Fuel sensor. A frightening looking thing.

I was preparing my fuel tank for repainting today when I noticed a bit of dampness around the fuel sensor so I thought I'd better check it out so I took it off.

Wow. This thing does not inspire confidence. A few naked contacts and a condensor or something on the end of a bar and all this sitting inside the fuel tank. Just one spark from any of it and....ooops!

Anyway I'm not trying to frighten anyone and I'm sure it is perfectly safe 8O

The thing is I'm not sure if it's working and can't test it during driving as the bike is now in bits. So any ideas on how to bench test it? However I'm not too worried about getting it working if it doesn't and might even blank off the hole for it altogether but if anyone has any opinions on the subject of getting it working or not, fire away.

 

Put the float in bucket with some water and run 12 Volts through it while adding or removing water. Should get a variable voltage as the water level changes. Unless bikes are different, in which case I'm probably worng.

 

I wouldn't worry about sparking. It only gets a few milliamps.

Use an ohmmeter between the two wires. It should show a variable resistance as the float arm is moved. My XJ750 fuel sender measures from around 30 to 300 ohms. Not sure if the XJ700 is the same.

 

It isn't a float. It only sends a signal to a light which I assume goes on when the fuel is low.

 

The thing on the end of the bar is the float maybe? In order to produce a spark, several hundred to thousands of volts are needed. There is a minimum voltage required to produce a spark, or electrostatic dischage (much less ignite fuel) and 12 V isn't it.

Interestingly enough, a similar incident to this is was caused TWA-800 to crash. The NTSB determined that the insulation of the wiring connecting to the low voltage fuel sensor system degraded to allow high voltage wires in the same bundle to arc to it. That combine with a nearly empty center wing fuel tank (more vapor) and an airconditioning system that ran coils near by (and the plane sat on the tarmac for a long time, heating the tank up) caused an explosion that destroyed the aircraft. So as long as you don't bundle your fuel sensor wire with your spark plug wires, I wouldn't be too worried.

 

"In order to produce a spark, several hundred to thousands of volts are needed. There is a minimum voltage required to produce a spark, or electrostatic dischage (much less ignite fuel) and 12 V isn't it."

you tell that to my jumper cables

 

they sure come in handy in town where you might not keep
as good a track on your fuel as you might when on a long ride.

i dont like running so low that the bike has to sputter for fuel before turning the petcock. mainly while waiting for the light to change in
a busy intersection.

 

hehe...I was wondering about that. Apparently hundreds of volts are needed to create an electrostatic discharge in atmosphere (when the electric field exceeds the dielectric field strength of e.g. air), but when connect a high current source, the current actually vaporizes the metal in contact and oxides it (burns), and vaporized metal has a different dialectric constant. So gitbox covered the high current case and I covered the low current case. Either way...nothing to worry about.

 

There is lots of bad information in this thread.

As Polock indicated, 12 volts will throw a healthy spark (in the atmosphere).

Milliamps will indeed ignite fuel (The spark across your spark plug is measured in milliamps).

Those sensors are scary looking. I've never opened one but I suspect there is a sealed pressure switch in that canister.

 

You need a proper amount of fuel and oxygen to get an explosion. There is an upper explosion limit. "UEL" and lower explosion limit "LEL". A tank full of fuel is not with in UEL or LEL. and is unlikely to explode. We all know when we try to start a flooded motor how hard it is to start. Modern vehicles have fuel pumps in the gas tank with no problems. The switch in the tank is on the ground side of the circuit and will not spark.

 

A ground side switch can too spark (after all, one side of it is hot).

A full tank will not light. An almost empty one sure will.

 

Multimeter across the two leads or the contact points where the wires connect. Set your meter to 1x ohms scale and move the arm up and down. The needle should go up and down with the arm.

The one on my 750J wasn't working a while back (it's got the fancy-schmancy computer with the LCD gas gauge), so I pulled the float. Shot a bunch of De-Oxit contact cleaner through the box that the float arm goes into and worked the float up and down until the needle started moving. Seems to have done the trick.

 

A: Agreed; and I can add that I strongly doubt any XJ owner has ever been blown to bits by his fuel sensor sparking and igniting an empty tank full of fumes (should we call MythBusters?)

B: I can tell you for sure that on the XJ there is no sealed pressure switch anywhere; the fuel level sender is one of the most primitive devices I've seen yet (although the reed-switch activated self canceler is close.)

Yamaha apparently has two "styles" of fuel level sensor, one that increases in resistance full-empty and one that decreases. I wish I could add more specifics, but I sold my spare to another member and my DVOM was acting up (low battery) at the time so I didn't record any accurate readings.

 

heres what's in there, the piece of fiber that the resistance wire is wrapped around is curved away from the wiper arm after all these years and it looses contact, i got it working again by a little bend here and there, it's been awhile
if you loosen that screw and move it on the shaft it works again, but for how long ? i didn't put it back in

 

Mythbusters did do something similar trying to ignite gasoline with a cell phone. They found that ESD from touching your car was possible, but not cell phones.

 

OK, here's some real info for you:

For the stock Seca 550 fuel meter, the readings are supposed to be from 75 Ohms to 85 Ohms FULL; and from 6-10 Ohms EMPTY.

That's from the 550 Seca factory book; others may be different (or just backward to that.)

 

Brainiac: Science Abuse did something similar with a travel trailer, except it was filled with propane, I think it was. They used multiple cell phones inside the trailer, dialed them all, nothing happened, didn't blow up.

So they used explosives.

 

Here you go, Dermot. From the factory service manual. I hope this helps.

 

Thanks for that Gitbox, it couldn't be more exact.

Seems the scientists have really come out in force on this thread regarding the chances of igniting gas (petrol in my part of the world). I'm sure these senders are quite safe or someone would have heard otherwise by now. Anyway the XJ isn't the only vehicle with electrical devices in the fuel tank as most cars and bikes have the electronic floats and senders built into the tank.

The only reason why I thought this particular sender was a bit frightening was because there is one or two bare connections and that cylindrical thingy that reminded me of a type of condensor.

Thanks all for your input.