[Tonkabot]

The other day it was 95 out and after using my tractor for a while I found that it would not start, just cranked away.
after waiting a day it started right up.

I am thinking that this is the infamous thermistor.

What I want to do is order the thermistor from digikey, where I can probably get 10 for $5, as opposed to greenparts which is like $10 for 1.

All that is keeping me from doing this is to get the correct part number from the thermistor. There are a few pictures of the part here and elsewhere, but none are good enough for me to read the lettering on the part. Does someone have a spare thermistor that they can photograph both sides of and post here? I need to see the lettering clearly, as I would like to know the exact specs for it - I suspect that I could choose a different one that exceeds the JD spec'ed part to make it more reliable. Worst case is I'll just replace it with the same part (except at greatly reduced price because of digikey).

Thanks!

 

[JD Driver]

Green Farm Parts has a pretty good picture of LVU10323 on their website. Hopefully it is accurate.

The picture is interesting, because it looks like a XU250 resettable poly fuse (PTC).

Even though the green tractor folks always call this device a thermistor in their documentation, I have always had a hard time believing a thermistor would be used in this circuit. A thermistor is a device that changes resistance with ambient temperature, and would not normally be designed into a circuit that passes high current and would cause self-heating of the thermistor. If a thermistor were really used in this circuit, if you started the tractor, then immediately turned it off, and tried to restart, it probably wouldn't start because the thermistor would still be too hot from the previous start to pass current and allow the fuel solenoid pull-in coil to activate.

It makes much more sense that this device is a 2.5A PTC instead of a thermistor.

The fuel solenoid pull-in coil is supposed to have a resistance of 0.4 ohms, which means that it would pull 30A at 12V, which would quickly trip the 2.5A PTC (after pulling in the fuel solenoid) and cause the K3 relay to activate. When the K3 relay activates in this circuit, it removes power from the PTC which resets the PTC immediately so it is ready for the next start - no matter how quickly the next start comes.

Did you check your K3 relay for proper operation? One of those failed on my 4610 early on.

 

[ttazzman]

i keep a thermister, fuel solinoid and relays on the shelf for my 4500 just in case ...picture shows one side of a new one in package...the other one is the reverse side of one i took out a few years ago

 

[JD Driver]

Thanks, @ttazzman

This seems to be the part. RUEF250

And here.

I haven't tested this, so no guarantees to anyone buying from this source instead of John Deere.

 

[Tonkabot]

Thanks a lot guys, I can buy a bunch from digikey or mouser and be set for life. I could also try the one step bigger part if the problem persists, and see if that would help.

 

[ttazzman]

Thanks a lot guys, I can buy a bunch from digikey or mouser and be set for life. I could also try the one step bigger part if the problem persists, and see if that would help.

it would be helpful if you aquire the thermistors and use them to know if they work as expected so others can gain from the experience......so it would be great if you follow up in the future

 

[JD Driver]

Thanks a lot guys, I can buy a bunch from digikey or mouser and be set for life. I could also try the one step bigger part if the problem persists, and see if that would help.

Tonkabot,

I would be careful trying to substitute a different PTC part with a higher hold current rating.

For the nominal fuel solenoid pull-in coil with 0.4 ohms resistance which pulls 30A at 12Vdc, the standard RUEF250 (2.5A hold current) will trip in 0.7 seconds at 20C. The purpose of this circuit is to briefly energize the high power pull-in coil and release it, after which the hold-in coil (which remains energized) will hold the fuel solenoid open as long as the tractor is running.

If you install a part with a higher hold current, it will take longer to trip, and this would be harder on the pull-in coil as it would stay powered longer than needed and have to dissipate more heat each time you start the tractor. These parts are cheap compared to replacing a burned out fuel solenoid.

 

[Tonkabot]

I got my new 'thermistors', actually resettable poly fuses as noted above.
I'll install it soon - I have had no problems starting lately but I expect that when I run it for a long time in the heat again, that the problem might come back.

It was something like $2.50 for 5 from digikey, a bit cheaper than john deere's price ($10 or so for one)
It is the same part from all the markings (the ones that may differ are likely the mfg date code)...

 

[Tonkabot]

Just to complete this post, I did have the problem manifest again (on another hot day). I pulled the fuse panel out and found the PTC behind it. Soldered in one of my new ones, and it started up perfectly after that, including after running on a hot day. The Job is a bit of a pain getting to it. If I ever have to do it again, I might figure out a way to bring it to the front side of the fuse panel maybe in an unused fuse or relay position.