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200 Temp gauge stops reading at high rpm

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pdh View Drop Down
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Joined: 13 Sep 2009
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Post Options Post Options   Thanks (0) Thanks(0)   Quote pdh Quote  Post ReplyReply Direct Link To This Post Topic: 200 Temp gauge stops reading at high rpm
    Posted: 17 Aug 2024 at 4:47pm
Working out a few glitches on a new to me 200 with righ at 3000 hours.  Temp gauge seems to work at low RPM - but starts bouncing as RPMS increase and falls all the way to cold at full RPM.  Looks to be original gauge.  Maybe 50 years it all its got in it and its time for a new one?  I usually start all electrical quests with the assumption of a bad ground and will check that out after I pull the cowl off - but other trouble shooting ideas while I have it off are welcome.  TIA.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote plummerscarin Quote  Post ReplyReply Direct Link To This Post Posted: 17 Aug 2024 at 7:04pm
Can I assume the coolant system is full? I know, dumb question but it first came to mind and is newly acquired
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 8070nc Quote  Post ReplyReply Direct Link To This Post Posted: 17 Aug 2024 at 7:06pm
Id check the wire from the sensor in the thermostat housing and make sure its not grounding somewhere
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Post Options Post Options   Thanks (0) Thanks(0)   Quote KJCHRIS Quote  Post ReplyReply Direct Link To This Post Posted: 17 Aug 2024 at 7:30pm
Check the wire to temp sensor, unhook from sensor and operate tractor what does temp gauge do at differing RPM's then use a jumper wire to ground the wire and repeat process.  
 IF temp gauge varies its readings I'd go with bad wire or bad gauge. 
 IF temp gauge stays in steady position either high or low, I'd replace the sensor. 
 I've replaced a few sensors in 185 & 200 tractors but never the gauge itself. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Lars(wi) Quote  Post ReplyReply Direct Link To This Post Posted: 17 Aug 2024 at 7:54pm
Increased rpm’s is creating more air movement, thus resulting in poor connection of the wire on the sensor.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote pdh Quote  Post ReplyReply Direct Link To This Post Posted: 17 Aug 2024 at 8:19pm
Yes coolant is full.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote pdh Quote  Post ReplyReply Direct Link To This Post Posted: 17 Aug 2024 at 8:21pm
Will focus on connections at the gauge and the wire from sensor to gauge and see what I find.  Thanks all.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote DaveKamp Quote  Post ReplyReply Direct Link To This Post Posted: 18 Aug 2024 at 7:43pm
I would start by putting a pressure test gauge on the coolant cap, and fire it up, idle for just a few seconds, then bring it up to full governed, then back down, then back up, and see what that gauge does...  cold coolant won't generate any vapor pressure, but combustion pressure leaking into the cooling system can cause bubbles to accumulate, and if they accumulate around the sensor, it will not read coolant temperature properly.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote pdh Quote  Post ReplyReply Direct Link To This Post Posted: 18 Aug 2024 at 9:01pm
Thanks for all the ideas.   Sensor seemed a little rusty and the push on connector seened  a little loose - so I cleaned up the sensor and tightened the push on a bit - ran it around some to bring it up to operating temp.  Erratic reading is gone and now getting a consistent temp reading that stays steady through all RPMS albeit a iittle on the cool side - so guessing the thermostat is stuck open.  I will put getting a new stat in it before I put it to work on the auger this fall - but for now i think that issue is resolved.  Much obliged.  
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