Converting to 12 volt

Just got a conversion kit and need a good wire diagram as I have a 4 post start relay and would a better understanding of what is to be done

WHAT are we playing with???

Going to need make and model of tractor.

How about a hint.

"Four post" relay means what? Has an "S" and "R" for small terminals (start and resistor bypass)? Or - two small terminals with one for chassis-ground or neutral safety switch, and the other for relay-engage?

A "4 post start relay" often (NOT always) has


Two big posts, one is the BAT INPUT which has a big battery cable to the ungrounded battery post, the other
STARTER OUTPUT leads to the starter post/terminal.

Two small terminals, One wires to the START terminal on a typical starter switch (hot when depressed or in
start position). It "may" be labeled "S" or some other designation ???? I cant say !!!

The other "may" be labeled "I" (like on some Ford solenoids) which is to by pass an
ignition ballast resistor when cranking

How it works (IFFFF "some" older Ford type 4 terminal relays) is when you send voltage to the small "S" terminal current flows through the coil and returns to the battery via the frame ground, and when activated the two big BAT INPUT and STARTER OUTPUT are connected together so the starter receives voltage and spins. The "I" terminal is hot when S has voltage if needed to by pass an ignition ballast resistor.

NOTE on some of the older Ford type 4 terminal starter relays with the small "S" and "I" terminals the ground return was via the frame ground mounting connection. Again S received voltage when a start switch was in the start position and the ground return was via the frame

NOTE I have seen a few 4 terminal relays on which the two small terminals were S and Ground IE the ground current returned via the other terminal NOT the frame ground

NOTE they make 3 terminal relays but since you said 4 I wont go into those

HOW TO WIRE if you have a starter with a big post/stud terminal NOT an onboard mechanical switch, and want to start using a start switch/position and have the 4 terminal older Ford type solenoid as discussed above:

BAT INPUT to ungrounded battery post
STARTER OUTPUT to starter
S or other correct small terminals wires to the start position on a starter switch
I (if so equipped) is for ballast by oass which you may or may not have
Ground return for activation coil is often via the frame ground mount BUT NOT ALWAYS.....

NOTE not knowing what you have and with such little information this is ONLY MY BEST GUESS and may be wrong as rain so don't anyone have a calf. If I knew your solenoid and starter I could give a better answer SO NO WARRANTY

NOTE this is onlyyyyyyyyyyy one part of a conversion kit, theres much more to the story. To convert from 6 to 12 you need a 12 volt battery and be equipped to charge it,,,,,,,,,,,,a 12 volt ignition coil or a 6 volt coil with a ballast resistor,,,,,,,,,,possible ground changes,,,,,,,,,possible wiring changes

John T

As John T explained, there are various types of starter relays. They all look alike, only way to know is to do some investigating.

If both small terminals are in use, try removing the wire from the terminal marked "I" or "R", see if it will still crank.

If it does, that is a "resistor bypass" type, the most common design available at any auto supply. It grounds the relay coil through the base.

If It does not crank with the wire removed, that is an "externally grounded" type. Not very common.

What you need to be careful of, a base grounded relay will work in it's place, but by doing so you may have bypassed the neutral safety switch. Some applications use the ground side of the relay to complete the circuit through the neutral switch.

Another option would be to just leave the 6 volt relay on. It will probably last a long time, replace it when and if it fails.

As for the rest of the conversion, the electrical is really simple, especially if you chose a "one wire" alternator. The kit should have instructions, if not search "12v conversion", you'll get lots of hits, also Youtube is a good source.

One thing to remember, the electrical seems to be the most intimidating area, don't let it!

Apply just as much attention to the mechanical aspect. Properly mounting the alternator, getting the pulleys properly aligned, and the right pulley width, are just as critical, even more so. A shortcut here can result in major damage if an unnoticed belt failure turns into an overheated engine!

Steve, and then Ford changed to what is a more flat round hockey puck looking starter relay but I think they functioned pretty much the same. Those older 3 terminal Ford looking relays with the single small stud/post were used on some of the older 6 volt trucks n tractors. Seems most used the frame ground mount for return as you mentioned. Being in the RV community I see some "rookies" try to use a regular Ford type "starter" solenoid as their isolation relay (house and engine batteries) not realizing its NOT 100% duty cycle rated then wonder why it stops working so soon LOL

Fun sparky chatting with you

John T