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1996 Honda Shadow VT1100
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Discussion Starter · #1 ·
Hey everyone new to this forum. I have a 1996 VT1100 that I’m in the process of bobbing. I am completely redoing all the wiring and installing the motogadget m-unit blu, motoscope tiny, m-buttons etc…… well my problem is the new gauge does not have a coolant temp light on it, I was planning on taking the stock temperature unit located behind the speedo and wiring it up and installing a LED light in place of the 194bulb. But before I go doing all that I wanted to test the system to make sure it is properly working. So the temperature unit has 4 wires coming off it. I have a black and brown wire that goes to 12v. I have a green wire that goes to ground and a black and green that goes to ground. Last wire is green and blue that goes to the temperature unit. When I hook up the black and brown to 12v battery and the green, black and green to neg. battery I’m measuring 12v at the green and blue wire going to the temperature unit. The light will come on for a few seconds then turn off. Seems like it’s working so far…. I then took a pot of water almost 230F and placed the tip of the temperature unit in the water to simulate the bike overheating to see if the light would come on. It’s not coming on. I’m having trouble finding a spec. For what temperature the coolant needs to get at before the coolant temperature light will come on. Not sure if it’s just the water is not hot enough or I have a faulty temperature unit or even a faulty temperature meter. I have had the bike for almost 8 years got it running then and it’s been sitting, just now am I getting back into this project so I don’t know if there was a issue with this system before I started bobbing the bike. Any help would be greatly appreciated
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'83 VT750C
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It’s an old sensor, but if it’s clean, I would still expect it to work. Can you get that water up closer to 300F?

(That sounds silly, but I thought I saw 130-140C somewhere… if that’s even possible… 🤔)
 

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I'm thinking that the temperature sensor on your '96 is not a switch, but rather a thermistor, that goes down in resistance as it gets hotter. That's what the manual for my '96 1100 ACE says anyway, and gives specs of 47 - 57 Ohm at 176 degrees F and 14 - 18 ohm at 248 deg. F. My bike's manual says to reject the thermosensor if these values are more than 10% out of spec.

This feeds a buffer circuit built into the speedo that looks at this varying resistance and decides when to turn on the overtemp annunciator lamp. I'm not seeing a temperature spec. for exactly when this is supposed to happen, but for my VLX it is in the neighborhood of 260 degrees F. Water boils at 212 degrees F. To get it to boil at 260F you'd need to pressurize the system up to 20psi or so. 50/50 coolant boils at 226 deg F, in an open pan.

When testing my VLX sensor, I gave up and just pointed a heat gun at it, and a thermometer held in proximity. This eventually turned on the light, but it was anything but a precise measurement doing it that way. I've since found information that pure ethylene glycol, at atmospheric pressure, boils at 386 deg F, so this would be a way of achieving high enough temperature to test the overtemp lamp in a controlled way.

If the thermosensor, connected to the speedo circuit works correctly, everything is good, but if not, the thermosensor and speedo would need to be tested individually to see where the fault lies.

It's more important in my opinion, that the thermostatic valve be working, and this is supposed to start opening around 175-185 deg F, and be fully open by 200 or so. This temperature can easily be achieved in an open pan. It seems to me that if the thermostat never opens, coolant will never flow through the radiator and things can overheat fast. The fan switch is at the bottom of the radiator, and that will never turn the fan on if coolant isn't flowing. The manual will tell you how to remove the thermostat and check that it is opening in a pan full of water on the stove.

The Fan switch is supposed to close at around 210 deg F.
 

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1996 Honda Shadow VT1100
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Discussion Starter · #4 ·
Thanks for all the input. I ended up using a heat gun like suggested and almost instantly the light came on. So now I know the system is working as designed. Now I’m gonna wire up the LED in place of the 194 bulb verify is still works then back onto the build. Thanks everyone who chimed in, greatly appreciated
 
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