Fan comes on late...diagnosing problem?

Ok so I'm in the middle of my CBR rebuild and the last mechanical thing I need to fix is this issue I have where the fan comes on late - around 111-112. It used to come on around 104-105 but slowly seemed to extend until the current cut-in temp.

Now given that I am using the temperature on the dash, and using that to measure when the fan comes on, my problems seem to be limited to one of:

1. The fan switch is out of spec
2. The temperature sender is out of spec

I had the fan switch on the hob today doing some testing. Held the end in hot coolant (keeping the temperature steady-ish for a few minutes before checking) and checked the resistance between the pin on the switch, and the screw base (which contacts the radiator, which in turn contacts the engine/frame thus grounding the switch). The spec for the switch seems to be 98-102 to turn on, and 93-97 to go off, according to the manual.

Now my testing showed that it wasn't on at 102-103, and it was on when I tested again at 105-106. When cooling down it went off by 99-100.

So...this is my problem. I need to figure out if I need to replace this switch or if I need to pull even more of the bike apart (oh joy) to get to the temperature sender on the thermostat.

Btw a few relevant things I've been thinking:

- There could be a difference between the thermostat temperature sender's reading & the actual temperature at the fan switch. Because the thermostat temperature sender is right next to the thermostat, where all the hot coolant would be swilling around, whereas the fan switch's coolant would have cooled down slightly. As a result I wouldn't be surprised if the temperature at the thermostat was 104-105 yet at the fan switch it was only 98-102 (and thus with an in-spec switch, the fan would turn on). Similarly 111-112 at the thermostat could equal 105-106 at the fan switch, which is how my fan switch is behaving now it's out and under test.
- 1) is more important than 2). If 2) is correct, then I'm just seeing an incorrect temperature. If 1) is correct, I might actually get overheating. So I could live with 2) but not 1).

Thoughts, inputs, anything appreciated, I'm not getting far with this! Prior experience of how these things fail, and how likely these things are to fail, is good.

Can anyone help with my diagnosis?

[yorkie_chris]

AFAIK fan switch is not anything to do with temperature sender unit in thermostat at all... completely seperate circuits

Yep, agree with that...I am just trying to figure out which one is wrong as they disagree (fan comes on at 111-112)

The fan switch is £35 from wemoto and that would make sure that only 2) could be the issue, and as mentioned...it's not *so* important (but I'd rather it was fixed if possible). I'm tempted to just buy that if no-one has any ideas other than pulling out the thermostat temperature sender (awkward) and testing that too

[yorkie_chris]

Point the expansion tank overflow at your knackers... when your spuds start sweating you know it has got too hot

Maybe I'll try it as a last resort, lol