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CB900 Hornet overheating

Discussion in 'Technical and Troubleshooting Torque' started by Tabmow, Jul 5, 2016.

  1. Hope someone can help.. I have a 2005 CB900 Hornet. I have recently noticed the thermo fan does not come on until the bike is very hot. Normally the fan would come on when the needle just gets above the "C" , now the fan comes on when it half way to the "H".
    I have also noticed the coolant in the overflow bottle is stone cold after the bike is turned off.. Dont think this is normal
    If I disconnect the plug from the temp sender, the fan will operate, its not a faulty fan..

    Q What would cause the fan to come on much later than it used to?

    Any ideas appreciated.. Thanks
  2. #2 chilliman64, Jul 5, 2016
    Last edited: Jul 5, 2016
    winter and cooler riding conditions may cause this.
  3. Hi Mark. nup dont believe this, been riding this bike for the last 11 years, must be something else
  4. Faulty coolant temp sensor?
  5. Temp sender
    Thermostatic switch
    All could be a cause.
    • Like Like x 1
  6. crap covering the temp sensor perhaps
    pop it out, test the switching temp in a pot of water or oil.. or just get new one
  7. I'm tipping the temp sensor has failed (or is failing) which explains the fan not activating.
    The cool overflow bottle temp has me stumped, though.

    My 8 year old Strom reaches and maintains the same hot running temp on a brisk 3 deg alpine morning, as it did on a 38 deg day in Feb.
    Just like in car cooling systems, the thermostat's one and only job is to control coolant circulation, in order to achieve the above.
  8. fluid in the tank overflow does not circulate.
    it's a tank to contain fluid expansion from the cooling system, and it gets sucked back in when the system cools down.

    why would it be expected it to be warm?
  9. #9 mike8863, Jul 5, 2016
    Last edited: Jul 5, 2016
    All the measurements you need to know in order to check the system.....

    BTW Aldi had a great infa red digital thermometer for $25 a couple of weeks ago...just what you need to check a cooling system.

    By the sound of it assuming the thermostat opens at the appropriate temps then your fan was coming on way too early suggesting a faulty temp sensor.

    The manual I found online is here HONDA CB900F SERVICE MANUAL Pdf Download.

    The coolant in the overflow bottle will only get warm when the coolant in the system gets sufficiently hot. Hot coolant expands and ends up in the overflow bottle. If your fan has been coming on so early that the system never gets warm properly then the coolant is not expanding enough for any excess coolant to go to the expansion bottle.

    hope that helps

    Attached Files:

    • Like Like x 2
  10. Thanks mate, I am guessing this might be the case as well. Interestingly i removed the thermostat and placed it in a cold pot of water and brought it to the boil, the thermostat opened fully just before the water was fully boiling, hence 82c temp range, so i have eliminated that as the cause.
    I noticed also, that while i was bringing the bike up to temp on idle, to speed up the process, i held the revs at 3000 and watched the fan, it ever so slowly rotates at the higher revs, then comes on, but will go off again as soon as i close the throttle. I am searching now for a new temp sensor, will post again after i have obtained one.
  11. I'm voting for a crook sensor too.....
  12. I would expect it to be warm after the hot engine has ejected some hot coolant, prior to being sucked back in
    • Agree Agree x 1
  13. Thanks, sort of helps, however the fan is coming on to late, not too early. The thermostat opened at about 90c, which it should do. Thx
  14. "now the fan comes on when it half way to the "H"."...this is what I would expect to be normal...

    Thermostat opens @ 84 degrees, and is fully open @ 95...around the 95+ mark is where the fan should kick in to stop the temp climbing too high.
    If your fan was coming on as the needle was just nudging off the 'c' then in all probability it is coming on way too early and preventing the engine from getting to proper operating temp ( approx 85 -90 ) ie. faulty sensor.

    This is where having a hand held infa red thermometer comes in handy.
  15. Thanks Mike, I picked up a used sensor yesterday, fitted it last night with exactly the same result. I have a 0-200c analog thermometer which is very accurate. I have had this bike since brand new, whenever i started the bike from cold, regardless of the weather, I used to let it warm up until the fan kicked in. Just above the "C" there is a small gap in the white line on the temp gauge, this is where the fan would normally come on, but now it comes on as I said half way up the gauge. As it is now if i turn the bike off when it is this high, it has struggled to turn over, just feels like its too hot and the engine has expanded a little making internals tight.. after it cools down it turns over easily again.
    When its hot, there does not seen to be any flow to the catch tank, No the line is not blocked, making me think radiator cap, i will get it tested today. I also carried out, per the manual, the test of the gauge where you unplug the sensor and ground the green/blue wire in the plug, turn on the ignition and the needle moves to the "H" quickly. This says the gauge is OK and refers the next test to be on the sensor. With coolant heated to 80c, there should be a resistance of around 2 ohms. I have an anolog multimeter, but test was inconclusive, think the multimeter was not reading correctly, howver i got the same result with the 2nd sensor i have, so i concluded they are both doing the same thing, either both working or both failing. It is a real frustrating problem, that i know. I will fully bleed all the air out tonight and try again, possibly with a new cap if mine fails testing. Will keep you all posted.
    BTW.. if anyone has a 900 Hornet, maybe you could do a quick check, with the bike cold or hot..remove the plug from the sensor and turn the ignition to on.. does the fan come on, mine does
  16. This sound pretty serious, then...
  17. When testing the sensor its the drop in resistance from 2 ohm @ 80 deg. to 0.7 ohm @120 deg that is the important part..... that drop in resistance is what the ecu interprets in order to turn the cooling fan on and off.
  18. is the fan switch and temp sensor pre or post thermostat (in terms of coolant flow direction)

    if switch is pre, and sensor is post, makes sense the fan would come on when the temp appears low (but may not be low pre thermostat), and after thermostat opens, fan appears to kick in at higher temp.
  19. The switch and the sensor are one and the same. I think its called an ETC (Engine Coolant Temperature ) sensor. It is located on the cylinder head.
  20. Just do the obvious coolant system checks. Does the outlet radiator hose get warmer once thermostat opens. If yes then your thermostat is working. Does the coolant piss out when you have the radiator cap off with engine warm and once engine is revved it starts circulating, that's meants water pump is flowing the coolant.
    If your temp sensor is not the culprit then the only other thing you could try is hooking up a digital read out and seeing if the fan turns on per factory specs.
    Unless your gauge is cactus.
    Also might help resetting the ECU in case it has gone bananas.