Testing solenoid

The old Ford type solenoids were marked "I" and "S" to indicate the use of the small terminals. The 2 large terminals were connected to the battery (+) and the starter lug. The 2 small ones: One marked "I" was for the ignition bypass. It put a full 12 volts to the points for starting the engine. When you released the key, the power to the points went back through the ballast resistor and lowered the voltage across the points to prolong their life. The "S" terminal was for "start" or to energize the solenoid to pull in and close the 2 large terminals to put battery power to the starter motor.

Solenoids can be designed in a variety of ways and all accomplish the same basic task. Some use two small terminals and require a positive and a negative and both of those can be controlled in different ways. For example you could send a positive to the solenoid through a key switch and push button and supply the negative through a seat switch, mower deck switch and neutral switch such as used on lawn equipment. Some are grounded internally and need to be bolted down to operate. Some even only require a ground to one small terminal because the positive small terminal is supplied internally from the battery cable on the large post. When a negative is brought it to the small terminal the solenoid energizes.

Some systems even use a solenoid to bring in a larger solenoid on things like larger starter motors. Another thing to mention since this pertains to starters is you should never beat on your starter because it may be a permanent magnet design. When starter brushes wear or hang up in their holder they wont make good contact with the armature so people will sometimes bang on the starter and it will start working. You can sometimes get away with it but sometimes it breaks the magnets and they collapse in a crumbled mess onto the armature. The magnets are glued into the starter housing.

I think Doug answered that. For some reason in a wise engineer's mind, they energize the solenoid by switching Ground. The typical "Ford Starter Solenoid" is built to be grounded. In other words it has to be installed, to metal grounded to the Battery, to cause the Starter to work when the Driver turns the Key. ONAN just didn't do it that way. I remember reading that a "common solenoid" will NOT work on an ONAN genny. Now I see why.

One thing I was wondering about. In a couple of videos on YouTube, they bench tested two solenoids similar to mine, with the exception that their small posts were bolts, instead of spades like mine. In those tests, they grounded the metal mounting bracket instead of the small "S" post. They got the solenoid to click. As you all know, on my solenoid, I had to ground the "S" post. Why the difference? Also, why do they refer to or label the small post as "S" if it is a ground post? Why don't they label it as "-" or "ground?"

Ok you now see how it works but there is another thing that happens with solenoids when they get many hours of use as they carry high current. The tips inside will become worn and not able to carry the current required by the starter. It's probably unlikely but it does happen. Some solenoids have a flat contact bar which connects the two big terminals and some have a round disc contact that connects the two large posts. As those contact tips wear they dont transfer power well. You could see 12 volts travel across the contacts or you could see a connection with your ohm meter but if worn they may not carry current to the starter.

A solenoid is a device that lets a little guy (small wires) tell a big guy (heavy cables) what to do. If you ever get ahold of an old solenoid break it apart and it all becomes very clear.

I forgot to add that 12 volt current is now going across from the large battery post to the other large starter post. Thanks for all your help.

Ok. I see where I erred. When I applied 12 volt power to the "I" terminal, I was getting the same voltage reading on my multi meter from the "S" terminal. I thought that if I grounded the "S" terminal, I would get a large spark because I would be applying a ground to a positive current, however, this time around, I grounded the "S" terminal as you all said, and voila, no spark. And guess what? I got the click. I tested both old and new solenoids the same way, and I got a click from both. So great news on two fronts. Now I know how to test my solenoids, and also, now I know that both are in good working order, and now I can move on to another issue, but that is a different story.

Onans use a GROUND to energize the solenoid. Doug

Here's Test from the 4KY Service Manual
***
A check can be made by measuring the resistance of coil terminals I and S (Figure 6-5). With the harness leads removed, the coil should read 3–5 ohms.
If an abnormal reading is measured, replace start solenoid relay K1.
If the coil checks good and a problem with the solenoid is still suspected, remove the leads from the side terminal posts.
An open circuit should be measured between the side terminal posts with the coil de-energized. With 12 VDC applied across the coil
(I and S terminals) the relay should be energized and continuity should be measured between the side posts.
***
This means you need Battery AND Ground on those two little spade terminals before the Coil will pull in and connect the two Stud Terminals.

If it has two small terminals one of them needs a positive and the other needs a ground. If it has one small terminal then the body of the solenoid is ground so you would attach a negative to the metal body of the solenoid.

When it clicks the internal magnet is pulling a contact tip across the two large terminals which sends the battery positive to the starter.