Been meaning to get this done. I have a spare starter solenoid and wanted to see what would happen if I introduced a voltage drop like one would see if there were bad/dirty/corroded connections in the start circuit. Using a voltmeter from the spade terminal of solenoid ( where the large yellow wire would hook up) to ground for voltage measurements. First used a large 12 ga ground wire to the negative side of the solenoid. Same point as where the starter motor wire would connect.
Then using a 12.2 volt car battery, hook up a small (22ga) wire to positive spade connector. Voltage = 7.0 and solenoid would not operate. Next used a 20 ga. wire , voltage = 8.4 and solenoid operated, but slowly. Next used 16 ga. wire, voltage = 9.6 and solenoid operated fast. Finally used an additional 16ga. wire to double current carrying capacity and voltage = 10.2 and solenoid operated very quick.
Trying to count all of the connections between the battery and solenoid spade, I have found that there are a lot, and all cars will not be the same. However, in order of the probability that they are bad, here they are.
1- the 4 lugs at the positive of the battery clamp. They are exposed and could be corroded inside the barrel of the connectors.
2 - Next bad place would be the spade connector at the starter solenoid with the yellow wire. Exposed to water/heat,etc.
3 - the relay board 14 and 12 pin connectors. Dirt, oil film, corrosion, and heat.
4 - the seat belt ( if your car has them) relay connectors or jumper wire.
5 - the key switch and/or connectors at the key switch.
Due to the possibility of corrosion inside the barrels of the crimp on connectors, I think it would be a good idea to lightly solder these connections also. Keeps dirt and corrosion out.
While I had this inside and testing, I hooked up a diode across the solenoid coil and it made a significant difference in the arc when I disconnected the battery during testing. There is still some, but much less. I will be soldering a diode across the solenoid coil of mine before I bring it out for spring. I think it can help and it certainly will not hurt.
Hope this will help some of you guys with this mean old starter circuit.
Tom

Nice test ! Here is a handy dandy calculator that sometimes helps to.

http://genuinedealz.com/voltage-drop.html

Nice work!

Aslo remember, the wiring harness itself is long past it's life cycle. Corrosion has already started to attack the copper throughout the length of wire causing further voltage drops. Our cars were never supposed to last this long.

When is Jeff Bowlsby going to make a complete new harness kit for us?

Good job, but it's old news (like 20+ years) to me. It takes 9 volts to kick in the bosch solenoid.
I've used fender mount Ford solenoids since the 80's, they only take 3 volts to kick in and then I have a full 12 volt right to the starter. I never cut any wires or drill holes, 20 minutes work and less than $20 in parts. Small mod to the bracket and it bolts right to the tranny.
Works every time.

underthetire, that is a nice link. Thanks.

kg6dxn, Yep, you are so right about the age of the harnesses. I think a new one would be great, but most of us wouldn't want to pay the price.

Mark Henry, Yep, old news to some, but we still get people on here who have a no start from the starter and think it can only be the starter. I'm just trying to show that there are other things that could be wrong. Just taking the starter off will "disturb the connections and when the new starter is put on, it works just fine for a while, then back to intermittent. Whenever you disconnect then reconnect connections in an electrical circuit, you may have just changed the symptoms. If guys will take a more structured look at why the starter isn't working all of the time, they could save some money. I would imagine that a good 50% of the time a new starter will only mask the problem for a while, unless some attention is payed to the connections. I agree the wire in the harnesses is degrading also, but the connections would be my first checking point.
By the way, I would agree wholeheartedly with the inexpensive fix using the Ford relay. By using the Ford relay, you are getting a much smaller back EMF arc across the key switch also. Actually, any relay that draws less amperage is better for that. The bosh solenoid I have as my spare has 0.6 ohms DC resistance. With 12 volts applied, that draws 20 amps. 20 amps causes a pretty good arc! I haven't tested a Ford relay, but expect it only draws a couple of amps or less.
Tom

Found a good link on crimping. It is a little long, but has some good information about why a good crimp is necessary. Just skim thru and pick out the good stuff.
http://www.molex.com/pdm_docs/ats/TM-640160065.pdf
Tom

In the Marine Corps we used a solder crimp on aircraft. Crimp, heat with heat gun, melted solder into crimp and the outer insulation was built in shrink tube. Awesome stuff. I still use Molex crimpers for the EFI systems I build. I crimp then solder.

Tom. Thank you for the info and extra effort, and tasking me on my starter relay w/ resistor diode, still working like a charm...

Another note on amperage and cabling, I do not have the link anymore but read and am using on the alternator harness I made is that double wire of a smaller gauge can conduct more amps faster than just one large one due to resistance characteristics .. pending gauge sizes compared of course but works great in some installations.. ..

You are welcome, glad to be able to help.
Two wires are better than one! Yep, as below!
The wire gauge is a logarithmic scale base on the cross sectional area of the wire. Each 3-gauge step in size corresponds to a doubling or halving of the cross sectional area. For example, going from 20 gauge to 17 gauge doubles the cross sectional area (which, by the way, halves the DC resistance).

So, one simple result of this is that if you take two strands the same gauge, it's the equivalent of a single wire that's 3 gauges lower. So two 20 gauge strands is equivaent to 1 17 gauge.

Tom