Jump to content

  • Join Today, It's Simple and FREE!

    As a member, you can post in our forums, upload your photos and videos, use and contribute to our downloads, create your own member page, add your ATV events, and even start your own ATV club to host your own club forum and gallery.  Registration is fast and you can even login with social network accounts to sync your profiles and content.


Trail boss thermister (oil cooling) mod

Recommended Posts

After riding my new (to me) 2000 trail boss, I noticed the fan NEVER ran, even after letting the quad idle for 15 minutes. I thought I had a problem with my fan or my thermister, so I did some checking and found both were working as they were designed...turning on at 260 degrees F. A further check on the various forums found that others noticed their fans not turning on.

I believe that this is a design error. The thermister, sitting on the downstream side of the radiator, is waiting for the oil to literaly cook before turning on the fan. I mean, if the oil has reached 260 degrees after going through the radiator, how hot is it in my engine? My belief is that the best oil temps are just above boiling (215 to 245 degrees F or so), so as to help eliminate any water build up, but keeps the oil from cooking.

I also noticed that the fan blows air through the radiator, then across the cooling fins of the head/jug. I feel it would be nice if the fan would come on at lower temps, to help with cylinder cooling as well.

So I came up with a mod that has really seemed to do the trick. Basically, I just placed at 10K ohm resister across the the thermister wires, so that it is wired in parallel (from one wire to the other). The result:

Before: Fan would only turn on at 260 degrees F, off at 240 F. Engine temp light activates at 290 degrees F.

After: Fan turns on at 245 degrees F, off at 225. Engine temp light activates at (estimated) 285 degrees F.

After making the mod, I tested it, and after idling for 10 minutes, my fan turned on!!!!

Oh, and when I placed the resister jumper, I was able to keep everything water tight. I pulled the pins out of the harness that attaches to the thermister wires. Once the pins were out, I used a needle to poke a small hole through each of the two "sealing bushings" that were attached to the wires. I then threaded the resister leads through these small holes, and soldered them to the back ends of the pins. Then I put the pins back into the harness, and for good measure covered the back of the sealing bushings with silicone, also covering the resister in the process.

If the temp figures are not low enough for ya, you could use a lower value resister. Go with a 5.6 K, and your fan would turn on at approximately 230 degrees, off at around 200.

If you already have a soldering gun, solder, and silicone, the mod only costs .99c for the resister!

This mod was done on a 2000 Trail boss. I'm sure it would work for other years and/or models. Check your owners supplement...if your thermister activates your fan at 260 degrees with 2519 ohms across, then it will work.

Edited by tawood

Share this post

Link to post
Share on other sites

Nice post, that is a really good idea. I am going to sticky this thread so that it will stay at the top of the Polaris forums so hopefully some other like minded Polaris owners can see what you have done. Nice job.

Share this post

Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Similar Topics

    • By Dra O
      posted a while back - it was running at one point - I have all the plastic shroud off - it's down to frame and engine now
      -fuel pump: when line from petcock is attached, no fuel comes out other side to carb; the line to the carb is open (can blow through it); I thought when I cranked engine that the pump would spit out fuel on other side but it does not; I removed fuel pump and opened it up - diaphragm looks good - but internal areas had lotsa calcification build up - I cleaned this out and dried it good - the clear plastic pieces (?diaphragms) aren't broken - the spring w/ steel ball on end moves freely; am putting it back on tomorrow; but can anyone answer about the pump? isn't it supposed to pump fuel through it... to other side... to carb? puzzling
      -along with fuel pump: I replaced petcock 3 weeks ago (gas tank had rusty fuel in bottom - cleaned it out prior to new petcock); when I turn petcock arrow NOT to on or reserve, fuel comes out of line; when I turn it to on or reserve, fuel doesn't come out; puzzling
      -I don't know what's on back of fuel pump - it's a valve that has a hose coming out of it - looks like it goes to engine/carb - I haven't traced this out yet - what is it?
      -can squirt gas on air filter and engine will run for 2-3 seconds then stop - fuel in top of carb (under diaphragm) does same
      -it likely needs carb rebuild - will get to this - but something isn't allowing fuel to get from fuel pump to carb
      -I THINK it has a mikuni model x carb - diaphragm is ID 26mm  OD 68mm - hard to find inexpensive diaphragm, I guess because it's a model x
      -last thing - I replaced solenoid due to old one clicking; I HAVE to be charging the battery or it won't start/turn over - battery just goes dead; but if charging, it'll turn over nicely; i'm guessing it's a bad battery
      -any help appreciated - I've never owned an ATV before - inherited this beast from grandparent in-law
    • By lanow
      Hi Everyone,
      I am new to this forum and hope that someone out there can help me, I have an early Suzuki King Quad 300 (280cc) which starts intstantly every time but at about half revs or there abouts it starts spluttering and will not rev any higher, this fault is present whether choke is applied or not, some one suggested that silencer maybe be clogged so removed baffle but no improvement have removed carb and stripped 3 times but still no joy.
      Any ideas would be most welcome.
      Cheers Rob
    • By Rich1028
      Hitting the Trail this  Morning.
      3 weeks in a row!
      The Wife and I are heading back out to Leota
      Why Leota you might ask,and not somewhere new?
      Cause there are lots of trails to ride,and my old ass has to get familiar with the trails so I do not take a wrong turn.
      ...and Yes I already took a few wrong turns...getting better tho.
      Have A Great and safe weekend everyone! 
    • By StolenATV
      1987 Suzuki LT 80 and 2007 Honda CRF 50 dirt bike stolen out of my driveway on July 4th. Anyone with info that recovers them will get $400 no questions asked.
      The post Suzuki LT 80 Quad And Honda Dirt Bike appeared first on STOLEN 911.
      View the full article
    • By oxidized_black
      1995 - 2000 Honda TRX300 Service Manual
      View File 5851d86071202_1995-2000HondaTRX300ServiceManual.pdf
      Submitter oxidized_black Submitted 12/15/2016 Category Honda ATV  
  • Similar Tagged Content

    • Guest
      By Guest
      Ok...I'm gettin' me juevos ready to do an oir filter clean....
      Went to my local harley/quad dealer and he had me purchase the No-Toil kit (Cleaner, Air Filter Oil, Rim Grease)
      What I've read is that the No-Toil cleaner will NOT clean
      other oils off air filters....just the No-Toil oil....
      So, I've also read that others have cleaned their air filters
      in the kitchen sink, used gasoline, or used flash solvent....
      Looking for advice as I've mentioned I'm very mechanically
      I'm going to wash the air box seal in the sink and give it a good dry....
      One more thing......No-Toil or Bel-Ray for filter oil?
  • Recently Browsing   0 members

    No registered users viewing this page.

  • Create New...