Thermistor

[KRH]

Hi All:

My brother has a 4600 and the fuel cutoff solenoid doesn't work after a number of starts. If you let it sit awhile, it will start. I have taken the solenoid out and laid in on the frame. It will pull the plunger in a few times, but then will stop. If you push the plunger in, it will stay. So obviously the thermistor is working as it "holds" the plunger in. Does the "pull" function also run through the thermistor? It acts like the "pull" function won't work after something heats up?

Thanks.

KRH

 

[DennisFolsom]

I have a 4600 also. I bought the Technical Manual that goes with it. Upon reading the "Fuel Supply Theory of Operation" on page 4-42, it sounds to me like the thermistor allows enough current to pass to operate the "pull" function before it heats up, then cuts the pull current back.

I believe there are others on the forum who know more about this than I do. Hopefully, one of them will chime in. The Technical Manual outlines a series of diagnostic tests. It would take a bit of study to fully understand what's going on. I'm the type of guy that would do the tests with the book in hand.

Good luck!

 

[DennisFolsom]

Another thought - do a search in this forum for "thermistor." You'll find several threads discussing similar problems. perhaps some of them will fit your situation.

 

[JD755]

Hi All:

My brother has a 4600 and the fuel cutoff solenoid doesn't work after a number of starts. If you let it sit awhile, it will start. I have taken the solenoid out and laid in on the frame. It will pull the plunger in a few times, but then will stop. If you push the plunger in, it will stay. So obviously the thermistor is working as it "holds" the plunger in. Does the "pull" function also run through the thermistor? It acts like the "pull" function won't work after something heats up?

Thanks.

KRH

The thermistor does not hold the solenoid in. The thermistor lets high current flow for a brief time during Start to pull in the solenoid. After that the solenoid is held in by a latch powered by a low current. Your problem is very typical of a failed thermistor.

 

[KRH]

Thanks JD755 for the clarification. Just after I started this thread, I found a old 2010 thread on this matter in which you did a great job of explaining the thermistor and how it works. After studying the wiring diagram and the various posts, I was sure I had it backwards as you pointed out. Ordering the part tomorrow.
Again many Thanks for your assiatnce. Much appreciated. Happy Holidays!!!!

 

[Rob-D]

Thermistors are temperature variable resistors used as sensors. They change with the temperature, usually NTC (negative temperature coefficient). That means as the heat rises their resistance drops. Circuits sense the change in resistance (usually comparators or microprocessors) and turn on/off control circuits. You can test a thermistor with a multimeter. They have two leads if you heat the thermistor (mildly!) while the meter is on its ohms scale you will see the resistance change.
That's all they do, they do not carry siginificant current, the control circuit does that. So if you have a thermistor in a circuit that is failing it may be the control circuit. The first thing to check is the leads to the thermistor, a broken lead will act like a bad thermistor. If you put an ohm meter across the thermistor and it measures infinite ohms or a short than it is bad or the wires running to it are bad.
The standard resistance for most thermistors is 10k ohms at 25C but other standard values are used, if you measure the themistor at 20C (68F) and it measures around 10k it is problably good.

 

[KRH]

Thanks Rob!!!

 

[JD755]

Thermistors are temperature variable resistors used as sensors. They change with the temperature, usually NTC (negative temperature coefficient). That means as the heat rises their resistance drops. Circuits sense the change in resistance (usually comparators or microprocessors) and turn on/off control circuits. You can test a thermistor with a multimeter. They have two leads if you heat the thermistor (mildly!) while the meter is on its ohms scale you will see the resistance change.
That's all they do, they do not carry siginificant current, the control circuit does that. So if you have a thermistor in a circuit that is failing it may be the control circuit. The first thing to check is the leads to the thermistor, a broken lead will act like a bad thermistor. If you put an ohm meter across the thermistor and it measures infinite ohms or a short than it is bad or the wires running to it are bad.
The standard resistance for most thermistors is 10k ohms at 25C but other standard values are used, if you measure the themistor at 20C (68F) and it measures around 10k it is problably good.

Hi Rob. What you say is correct, but only for negative temperature coefficient devices. The device used in the fuel solenoid circuits in these tractors are positive temperature coefficient. This means they do get hot and do carry significant current. Their room temperature resistance is near zero so the heavy current will flow at first to operate the pull-in solenoid. Within a couple seconds it will get quite warm and its resistance will rise such that very little current can flow and the pull-in will release by which time the low current latch will be holding the solenoid in.

While JD has used several different methods of activating the solenoid through use of the thermistor, my 755 has the most basic. The starter switch simply connects power to the thermistor and on to the solenoid. No matter how long you may crank, the solenoid only has power during the brief period the thermistor takes to heat up. After releasing Start, then all remaining low level thermistor current is removed.

The 4600 tractor uses a relay coil across the thermistor. Here the thermistor also provides direct heavy current to the solenoid, but the relay in a bootstrap fashion will turn on and remove power from the thermistor and thus the solenoid after the thermistor has heated up.

Quote from the 4600 service manual:
NOTE: The variable thermistor will heat to a maximum
temperature of approximately 180ｰ F (82ｰ C)
during the brief period of time required to
overcome the fuel shutoff solenoid spring.

 

[Rob-D]

Hi Rob. What you say is correct, but only for negative temperature coefficient devices. The device used in the fuel solenoid circuits in these tractors are positive temperature coefficient. This means they do get hot and do carry significant current. Their room temperature resistance is near zero so the heavy current will flow at first to operate the pull-in solenoid. Within a couple seconds it will get quite warm and its resistance will rise such that very little current can flow and the pull-in will release by which time the low current latch will be holding the solenoid in.

While JD has used several different methods of activating the solenoid through use of the thermistor, my 755 has the most basic. The starter switch simply connects power to the thermistor and on to the solenoid. No matter how long you may crank, the solenoid only has power during the brief period the thermistor takes to heat up. After releasing Start, then all remaining low level thermistor current is removed.

The 4600 tractor uses a relay coil across the thermistor. Here the thermistor also provides direct heavy current to the solenoid, but the relay in a bootstrap fashion will turn on and remove power from the thermistor and thus the solenoid after the thermistor has heated up.

Quote from the 4600 service manual:
NOTE: The variable thermistor will heat to a maximum
temperature of approximately 180ｰ F (82ｰ C)
during the brief period of time required to
overcome the fuel shutoff solenoid spring.

Yes, that's right a PTC thermistor will carry more current, thanks for the clarification on the circuit Deere uses.

KRH, testing the thermistor is relatively the same, there should be a resistance when the device is cold, but not a dead short or an open. If either of these conditions is measured than check the wiring first. Also because a PTC thermistor carries more current it will be more prone to failures, don't expect to see a short on a bad PTC thermistor, while not impossible it's probably not likely. A bad thermistor will usually be an open.

 

[KRH]

Thanks JD755 & Rob. I understand what both of you are saying. However based on the fact that the solenoid only acts up a few starts, might not the relay coil be acting up...not opening up to let juice flow to the thermistor/soleniod on suceeding start attempts? Just a thought!!!!