solenoid mystery

Hey Guys, I just finished putting my 1939 Farmall A back together and I wired it up this weekend. It is all converted to 12 volt, alternator,regulator,etc. As soon as I hook up the battery the starter begins to crank without the ignition switch being touched. I disconnected the small wire from the switch to the solenoid thinking I might have miss-wired something and got the same results. It has a three terminal solenoid two heavy terminals (one to the battery and one to the starter) and the small switch terminal which goes to the "s" terminal on the ignition switch. with only the two heavy terminals hooked up the starter cranks as soon as you hook up the other battery cable so I figured the solenoid contacts were stuck together. but when I remove the solenoid and check for continuity across the two terminals with an ohm meter I get an open circuit. I then tried another solenoid I had laying around and it does the same exact thing. so, either both solenoids are bad, both check open when checked with an ohm meter, or I"m out of ideas. before I go shell out $25 for another solenoid does anyone have any ideas on whether there is another test I can do on the solenoids to se if they are bad? I sent a couple of pictures for reference

Which battery post are you connecting to the tractor chassis? You should be connecting the - post to the tractor chassis, unless you specifically went out and bought a positive ground alternator.

You also mention alternator AND regulator? Most alternators have a regulator built in, and an external regulator is unnecessary.

The only possibilities I can think of are:
1. Wrong polarity is causing the solenoids to energize.
2. Main battery cable is hooked to the wrong post on the solenoid, "backfeeding" it and causing it to energize.

Easiest thing to do is swap the heavy wires on the solenoid.

I'm thinking of TWO kinds of starter Solenoids

Most Common Type A) Normally, far as I know (NO Warranty), if a Solenoid only has the two big INPUT (from battery) and OUTPUT (To Starter) Studs PLUS a very small terminal, it only engages if there's hot voltage applied to the small terminal which runs through the coil then to case ground. I cant envision a Type A solenoid that has closed contacts until voltage is applied to the small terminal and ground.

Type B) If I recall correctly (Again NO Warranty) some Fords used a solenoid that looked the same as above HOWEVER one of the big stud terminals (Hot Input side from battery) was wired to the coil and the solenoid engaged WHEN A GROUND WAS ATTACHED TO THE SMALL TERMINAL via the push to start button like old Fords had.

You asked for tests: Id say there ought to be continuity from the small terminal to case frame (if a Type A) and its resistance is the activation coil. See if there's continuity from the small terminal to either of the big studs??? Unless its an old Ford type there shouldn't be in my opinion.

Okay, I'm done now lol where am I going with this ??? I forget. I doubt its an old Ford type Solenoid (needs a ground to small terminal to engage)

Darn if I know, but I'm in a hurry for now maybe it will come to me in a vision lol

PS I have one "continuity tester" that's the ultra sensitive type with a light that will glow even if my fingers are on the device, so it glows continuity even when a set of contacts would be open. Just another thing to consider. BUT YOURS SHOWS OPEN AS ID EXPECT not continuity???

John T

Some of the 3 wire solenoids have a battery side and a starter side and if hooked up back wards they do funny things plus if it is a solenoid made for a ford like the 8N then S post has to be grounded to work so yes you could have the wrong solenoid. Also just to let you know when new it did not have a solenoid it had a starter button

Thank you guys. My pictures didn"t make it on but I still have the
push start button on my dash so If I don"t figure this out I could still
use that. I do have continuity between the small post and ground
but nothing between either big post and the small terminal. and
lastly I haven"t checked on the big posts to see if they are marked
but that may be the answer because all the mounting holes are the
same before I took it apart last year, so maybe I only have to flip the
solenoid over and the poles would be reversed. I"ll keep you
posted when I give it a try. Also it is a negative ground. Thanks Again Tom

If you take the solenoid off, touch the base to the - terminal on the battery, and use a test lead to touch the + terminal from the battery to the small post on the solenoid, it should activate.

Given your description of continuity, though, I suspect that all it will do is create a short.

If you take your test lead and touch either large post I bet the solenoid will activate, which is NOT what you want to happen.