In the for what it's worth department:
I had the fuel shut off solenoid go in my
B21. It was a 3-wire (i.e. two coil) solenoid. When there is no power to either coil, a spring pushes a bar out and that cuts off the fuel. The 1st winding is used to pull in the solenoid. It has a very low resistance (.39 ohms in the case of my
B21). It was only energized (on the
B21) when the starter was cranking. The 2nd coil is a "holding coil". It had a resistance of 11.6 ohms. Once the solenoid is pulled in (via the 1st high current coil) and there is a metal to "coil" contact, it takes substantially less current to hold the solenoid in. So the 2nd coil is on all the time that the tractor is running. On the hold in solenoid, I put the "coil" in quotes because there is really "cut off bar metal" to "the metal that the coil is wrapped around" contact.
Looks like this is what's going on here. So with these designs, the goal is to hit the kick in solenoid for a short period of time when starting, and then have the holding solenoid take over. The holding solenoid is energized whenever the tractor should be running, and cutting power to it is how you shut of the engine on the tractor. Because the kick-in solenoid coil has such a low resistance, it draws a lot of current and therefore generates a lo of heat, so it can't be left on for too long. On my
B21 if you crank to long I'm sure the fuel cutoff solenoid gets very hot. As others have mentioned, Deere has tried various methods of controlling how long kick-in solenoid is energized, such as timers, electronics controls, and the PTC (positive temperature coefficient) thermistor that is being discussed in this thread.
I like to think of a PTC as a "plastic fuse". As you pass current through it, it heats up. The conductive plastic inside the PTC reaches a melting point at some temperature. When it melts, the conductivity of the plastic drops and less current can pass through it. Typically, the part finds some happy medium balance point where it draws some current, keeps the plastic liquid, but still limits the current to the outside circuit. While in this state, the part is very hot- up in the 140 to 220 degree F range depending on the part. Once the high load is removed, the part cools down and is conductive again. So a PTC is a self reseting fuse that is full of conductive plastic. Isn't technology a hoot :laughing:! Note that even when PTC fuse has blown, you'll se some current flowing (and if you were to measure it, a bit of voltage such as maybe 2 volts on a 13 volt circuit).
Now in the case of the tractor, we have a PTC fuse that drives the pull in solenoid on the fuel shut of solenoid (which really should be called the "fuel cut-off solenoid
s"). If that circuit is driven at the same time as the starter solenoid, once the tractor starts and the user releases the key from start/crank to run there is no power and the PTC cools down and is ready to go again on the next start. When you start cranking the pull in solenoid is engaged until the PTC fuse kicks in and drops the current down, and by that time the solenoid is engaged (the metal bar is pulled all the way into the solenoid) and the hold-in coil keeps it in place.
So if this is a brief time, and if the tractor wiring is rated correctly (as in it's 105 degree C wire) then for the brief time the outside of the PTC is hot there should be no problems. It's best if it's not pressing hard on any of the wiring if you can do it. That way if something goes wrong (like the starting relay has it's contacts weld shut), the hot PTC won't cause problems.
I have seen problems with both surface mount (not used here) and leaded (used here) parts in that you are depending on the outside case ("Grey Poupon" yellow in this case) to hold the leads in place while the fuse is in the melted plastic state. So I would avoid excessive stress on the leads by making sure the wires to them are not pulling on the case too much. The picture shown looks great. Note that if the parts shown in the picture are in the "blown and hot" state for a long periods of time they do change color and get a bit darker. Just another clue and piece of data to be used to help diagnose problems.
These parts are used in a huge number of devices these days. You get fuse protection without the pain of a user serviceable fuse. Alarm systems with batteries use them, battery chargers use them, any almost box that feeds power to something uses them.
I hope this long winded explanation helps. Knowledge is power
(in this case, power to diagnose problems _and_ power from your diesel engine :thumbsup
.
Pete
