Erratic temperature readings on 2002 VFR800

Discussion in 'General VFR Discussions' started by dan.sideen, Mar 20, 2021.

  1. dan.sideen

    dan.sideen New Member

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    My temperature readings are all over the place & change every few seconds. Range from 35C to 132C. When 132C, the temperature light comes on. But the bike isn't running hot. I suspect the temperature sensor, but wouldn't that generate an EFI error?
    Any suggestions before I tear it down & remove the throttle body to get at & replace the sensor? And just at the beginning of riding season here!

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  2. RllwJoe

    RllwJoe Insider

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    Is the level in the "over flow bottle" normal?
    It almost sounds like the coolant is low and the temp changes when an air bubble hits the sensor.
     
  3. dan.sideen

    dan.sideen New Member

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    Thanks for the tip. I dont tbink its low, but ill check

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  4. squirrelman

    squirrelman Member

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    it's best to check coolant level under the rad cap, not the overflow tank.
     
  5. Terry Smith

    Terry Smith Member

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    That has to be an electrical fault in the ECT sensor. There's actually two circuits in the sensor, one for the dash readout and the other for the computer to monitor. I'd try unplugging and reconnecting the plug on the sensor as a first step. From memory the sensor is screwed into the back left of the front head but I may be misremembering.
     
  6. Terry Smith

    Terry Smith Member

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    upload_2021-3-22_10-19-19.png
    looks like the Green/Blue wire heads to the dash so that would be the one to be checking. The service manual also has a page on diagnostic testing of the ECT, basically the resistance drops as temperature increases. If you see sudden extreme temperatures then that would suggest you are getting a short to ground (bypassing the ECT resistor) so a bare wire/frayed insulation somewhere on the G/Bu wire to the dash.
     
  7. dan.sideen

    dan.sideen New Member

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    Can I access the sensor connector without removing the throttle body?

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  8. Terry Smith

    Terry Smith Member

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    You definitely need to pull up the TB's to get in there, sorry. You probably don't need to disconnect too much however, I did this while changing out the thermostat and just lifted the TBs out of the way with the throttle cables etc still attached. Not a fun job however. If you haven't replaced the thermostat or any hoses now might be a good time to do so.

    It is always a good idea to try the simple stuff first, checking the coolant levels and radiator is full, but unless the fault developed after a drain/refill or other obvious intervention, my electrical diagnosis would still stand.

    Before you get into the TBs, you may be able to do some diagnostic work from the instrument unit back that would help to isolate the fault. You could check resistance to ground at the green/blue wire at the connector nearest the dash, that connects through the ECT to ground, and you should see a steady resistance when cold, that drops with rising coolant temperature. Wriggling the wires should not make the resistance change suddenly and would help to find where any chafing is. Seems more likely that you would have a wiring fault near the front of the bike where the steering etc is, than at the sensor.
    upload_2021-3-22_14-21-19.png
     
  9. dan.sideen

    dan.sideen New Member

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    I'll try the easy stuff first. Does the tank need to come off, or just raised?

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  10. Terry Smith

    Terry Smith Member

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    Just to check the radiator and reservoir, you need to take off the right side lower fairing to access the radiator cap, the reseroir is accessible by your left knee. If you want to get into the TBs then you need to unbolt the tank, I recall you can flip it completely over and lay it on the frame rails (with towels for padding), the cap won't leak when you do this.

    You can download the service manual from the other VFR site:

    https://www.vfrdiscussion.com/index.php?/files/file/246-98-01-vfr-service-manual/
     
  11. RllwJoe

    RllwJoe Insider

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    BTW how many miles are on this VFR?
     
  12. GreginDenver

    GreginDenver New Member

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    Getting weird electrical problems on a VFR800 but not getting EFI error codes is often related to a poor or problematic/compromised path-to-ground for the wiring harness.

    You should definitely look at this first (because it falls under the heading of "I'll try the easy stuff first")
     
  13. dan.sideen

    dan.sideen New Member

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    The bike has 50,000 KM. And today's project is to trace the wire from the display to the sensor. And hope to find a frayed wire. But I'm not optimistic. The erratic readings only start after the engine starts to warm up (ie over 35C) & get worse the closer it gets to operating temperature.

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  14. dan.sideen

    dan.sideen New Member

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    I traced the wiring for the temperature readout, but couldn't find the short. Didn't want to tackle taking off the throttle bodies at the start of riding season. So took it to my trusted local shop,.

    He replaced the sensor, but the problem didn't go away. He also traced the wiring but couldnt find anything. He suspects the ECU. (Although the bike runs fine) And this wasn't cheap. He confirmed that the bike isn't overheating & that the fans are working properly. (I couldn't get the fans to come on in the cold, early spring weather.

    My Haynes wiring diagram for the 2002 North American model shows a direct link from the sensor to the instrument panel & a separate wire to the ECU, so I'm a bit confused.
    I think it will be a winter project to redo the trace.
    Any words of wisdom?

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  15. Diving Pete

    Diving Pete Member

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    check your earths.
     
  16. raYzerman

    raYzerman Member

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    Yes check the grounds for sure. And with the bike on the side stand, check coolant level in the rad. Clean/fill coolant reservoir if necessary. With all that good, see if you still have erratic readings.
     
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