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Discussion Starter #1 (Edited)
Hello folks

I'm hoping you can help me pinpoint the cause of a peculiar looking issue with the 700.

A bit of background; I've had the bike for a couple of years and recently had the engine rebuilt by a Honda dealer to repair a stripped output shaft. The splines the sprocket mounts onto had basically worn away to the point where it was barely hanging on. Could've done it myself, but didn't have the inclination given that I've got a few other bikes about the place and the shop gave me a loaner. The (expensive) lessons I learned from that little exercise are that the output shafts don't seem to have been particularly well case hardened and to use Honda sprockets. That's the only significant issue I've had so far and I don't think any of this is particularly relevant, but thought I'd mention it anyway.

Here's the issue mentioned in the title. Yesterday, the bike's temperature gauge lost its mind, by which I mean it's recording entirely erroneous temperatures and updating every five seconds or so. It'll jump from somewhere in the mid-seventies to the mid-nineties, then down into the late-sixties within the space of a couple of hundred yards from takeoff at traffic lights. There doesn't appear to be any particular logic to the way it's acting; it's all over the place, and the only time it looks remotely accurate is from cold start to about 60 degrees. After that, all bets are off.

Unlike the older bikes, the gauge on the 700 is digital, so it gives the reading in, um, digits and normally, the numbers it reports are pretty predictable - they go up when it gets hot, and down when it cools. In normal operation, the gauge doesn't (or doesn't appear to) update every five seconds; it looks to me as though it smooths feedback/takes average readings over fifteen-thirty second splits and updates accordingly. From what I've read, another difference between mine and older models is that mine gets hot - properly hot on my run in to work (central London). It's unusual for me to make it into the office without the temperature reaching over 100 and the fans kicking in at least once.

From what I can tell, the fan isn't dependent on the temperature readout - a design choice I'm very grateful for right now. If it was, the bike would be overheating badly. But as I say, the temperature gauge readings are all over the place. And with after all that, I guess my main question is what's most likely to have broken? From what I've read, I reckon is it's either the temperature sender or something in the wiring loom rubbing against something it shouldn't. I'm praying it's the former - I really don't want to start a fight with the loom - so supplementary question, what's the best way to test the sender?

Thanks for sticking with me . . .

Neil
 

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Neil I have got 700 as well and it does not like traffic in the summer. Fan is on all the time but this is normal as long as it comes on at 103 C and turns off at 98C or thereabout. If this is the case it rules out the thermostat fault and leaves you with the electrical gremlins. I don't know how to test the sender but I am sure someone will be along......
 

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Discussion Starter #3
Thanks Queasy

Well, the fan's come on and gone off again, but as to what the temperatures were, I have no idea because of the crazy readings. The fan kicked in towards the end of my journey, which is consistent with usual behaviour, but the temp was reading somewhere between 75-88 deg. I know from experience that's wrong; it should have been reading way hotter. But like you say, so long as the fan is kicking in and switching off, it's good and not the thermostat.

I'm wondering whether there could be an airlock around the sensor, so in effect it would only be sampling the air temperature and periodic splashing from the water, but even if that were the case, it wouldn't explain the speed at which the temperature reading changes.

I'll probably pick up a new sender since they're not all that expensive and change out the current one. It could be that simply by undoing various bits and pieces to get to it I'll see something out of place . . . maybe.

Thanks again.
 

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Discussion Starter #4
Thought I'd round this out for anyone who has the same issue in future.

As I suspected, the temperature sender was knackered. Comparing the replacement to the busted one, the parts looked identical but the replacement had a significantly lighter colour to it. The old one was very dark at the tip. Dunno whether that has any bearing on anything; I suspect not because it's what's inside it that counts.

Anyways, it's not a particularly difficult job. The reason it's taken me a while to do it is 'cos I had several jobs to do at the same time - head race bearings, drive chain and cleaning the air filter - and so I had all sorts of bits off the bike making it a bit easier. But if you were doing this job on its own, I'm pretty sure all you'd need to do is remove the left side panel to get to the thermostat housing. The housing is the point where several radiator hoses converge, so once you've drained the coolant you basically undo the hoses and one mounting bolt and it'll come away. The sender screws into the side and plugs into the wiring loom. Sender part # is 37870-KRJ-901.

And the temperature gauge now behaves itself again. Yay.
 
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