noticeable. So while it is possible the solenoid is at fault, I would suspect the switch or wiring first.

I know it's not a heat soak issue, although that may have contributed to the problem, because I get nothing even after it sits for days. I have new cables as of last year. It is a purple wire to the solenoid along with the one hooked to the fuse box. The ignition switch on the column is new last year or the year before. Neutral safety switch hasn't been replaced or isolated yet. I was going to take the console lid off to tinker with that one day. Should be able to see which one is hot when I turn it to start and go from there as far as voltage. If I connect the hot wire when starting to the wire going to the solenoid with a jumper wire, that should tell me if it's the wiring or switch by checking voltages and isolating the switch. I'm not real mechanical, so this is going to be something I may ask more on. It's not an original starter, but works fine when I hit the button for the fuse panel power to it. I kind of don't want to replace the starter because I'm not sure how to shim it if it needs to be shimmed. Removing and replacing parts is fine as long as they match up old and new.

Chevy starter shims are made to be swapped without dropping the starter. It is done by removing the outer bolt and loosening the inner one. The shims are open on the inner end and have a bolt hole on the outer. The idea is for the starter to engage without grinding. https://www.amazon.com/chevy-starter-shims/s?k=chevy+starter+shims

Starter shim gap is set by sliding the allen wrench that comes with a new starter between the flex plate teeth, and the starter gear shaft on the flat parts of the wrench with the starter torqued to spec. Many people don't do this, or don't understand how to so they skip it only to get bit by it later. The starter should not make any strange sounds if the gap is set properly. You may have to cut shims in half to get where you need to be. Some instructions will show what the incorrect gaps look like, and on which side of the starter the shims need to go if it comes down to doing the half shim thing.