Thermistor, John Deere 4120

[jsteven]

Hello,

I'm new to this site and was following a thread from 2011 about a John Deere 4320 with starting problems. The thread describes my problem
exactly. The tractor will be running fine and when I shut it down it might or might not start.
The thermistor sounds like the answer but there is not a thermistor in the wiring diagram. I researched it with my local dealer. Because this seems to be a common problem with the compact series I'm hoping someone might have a fix for this issue.

Thanks - Steve

 

[Blaneypark]

I had the same problem with a JD 4100. I cleaned every connection I could find from the Battery to the starter and also the ignition switch. When it happened to me everything would shut down. No lights, heater fan, wiper, nothing. After I cleaned all the connections never had the problem again. I am knocking on wood as I type.

 

[jsteven]

I wish it was that simple. I have battery power, the starter turns full speed. It just won't fire. I've been dealing with this for months now and the only way to start it is to pour a little gas on a rag and put it in front of the air cleaner intake and as soon as it fires (a matter of seconds)remove it. A mechanic told me it that this way is much kinder to the engine than using starting fluid. I've drained the fuel tank, replaced all the filters, also replaced the fuel solenoid. The thermistor or something of that nature seems to be the issue. Went out yesterday to clean up a downed tree, started fine, I shut it off while sawing the tree, and as usual at random times and places, it would not start.

 

[JBlk]

jstevens your problem sounds like more of a fuel problem than an electrical problem.You don't say if your tractor is gas or diesel.As I understand a thermistor it is nothing more than a resistor that changes to lower value when it is heated by amperage.This could be a small unit that looks like a ceramic capacitor but it is more than likely a wire in the harness that has thermistor specs.If your engine is diesel it could be injectors that need replaced or cleaned.If it is gas try isolating the cause by feeding gas directly in the carb and working back from that point to the tank.You can also check the ignition system by using a hot wire to the cold side of the coil.One thing for sure is that if is starting with just a rag with fuel near the intake breather you problem is minor.I have a 4010 diesel that the wrens got through the pre intake and filled the pipe up with a nest of twigs.Pull the bottom off the air cleaner and see if your pipe is clear.

 

[jsteven]

Thanks for the reply JBlk, The 4120 is diesel machine with fresh fuel, fuel filter, air filters, no bird nests. Hmmmm! A wire in the harness with thermistor specs, I'll try going down that road. I'm not sure about jumping across the coil, do you mean connecting 12V + to the negative side of the coil? Thanks

 

[JBlk]

You can forget about the coil, because a diesel fires from compression and doesn't use a coil.I read the similar thread on the 4120 and it sounds like the thermistor is a seperate unit and not a wire.That thread stated that the purpose is to supply more voltage to the fuel selonid when starting the engine.I did a little research on thermistors and discovered that they are made with both positive and negative heat values.The whole thing sounds like you could trouble shoot it with a dvm once you learn the voltage values are supposed to be at the fuel selnoid during a cold start.You could probably call a JD dealership and ask to talk to a mechanic and he could probaly tell you the correct values.

 

[JN4310]

I have a 4120 that had a similar problem. It turned out to be the wiring harness for the PowerReverser switch. On E Hydros like mine they install a jumper into the plug since there is no reverser switch. It can come loose or fall out and cause the problem.

In my case it was still there but was sandwiched between the chassis and the left side floor board and would short out when I got on the tractor (but not off, since I step in a different spot!). It was tough to find since it was completely hidden from view. As soon as I moved it, the problem went away completely. Now if I can just get my headlights to work...

 

[jsteven]

Thanks JN4310 & JBlk, thanks guys, I used it 3 times today with no problems. The starting issue is intermittent and you never know when it's going to happen. It's a 2006 model with 1150 hours on it and I bought it new. I started having the starting problem about a year ago and have talked to the JD dealer and JD mechanics. First we thought it was a fuel issue or injectors. The dealer confirmed that this model does not have a thermistor chip. @ JN4310, mine is an E Hydro also so I was pretty excited when I saw your post. I crawled under the tractor and searched every connection and could not find one with a visual problem. Although the JD mechanic said it sounded like a grounding issue and a short, I'm Still puzzled.
In addition, when this has happened before I have disconnected the battery and reconnected it to see if something would reset, like an open circuit would now close. No luck. The only way to start it is to pour a little gasoline on a rag and insert it into the air cleaner opening and when it fires once, pull it out, Otherwise the starter will grind until the battery runs down.

 

[JN4310]

Yeah, the gas on the rag bit makes it sound like something else. If I got off of mine while it was running it would sometimes shut off as I got back on if I stepped on the right (wrong) spot. It did that a few times and then finally refused to restart at all.

The wiring connector was not visible at all until I moved some things out of the way. It's underneath the left floor board where it branches up to go to where the switch would be and was a bear to get to. All I had to do was move it an inch and it has been fine ever since. Good luck to you finding your problem!

 

[JN4310]

BTW, do you get any codes on the display when it happens?