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Help VT 250 Spada Cooling System

Discussion in 'Honda 250cc Twins' started by Gizziracer, Jul 17, 2019.

  1. Gizziracer

    Gizziracer Active Member Premium Member Contributing Member

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    A question for those who understanding the intricacies of water cooling. The mighty Spada, and probably heaps more water cooled bikes has a temperature switch on the radiator and a temperature switch that runs from the thermostat housing. The radiator switch controls the radiator fan and I'm guessing is a simple on off type design. The smaller switch in the thermostat goes to the temperature guage in the instrument cluster. Does anyone understand how this single wire conveys constantly variable signal to the temperature guage? Its beyond my tiny brain.
     
  2. my67xr

    my67xr Bike Enthusiast Staff Member Premium Member Contributing Member

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    The temp gauge sender is a Thermistor, it change's resistance as it get's warmer/colder.
    So the temp gauge has power going in one side and it's earthed through the temp gauge sender, as it warm's up the resistance may go from say 20 Ohm's cold to 280 Ohm's when it's overheating

    If you are having trouble's with no reading then first check the thermostat housing's earth,
    check that it goes to clean bare metal, and that there's no corrosion in the wire's that the terminal is crimped onto.
     
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  3. Gizziracer

    Gizziracer Active Member Premium Member Contributing Member

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    Thanks, that makes sense. Not that type of trouble though, very minor issue. Temp gauge seemed to be reading a bit high on latest project. Reading about half way on gauge, (well clear of red zone, just higher than other Spadas), then the fan would kick in and reading dropped slightly, fan stops, then the process repeats,, but still reading slightly higher than other spadas. I flushed the system, changed the thermostat and switch and the radiator switch, now the temp is reading about the same(slightly below 1/3rd), that other spadas read on a mild day, but the switch is still kicking in when the bike is nowhere near overheating, then fan is stopping as it lowers the temperature slightly. As I said, not a big issue, just trying to understand whats happening.
     
  4. my67xr

    my67xr Bike Enthusiast Staff Member Premium Member Contributing Member

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    Have you checked the reading's out from your switch, and compared them to the spec's in the service manual ?

    As an example, this is from the Suzuki GSX250F Across service manual, so will be different to the VT250 Spada spec's i'm guessing
    The standard fan switch come's on at appox 105° C and turn's off at approx 100° C going off the service manual.
    The service manual has the spec's for the fan switch listed as-
    @ 50° C it should read approx 167 Ω
    @ 120° C it should read approx 17 Ω

    Do you have a non contact Infrared thermometer you can point at the radiator and check to see what the actual temp is?
    And if they are close to what the service manual says
     
  5. Gizziracer

    Gizziracer Active Member Premium Member Contributing Member

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    Thanks very much. Is a non contact infrared thermometer usually available from J car or supercheap Auto?
     
  6. Gizziracer

    Gizziracer Active Member Premium Member Contributing Member

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    Yep, $60 at supercheap, about a million on ebay, Thanks so much for all your help.
     
  7. Jethalter

    Jethalter Active Member Premium Member

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    To check the single wire on the thermostat which operates the gauge, simple remove the wire off the sensor and ground it, this will test your gauge and move its sweep direction from cold to hot. If it does this the gauge is fine and sensor most likely faulty if no reading whilst engine hot.

    The radiator 2 wire sensor is the electric fan sensor, this is common to honda's fitting them in the rads. The sensor will be model dependent but a particular sensor could be used over multiple models. Best bet is finding what the sensor part number is and cross referencing it with other honda items, chances are its in other models too and this also means you can get the resistance readings for it and the temps it turns on and off at.
    These two wire if you short them together whilst not connected to the sensor should turn on the fan to tell you that the fan circuit is all working.

    As far as it position on the gauge, it may be higher than other spadas but doesn't mean something is wrong. After sorting a mates cx500 his needle goes to middle, fan turns on, drops back down to 1/3rd and fan off.
     
  8. Gizziracer

    Gizziracer Active Member Premium Member Contributing Member

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    Thanks very much for that, everything is sorted. Yes I came to the conclusion that it was all down to a slightly different calibration of the temp needle, possibly due to age or lack there of. I purchased an infrared thermometer that let me know the fan was clicking in at around 100 degrees and clicking off when the temp dropped a little so all good. Thanks very much for all the helpful info.
     
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