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Discussion Starter · #1 ·
There is a temp sensor on the thermostat housing on my VT700. But I can't find it in the electrical schematic. I was wondering what it does and where the wire goes.

Plus, is there a way to check if it is working?
 

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The temp sensor is not on the schematic because it is not connected to the electrical system. The system most likely uses a thermocouple to move the needle on the temperature gauge, and so it draws no power from your bike's electrical system. Because of this, it will continue to register a temperature even after the bike is shut off. Mind you, the temperature drops quickly because that particular area of the cooling system does not hold heat for very long.

You should see one cable going from the sensor, directly to the gauge in the tachometer housing. That connects the therocouple to the gauge, and that's all it needs!

So, that's why it's not on the schematic.

Hope this helps!

--justin
 

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Oops! Forgot to mention the other question you had. My 1968 VT700 barely registers a temperature on the gauge when I'm riding. Only If it's 85 degrees out, and I'm stuck in traffic will the needle move. The fan kicks in when the needle gets just to the red portion of the gauge, so I know it's working right.

Best way to determine if yours is working is to start the bike on a windless day, and let it idle for 15 to 20 minutes. By that point, the fan should kick on, and the needle had better not still be at the bottom of the temperature scale. If it hasn't moved, it's dead. Some of these meters, you can replace the sensor, or replace the gauge. But a lot of them come as a single unit with the gauge, cable and sensor all sealed together, so you have to replace the whole thing.

Good luck!

--Justin
 

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Discussion Starter · #4 ·
Thanks that makes sense. It has been a while since I rode the bike and I only rode it twice since I bought it. I remeber now seeing the gauge move so that sensor is working.
the temp sensor for the fan I'm not sure about. the guy I bought from said it didn't work, so he disconnected it and then hooked up a switch that directly turns on the fan. I'm wondering if he just didn't see it come on and then assumed it didn't work.

Away, I have the radiator out for cleaning. I would like to check the fan temp sensor before putting it back together. I figure I will just hook up a volt meter to it while putting it in boiling water.
Do you happen to know approx what temp it should turn the fan on?
 

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According to the 1986 VT700 Shadow Service manual, the switch should be an open circuit below 199 degrees, and the switch should close between 199 and 207F. The manual said that the switch is sensitive to rapid temperature changes and will not function correctly if the temperature changes quickly. They said not to just dunk it into boiling water and see what happens.

Also, it said not to submerge it in water. Rig it up so that the part that goes in the radiator is under water, but only up to the threads. Put it in a pan of room temperature water and put your ohm meter between the terminal and the housing. Make sure you have good electrical contact between the meter and the housing and that paint isn't acting like an insulator in any way. Then put the assemble on the stove, and bring the water up to boiling. Have a candy thermometer (or any thermometer that's fairly accurate and will register up to at least 230 degrees) in the pan also, and watch the temperature come up. You should show infinite ohms on your meter until you get somewhere between 199 and 207 degrees. Then the meter should show zero ohms (or at least no more than 3 or 4 if you have a digital meter!). In any case, the switch should switch on before the water starts to boil at 212 degrees! (or thereabout depending on your elevation.) Actually, if you're at really high elevation, it's possible that the water may start to boil below 207 degrees, and if it does, you may have to add something to the water to raise it's boiling point, but that's a whole other topic, so I'm not going to worry about it unless this happens to apply to you!

In any case, this ought to let you know if that switch is working or not!

--Justin
 

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tubes_rock is right about checking the temp sensors - both fan switch and temp sensor.

I checked both of mine (87 VT700) last spring when I bought it - fan wasn't working - turned out to be dirty brushes/commutator on the fan motor blowing the fuse. Fan thermo switch was good, thermo sensor for the gauge was off-spec, but I still got a low reading on the gauge. New sensor from Service Honda was $34 - this is consistantly lowest price place I've found for parts. Didn't really need it, but that's just me, if it ain't TOTALLY BROKE I might fix it anyway!

I just checked the schematic in the factory manual, both sensors are shown. Top center on the page - Thermo Switch is shown along with fan motor and radiator callouts. To the right of this is the Thermo Sensor for the gauge (right next to Pressure, Overdrive, and Neutral switches.

Gumpy
 

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Discussion Starter · #7 ·
Ok, found the coolant temp sensor in the schematic. I guess I'm still thinking about my old bike, because when it said "temp sensor' next to the oil pressure sensor, I thought it was the oil temp sensor, like on my old bike that was air cooled.

Away, I checked the radiator temp switch, I'm out of luck. The switch did not close even when the water was boiling.
Guess I will try to find one on Ebay since Bike Bandit wants $80 for one!
or maybe I'll just stay with the manual switch.
 

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tikicarver

Try Service Honda [/url]http://servicehonda.com. About $46 for the thermo switch....ing the dreaded "discontinued" parts. Gumpy
 
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