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Fuel Sensor Issue - Electrical Issue?


Zero Knievel

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Okay, a summary of where I'm at....

 

My fuel gauge (dash) reads 1/4 when "empty" and 3/4 when "full."

 

Disconnect the sender unit and test the harness.  Reserve light works.  Disconnected and power on makes the needle drop to the bottom.  Jump the terminals and the needle "slowly" goes up just past the full mark (as it should).

 

So, I know it's not the gauge.  I know it's not the harness (or should not be).

 

Testing the old unit, manually having it at full and empty reproduces the problem.

 

Ordered a new unit...shipped from Germany.  Plugged it in and the problem remains.  Odds of a new part having the same problem as the existing part...my guess is very slim.

 

I don't know where else to look or what else would explain the problem.  Since F/E = current allowed through the harness (the sensor basically being a rheostat), something bleeding off power from the return wire?

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Howd I guess it wouldn't be the sender?

 

So your open/closed circuit test told you that the gauge & wiring is good but senders don't sweep from open to closed circuit, they sweep from one amount of resistance to another.  A few are closed at one end or the other, none are ever open at either end.  You were basically sending overpowered signals to the gauge so you got max and min readings even tho there's a fault.  You can verify buy testing the resistance of your senders and see what their range is which will pretty much verify that the old one is good if it matches the new one.

 

I doubt it could be wiring, a resistance or power issue should make it read too high or too low all the time.  I assume it's a bad gauge unless the sender arm was sticking and unable to make its full swing when in the tank, pretty unlikely if its smooth feeling and makes a full stroke out of the tank without major resistance...unless you have added anti-slosh stuff or something to the tank.

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The problem is that there's no instructions on how to test if the gauge is bad or good.  If I was going to replace it, I'd like to confirm it's gone wonky before laying down the $$$.

 

The test for the sender is to measure resistance at each extreme, if it's in spec, it's working properly.  That's testing fine.  I'm trying to see if something's throwing off the resistance the gauge is seeing.  I don't see anything burnt, corroded, or otherwise contaminating the gauge component or it's connectors.

 

I doubt multi-meters are designed to create artificial resistance for testing.  I'm not sure what kind of tool would do that.

 

***

 

Well, the 5 watt bulb in my head went off.  I realized if I could find a way to ID the terminals for the dash connector, I could measure the resistance of the sender at that point.  Took a few tries, but F and E positions register within spec, so it has to be the gauge that's gone wonky...unless there's a way to test the gauge directly...which I doubt.  At least this eliminates an intervening problem in the harness.

Edited by Aunt Zero
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There's always the possibility that a ground from the cluster isn't working right...throwing off the gauge's accuracy, but that's a whole other needle in the haystack.  I'm not even sure it should be an issue as it appears the gauge is a 3-wire setup that goes to the sender.  If there's a ground issue, I'd think my harness test would show something amiss.

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2 hours ago, Aunt Zero said:

The problem is that there's no instructions on how to test if the gauge is bad or good.  If I was going to replace it, I'd like to confirm it's gone wonky before laying down the $$$.

Like you didn't do with the sender.

 

The manual confirms my belief, if the sender and wiring is good it's the gauge.  What it's doing is odd, but it's something that a wire issue just couldn't do as far as I know.....BZITE.

 

Someone here may have a gauge laying around to sell ya cheap.

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Don't know what year bike you are working on, but if it's the ground connector corroded you're going to be pissed.  And if it's the positive connector like all my 99's have (identical to the ground one but all hot wires, located on the clutch side frame rail by the valve cover) you're going to be even MORE pissed as you have to pull the tank AGAIN.

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15 hours ago, Furbird said:

Don't know what year bike you are working on, but if it's the ground connector corroded you're going to be pissed.  And if it's the positive connector like all my 99's have (identical to the ground one but all hot wires, located on the clutch side frame rail by the valve cover) you're going to be even MORE pissed as you have to pull the tank AGAIN.

 

Do you have a pic or diagram of what this looks like...to help me spot it.  I will try to check that if I can.  It's a '99.

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It's been a long time since I had to do that repair, but I'm pretty sure it's in the main wiring harness, inside the left rail (clutch side) somewhere near where the coil bolts onto the airbox.  I stumbled upon it chasing corroded wires in the dragbike and it is nearly identical to the one for the grounds, which is back behind the battery box.  IIRC, the ground one is brown, the power one is green.  Both are taped up into the harness.

