Low Fuel sensor

CX500 & CX650 Turbo Forum

Low Fuel sensor

Brian FORD 05-18-2008, 12:41 AM
Does anyone have a broken CXT Fuel Level Sensor that still has a working thermistor for the Low Fuel Level light? Upon removal from my tank, the little metal case for the thermistor had "exploded" and looked like a flower that had just opened its petals. The thermistor inside was no where to be found.

Any theories on what might have caused this? This particular bike has certainly had a 'colorful' history, so anything was possible. When I removed the assembly from the tank, I noticed it was in sideways. I can probably guess that one myself.

Thanks,
Brian

Re: Low Fuel sensor

CXTURBOBOOST 05-18-2008, 6:33 AM
Brian,

I may have the whole assembly. I will need to look later this evening.

Pete

Re: Low Fuel sensor

George in Indiana 05-18-2008, 8:33 AM
Someone disassembled the capsule to get at the thermistor.

Best solution is to get a sending unit from Pete. If he doesn't have one you are out of luck as they have been long discontinued from Honda.

There is something I did with MY sending unit. I can give you a link and a number and whatever you do with that information is 100% up to you. I will not tell you what to do with it!

I'm not trying to be an ass by withholding information. It's just that:

1. I'm not an R&D guy employed by Honda.
2. I don't know others mechanical abilities.
3. We're talking about a tank that holds 5 gallons of gas next to your crotch.
4. This is a sue happy world.

We'll see if Pete can dig one up first.

CX500 TURBO
CX650 TURBO
'86 Honda Elite 150 Deluxe
'04 GSX1300R Hayabusa

Re: Low Fuel sensor

Don in Oz 05-18-2008, 4:07 PM

George, I sure can guess why you're a bit sensitive about information on the subject. That fuel-level sensor - and many others in cars and autos - seems to be a VERY "Heath-Robinson" device to have in a volatile fuel environment. BUT, provided you don't let too much oxygen into the tank, then you just don't get the right fuel-air mixture for a spark to cause an explosion, PROVIDED.........

I see that Yamaha now sense fuel level in some bikes by virtue of a float that carries a magnet, which slides up and down a pillar in the tank, past reed switches at various levels on the pillar - to me, a much more esoteric solution, and basically a lot safer also.

Re: Low Fuel sensor

Brian FORD 05-18-2008, 8:10 PM
George,

I believe the capsule is the case for the thermistor, so if they wanted to cannibalize it, they should have taken the whole capsule. The case looks like it burst open and was in long pieces now bent outwards like many peels of a banana. The base of the capsule looks a lot like the base of a single filament marker light, in that a single wire runs through the round PC board base and is soldered on the inside. I think that the wire of the thermistor was then soldered to it and the outgoing wire was soldered to the inside of the case for a ground return. It's the simplest of circuits designed so that as the thermistor element heats up, it conducts more current. The heat generated by it being always conducting is dissipated by its submersion in the gasoline. When it is no longer submersed (low fuel) the heat build up causes greater current flow in the circuit that activates the low fuel light. Probably any thermistor would do, but the industrial ones (the only ones Ive found) are relatively expensive when you only buy one.

If this gets to be too difficult to pursue, I'll just design a different way. I saw a Porsche fuel sensor that was designed the way Don mentioned, but nothing for an automobile will fit in my tank easily. I do have the threaded hole opposite the petcock, that might be adaptable. My KZ-550LTD has a low fuel sensor as well and I might be able to find other m/c sources.

OR I'll take a look at your design and try it instead. Where should I look?

I noticed there is a possibility of fuel being trapped on the right side of the tank and not getting to the petcock. It doesn't look like much but I was thinking more of trying to drain out sediment and water at the bottom on both sides equally. Perhaps that is why so many of these tanks rust inside. I had a Kawasaki H1 500cc triple that had a gas tank with a hose running underneath connecting both sides of the tank, so it would drain completely. Maybe I can apply that here.

One possible explanation for the condition of the thermistor casing is corrosion caused by water trapped in the tank. But I envisioned that ending with an "Apollo 13" scenario instead of just a small burst.

Re: Low Fuel sensor

George in Indiana 05-18-2008, 8:55 PM
Odds are a previous owner pulled the capsule apart to try and fix it. It's just a baffle with holes in the side to keep fuel from splashing on the thermistor.
Your theory of operation is spot on.

Thermistors do get hot. The one in MY 650T got up to 180 degrees F after being engerized in a mock up circuit all day long and placed directly above a shallow pan of gas. Flash point of gas is close to 500 degrees and as Don pointed out you need the correct air/fuel ratio. The millions of electric fuel pumps inside gas tanks are proof...I once pulled a really burnt one out of a Hyundai.

What I type below is for informational purposes only. Any part you buy and use from that company for any purpose is done so at your own discretion and risk!

www.newark.com

20F735

CX500 TURBO
CX650 TURBO
'86 Honda Elite 150 Deluxe
'04 GSX1300R Hayabusa