Hi everyone! First post so bear with me, had a helluvah day yesterday working on my old truck and I'm a bit lost so looking for some electrical advice.

I'll start from the top that I'm a mechanical novice that has owned this 73 F-100 for about 8 months now and it's been a struggle at times but I love it to pieces.

The latest struggle has been with the electrical system. Over the last few weeks the battery has been consistently failing to charge itself while the truck is running, leaving me to yank it out and charge it myself every week or so to keep the voltage high enough to turn the starter. I had checked all the connections working through voltage regulator and alternator and found no wiring faults, but I'm not very good at this so may have missed something.

My suspicions quickly turned to the voltage regulator as the entire case and especially the connections were very rusty. I swapped that out and saw a modest jump in the voltage across the battery terminals while the truck was running (got up to 13.2V or so whereas before the running and non-running voltage were both in the 12.4-12.7V range). So I thought I had actually solved the problem, but give it a couple days and the battery is down to about 12.2V and unable to start. (I had btw been regularly checking for a parisitic drain off the battery and finding no current draw, and also disconnecting the battery terminals when leaving it overnight because I guess I'm paranoid like that haha!)

So yesterday I drove my ailing truck into work to get this sorted out (I don't have a garage to work in myself but I can sometimes get into my cabinet shop on weekends to get some nice dry, bright workspace). re-checked every connection I could find between alternator, voltage regulator, and battery. Couldn't find any faults but again might have missed something. So I ended up finding a replacement alternator and swapping it out. Hooked everything back up, re-checked the connections, and go to start it up and I get no cranking at all. The lights work, and with the key in the "run" position I get turn signals, wipers and can hear the electric fuel pump running.

So this where I start to get a bit desperate/creative because I have to get the truck rolled out of the shop somehow because tomorrow morning my boss won't be too pleased if my truck is still stuck beside our new panel saw!

My attention ended up on the starter relay/solenoid mounted next to the battery. I've attached a picture of the relay here (I think):

I tried bypassing the relay (from the battery connection on the left to the starter connection on the right hand side) with some heavy-gauge rod but all I got were big sparks. That's when i tried switching the black plugs on the middle (I and S) terminals with each other (I hadn't touched them at all that day but at this point I was trying anything to get power to the starter). With those plugs switched, the engine cranks just with the key in the "run" position. I don't even have to turn it to "start" and it's cranking and I got it running.

The good news is I was able to back the truck out of the shop and saved myself some grief come Monday morning. The bad news is the starter stays engaged while it is running in that condition so I only had it running for a few seconds before I shut it off.

So I don't know much but I'm guessing I'm either looking at something bad in or to the ignition switch, or something bad in the starter relay/solenoid. I'm not really sure where to go from here to diagnose and fix this but any ideas or suggestions would be greatly appreciated! Sorry for the long read but tried to cover all the bases and note any relevant info.

Oh and here's a pic of the truck itself in a fairer season:

 

Welcome to FTE, lots of good, helpful people here.
Just from the overall description of your problem I would have the battery tested to make sure it's holding a charge.
On the alternator output, it should be closer to 14.5-14.7 volts to keep the battery charged.
In the picture you posted of the starter solenoid I see a couple of problems, probably not related to your current problem though.
The two small terminals, the "S" and "I" terminals, the red/blue trace wire goes to the "S" terminal. This is the wire that actually signals the solenoid to start. The brown wire goes to the "I" terminal. This wire goes from the solenoid to the + terminal on the coil and is hot only when the key is in the start position.
Ford ignitions run at around 7 volts when running. There is a resistor wire that goes from the ignition switch to the coil that drops the voltage, however, to ease starting the brown wire sends 12 volts to the coil while starting. In the picture you posted I see two wires, a red wire and a yellow wire with a fuse attached to the "I" terminal. There should be no wires, other than the brown one attached to this terminal. I don't know where they're going but if you need them, find another power source.
Whats happening now is when you start, the wires will be fed 12 volts just like the coil, however, once you release the key, the 12 volt feed goes away and these two wires will now be backfed from the + side of the coil. This not only drops the voltage, but now is overloading the resistor wire and could cause it to burn out.
Nice looking truck BTW.

 

Sharp truck !
1 - Put the I & S solenoid connectors back to the way they were.
2 - Remove the red wire and yellow wire with the fuse from the solenoid I terminal; not
a good place to tap + 12v beacuse the purpose of that terminal is to feed +12 v to
the coil on start, after the engine starts the voltage at that terminal drops because
it is feed thru a resistor wire.
3 - Spray both large solenoid terminals with WD, remove the Pos battery cable from the
battery, remove the pos batt cable and all other wires from the solenoid, scrape
clean and reinstall. The do the same with the other large solenoid cable that goes
to the starter.
4 - Check the black wire with the rubber block on it, that is a fuse link that connects to
the large terminal on the alternator and remove the wire that is added under the
tape.
5 - Are the battery terminals the clamp on / replacement type?

 

Looks like the rats nest i had in my 73 all due to some body adding things in and no knowing what they are doing. I can't add any more advice at this time other than what you got from others. I did end up replacing my starter solenoid 4 times before finding a new one that was not junk , offshore crap.

 

Hi again and thanks for your advice so far!

Going in tomorrow to work on the truck again so I've got a list of things to check and clean.

The battery terminals are both clamp-on type that I tighten down with a wrench when it's running.

When I remove the two extra wires from the "I" terminal, where is a good place to hook them up to get a proper 12V connection? I don't properly recall where they go so that may be something I have to investigate tomorrow.

Thanks again and will update when I get it going tomorrow!

 

You said you have an electric fuel pump so it might be the red wires are for that.

 

Battery terminals - Are the battery cable ends part of the cable or are they the ones that clamp over the cable, held in place with 2- 1/4 X 20 bolts ? The ones that clamp on are not that great because corrosion can build up on the cable and you can't see it.

12 v connection - Probably the quickest and easiest way to provide a good accessory power source would be to purchase an accessory relay (available at auto parts stores), feed it with a 14 ga. or 12 ga. fused wire from the battery side of the solenoid and a 14 ga. or 16 ga. fused wire from the fuse block (fuse taps are also available).

Be sure that any wire tapped off the battery side of the solenoid is fused or is a fuse link type wire - don't need any fires caused be a shorted wire !

As Mikeo0o0 stated - test your battery first to make sure that it is a good one.