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Here you go, pulled these pics off of bird#3.  If you pop the filler panel off the left side of the tank and look down at the harness, that's where you should see this big square lump.  Picture 3 shows you the throttle body boot clamp bolt as a point of reference.  So it's actually just behind the engine, not beside it, like I originally thought.  Like I said, it's been about 10 years since I had to bypass it on the dragbike.  Sorry for the insanely small pictures, I had to take these with my borescope.

testconnectorbottom.jpg

testconnectorclose.jpg

testconnectortop.jpg

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Thanks.  I'll take a look.  Gauges are "stupid simple" and really shouldn't just stop working...or at least not suddenly lose accuracy.

 

I had a thought that it might be a weak battery, BUT that didn't add up as the problem occurred while riding with voltage well within spec when the motor's running...as compared to running off just the battery.

 

For a while now, before the failure, I noticed the gauge reading near midpoint when powered off then going to correct reading once powered.  That always struck me as odd, but I figured it as particular to the XX.

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When powered off some gauges stay put, some drop, some just kinda float.  My recollection is that my analog XX dropped.  The gauge is a simple thing, as are the sender and wires, but obviously there's a flaw in one of those simple things.  It's a good idea to check out the connector that Fur pointed out, especially because it can save the harness from major damage, but my bet is with the gauge after the testing you did.

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Why would the sender, as simple as it is in design, go bad?  Why would a wire connection, as much more simple as it is in design, go bad?  The same could be asked about thousands of things that just wear out or fail, sometimes with no visible signs or warnings of impending doom.  Just sticking to the bird I can think of a few quick examples of things that frequently fail without notice and without visible signs of having failed, CCT, stator, R/R....the three most common failures on a bird.

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Didn't install it (yet) as the old one reads as okay.  I'll double check this weekend to be sure.  If the old one's good, I can part with the new one.  Better than waiting 4-6 weeks for one from Germany.

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6 hours ago, Furbird said:

My sender is bad in the flat black bird, but that's because it sat up for a year with a dead injector.  Guess I'll have to get a used one on ebay because I shudder to think what an OEM one would cost.

The sender shouldn't go bad from sitting, unless it rusted out.  You might be able to clean it up and make it work again, or buy Zero's new one and be set for life.

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Oh, its rusted pretty bad.  I had to scrub the tank out as well.  Pretty sure it had E10 in it when the injector went out and it was low on fuel.  Quite the mess.  Probably can't afford a new one on government salary.  I don't have that Dave money!

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Maybe he'll subsidize you.

 

I don't recall the resistances, but maybe you can graft in a universal replacement.  Or maybe Zero wants to keep his new one and sell you a working used one at a decent price.  I assume it can't be returned, or at the least would be subject to shipping and a restocking fee.

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Okay, here's where I'm at.

 

Took off the airbox and looked around.  I see the "wire bundle" and it's wrapped up tight.  I'm simply not inclined to cut it open unless I eliminate everything else because I know I won't do as good of a job wrapping it back up.

 

Removed the main ground-to-frame wire and did some work with a file to ensure good metal-on-metal contact.  Then I looked about the instrument cluster.  There's a big blue wire connector on the left side that apparently feeds the instrument cluster.  I opened it up, and here's what I found.

 

IMG_3440.jpg.8db0f84a1383d0b85923be1104a70c56.jpg

IMG_3438.thumb.jpg.3fc97ebfecafe1b1c644fd2bdbed65de.jpg

IMG_3439.jpg.0a9c79dd95cfb78a108680c55936707e.jpg

 

From what I can tell that's probably a ground wire.  How or why it's done that to the harness connector, I don't have a clue.  Since they don't sell parts to a harness, any suggestions on how to fix it?

 

I should note that the exposed part is encased and not exposed at all.  I had to separate them to see this, and last time I worked on it (years ago), I used dielectric grease when I put it back together.

Edited by Aunt Zero
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The connection was probably loose or dirty causing it to get hot.  You could just cut that wire and its mate on the other end and put terminals on them to reconnect outside the factory connector.  If you can hunt down the right terminals you could fix it to remain within the connector.  If that connector plugs straight into the instrument cluster and not another connector then you might have to get more creative.

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