Fuel guage wiring/type - Honda RC51 Forum : RC51 Motorcycle Forums
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post #1 of 11 (permalink) Old 07-13-2018, 08:13 AM Thread Starter
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Fuel guage wiring/type

How does the stock fuel guage run? Is it only two wires?

Does it send power through the float to sense level?

Need to know if it's compatible with the existing fuel guage on my lower tank of my '85 Shadow..

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post #2 of 11 (permalink) Old 07-13-2018, 06:52 PM
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An RC doesn't have a gauge , it's got a light that comes on when there's about a gallon and a half left.
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post #3 of 11 (permalink) Old 07-13-2018, 06:59 PM
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The 51 senses low fuel with a thermistor
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post #4 of 11 (permalink) Old 07-13-2018, 07:21 PM Thread Starter
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Ahhh, grooooovy. I'll fab up a way to keep the OEM guage. Thank you ☺️

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post #5 of 11 (permalink) Old 07-13-2018, 08:05 PM
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Originally Posted by Wibbly View Post
The 51 senses low fuel with a thermistor
I know what a thermistor is , but how does temperature relate to fuel level ?
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post #6 of 11 (permalink) Old 07-14-2018, 09:54 AM
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The negative from the dash indicator goes to the thermistor in the fuel tank, which is in turn connected to ground. There is always current in the circuit, but not enough to light the dash lamp when the thermistor is cold. So long as the thermistor is submerged in fuel, it stays cold and its resistance stays high, keeping the light extinguished.


When the fuel level goes below the thermistor, the small amount of current in the circuit heats the thermistor which reduces its resistance. As it gets warmer and less resistive the circuit current increases until the dash lamp illuminates.



This is why, even with very low fuel, it takes a minute for the low fuel light to turn on when you first turn on the bike.

It's also why you should not fill your tank immediately after turning the bike off if the fuel light was on. Best to let the thermistor cool off for a bit before shocking it with cold fuel.
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post #7 of 11 (permalink) Old 07-14-2018, 10:02 AM
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Thanks for that , I'd have never thought of that.
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post #8 of 11 (permalink) Old 07-14-2018, 11:15 AM
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i had a headache with it because i had to build my own fuel light when i changed my dash out for my new unit. it's tricky to get an LED to stay out just with the thermistor as they need SFA to light them up. i ended up running the thermistor circuit through the coil on a relay, and the LED off the contacts. the coil will pass enough current for the circuit to work, but not so much that it will latch the contacts.
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post #9 of 11 (permalink) Old 07-14-2018, 11:27 AM
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I'm used to 10K ohm thermistors for temp sensing.
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post #10 of 11 (permalink) Old 07-14-2018, 11:29 AM
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They're very commonly used devices. The one in the tank is a 10k iirc. But reverse acting where it loses resistance with heat.
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