Exactly how does a Special fuel sender work?

[fredintoon]

Or, why does mine not work?
Right now it's out of the tank and my multitester reads 1850 Ohms where Clymer's sez 1600 Ohms is the maximum.
But WTF happens inside the thing? Something should happen? OK, if it's fritzed I'll look for a replacement. Not knowing how the thing is supposed to work puts me in the position of someone giving a chicken to the Witch-Doctor so his crops will grow.

[ross]

hey there fredintoon.....sorry no chickens here in michigan, but i gotta nice white cockapoo that i would galdly sacrifice for the cause.....sorry but after havin malamutes, dobermans, labs, and "taco bell" dogs, this this is like riding on a honda 70 right after you took your xs out and found out just how fast she is in 5th gear revved 2 the redline....just another mystery found within the world of xsiveness....just my 2 cents...ross

[Bob Falter]

Fred,

Someone told me, or some where I read, the fuel sensors resistance changes when it is exposed to the air as the gas level drops. The gas evaporating off the sensor cools the sensor and its resistance changes (drops I would assume). Kind of a rinky-dinky idea, at less that is what I thought when I hear it. You could test this by putting your sensor in a freezer for a while and then checking the resistance. You also could dip it in, and out, of gas while reading the resistance (be careful, alcohol may be safer). I would assume the resistance drops turning on the light (though with electronics it could turn the light on either way). Since yours is reading high to start maybe it never gets low enough to turn on the light. If you test it this way let me know the results.

Bob

[Dragon Tamer]

Fred,
They run on some sort of magic, you will need several chickens. Mine is very dependable, comes on everytime right at the same time I run out of fuel or very shortly thereafter. Therefore I depend heavily on the trip odometer and the reserve function. If you get it figured out let me know how many chickens it cost and where the guy is at.

[fredintoon]

Panic over

Um, I guess my chicken stays in the freezer until the next crisis. Using the search feature I saw a post from Dragonrider saying that the el-weirdo sender needs time to work. This in reply to the last guy who whined that his sender didn't turn the red light on. The last guy's test procedure was the same as mine, with the sender out of the tank, plug the sender in and turn the iggy on. Everything lights up except the red light. Conclusion?
The sender's f**ked. Wrong! It's just lazy.
This morning I plugged the sender in again and turned the ignition on and no red light. One coffee and cereal later and the red light is glaring bright. Who'da thought it? Now to extend the sender wiring so it can fit in my Standard tank backwards instead of upside down.

[John]

It shouldn't take that long to illuminate the light. When mine comes on, and I take a sharp turn, the light will go out, but will come back on in about 30 seconds. It will come on very dim at first, but will brighten quickly as it heats up.

[fredintoon]

Shorter than a quick breakfast

Hi John,
my previous tests were:- turn the key, see no red light, mutter obscenity, turn key off again. This morning I turned the key, saw no red light and then had breakfast while leaving the key on. An estimated 15 minutes later when I checked, the red light was on full bright. How long it took to light up, I dunno. Just that it wasn't the "instant on" that I was expecting. The next procedure will be an observed time test.

[XSPastor]

Thermistor in the tank

The special's fuel light is attached to a thermistor ( thermal resistor) in the tank. When the thermistor is submerged, it stays cool and the circuit is open; when it is exposed to air, it heats up and the change in temp closes the circuit and triggers the warning light.

[egsols]

On my first special the light would not come on until well into the reserve portion and perhaps 3 miles left to go. On my second special it lights within about 10 miles before I need reserve?

[XSPastor]

Yeah, the thermistors probably vary a bit. I've never take mine out to study it. Maybe the level at which they are mounted in the tank varies too? That might explain the differing warning light onsets.

[egsols]

Yeah, the thermistors probably vary a bit. I've never take mine out to study it. Maybe the level at which they are mounted in the tank varies too? That might explain the differing warning light onsets.

That's what I was thinking. Funny thing though, ever since I started riding, especially the specials I have never relied on gauges and have always used the odometer to judge how much fuel is left. Even on my new truck, with the computer that tells me "distance to empty", I use the tripmeter.

I knew I was close the other day and as I was approaching the gas station I toggled the "DTE" button and was presented with 0 kms to empty! Made it to pump, no problem.

[fredintoon]

Timed response

This time I plugged the sender in, turned the key and waited, watch in hand.
First glimmer, ~35 seconds. glimmer to full bright, another ~15 seconds. Total, ~50 seconds. Restated to better represent the level of accuracy, "not quite a minute."
Then I put the sender in the Standard tank (backwards with longer wires rather than upside down, WhyTF they put the sender hole on the other side I dunno). With the new tank in place and the fuel & vacuum lines installed and the ignition turned on so the red light was lit, I partially filled the tank, adding the ~ 2 gallons that I'd drained from the Special tank. It took perhaps a gallon being poured in there before the light went out. Assuming the thermistor (hey! a new word for me, thanks Tim.) to cool down by being drowned in gas a lot quicker that it heated up via a 12V 1850 Ohm resistor, that gives me mebbe a gallon left in the tank after I see that red light. On my next long ride I will strap my trusty jerrycan on the rack and check out how far the Standard tank will go from full to red light, to needing to switch to reserve and to running empty. This will also check if Mr Stupid epoxied the gas tap filter/reserve towers in backwards or not.

[XSPastor]

I can usually get about three gallons in my 4 gallon tank if I stop very soon after the light comes on. The delay fredintoon stated sounds about right. That delay is neccessary so that it doesn't give false readings by flickering whenever the gas sloshes. I usually have to switch to reserve within a few miles of the light coming on or she starts to sputter... YMMV

[BrianB]

Mine did not work when I first got my bike either. I took it out and tested the ohms readings. I use about 70 degree water and after submerging, no change in readings.

I cleaned it really good in Carb cleaner and retested. Now the Ohms reading dropped quickly as I removed it.

Put everything back together and it works great. My delay time is very close to Freds last readings. Also it does take longer to come on than it does to go out.