clarification on starter wiring diagram

Edited by bikerfox - Aug/07/2019 at 12:45am

motorhead_1 wrote: 10ga will be fine for s terminal. i used that wiring up my robb mc starter

10 gauge is overkill for the S terminal IMO.
All you are doing is energizing the coil inside. There's no load at all except for that coil and the load is small. Look at the stock wiring of a GM or Chrysler with a solenoid on the starter - or the wire going to the solenoid in a 4.0 liter engine which has a starter solenoid that would have a similar draw - what gauge is that? 14 maybe Why not use something more manageable? 
The coil in the starter's solenoid doesn't take a huge draw either - more than the fender mounted but still..... it's only as couple feet from the fender solenoid. You need to figure distance as well as load. 
Overkill is fine but then you deal with really large stiff wires in a tight spot at the starter. 

Cable from Battery + to A on ORIGINAL solenoid on fender, then from that A terminal on down to the starter's battery terminal. In this case A is simply a way to connect battery to starter's batt terminal AND when energized by the S wire, the fender mounted device supplies the power to energize the solenoid on the starter. 
You are using the original solenoid as a junction AND using it to be a relay to the S terminal on the starter so you don't have to run your ignition switch S wire all the way down to the starter. You can leave your original ignition switch to S terminal wiring alone and simply use that fender mounted solenoid/relay as a relay.

I would wire it like that picture. It allows you to keep your STOCK wiring for the most part and simply run the wires to the starter as shown in your picture above. 
I would make one change if it was possible and easy - I'd run battery directly to starter and then from starter up to the fender mounted solenoid A terminal. That keeps the run to the starter a bit shorter BUT - and this is up to YOU - it means TWO BIG cables at the starter's battery terminal. 
SO if that is a problem, then do like the picture. 
IF you have space run battery to starter first, then continue up to the fender solenoid. 
That means two big cables up there - however, you don't need as bit a cable from the starter up to the solenoid A terminal that way because ONLY THE CAR's LOAD would be fed from that so you could use a much smaller cable from the starter. 

In it was me - I'd run trunk battery big cable to starter BATT stud.
I'd run a smaller cable from starter BATT stud up to solenoid on fender A terminal and have it big enough to run the CAR's systems and CHARGING system but you greatly shorted the battery to starter connection.

The pic above assumes front mounted battery, IMO.

The simple bit first - YES to run wire from the S terminal on the STARTER SOLENOID up to the B side of the original fender-mounted solenoid. This way you are using it like a RELAY. 
Your ignition switch will still be wired to S on the original - then when the original is energized it connects your A terminal to the B terminal and that energizes the S on the STARTER itself. 
This way you don't have to move your original ignition switch start wire/feed down to the starter. You leave it alone - which is a good thing, IMO, and use the original fender-mounted solenoid/relay simply as a junction block for the battery cable to car's load and as a relay for the START circuit. 

I've not checked the draw of a typical starter solenoid (the starter mounted type, but hear it's 8-10 amps while the "ford type" fender solenoid is no more than 5 amps. 
So from B side to S on starter you could easily use 14 gauge which will carry 15 amps up to 6 feet or so. If the starter-mounted solenoid took 10 amps, and it's only 3 feet from the fender relay - 14 would be more than enough for that starter's solenoid. It's half of 6 feet and much less than 15 amps.

Now this part - 
>>Smaller cable from starter BATT stud up to solenoid on fender A terminal--what gauge wire do you recommend?<<

What size alternator do you run? You will be keeping the battery topped off via alternator through this wire AND running the car's accessories like lights, radio, wipers, and other fun stuff through this.