Fuel gauge bouncing

[skunk]

08 Roadglide, fuel gauge fluctuates while riding. It doesent full sweep, rather moves about 1/4 tank. It does not stay in one place very long, kinda a constant movement. Would you suspect the sender or the gauge? It does track fuel level. thanks in advance

[smoserx1]

if it behaves when still but bounces while in motion it is most likely the gauge.  Fill tank and watch gauge movement when you turn on the ignition.  It should move slowly and smoothly from E to F.  If it slams against the F peg replace gauge, it has lost its dampening function.

[Ironpig9]

  Thanks for the answer. Read this post cause I'm having the same situation on my 01 Ultra. Will check it out.

[skunk]

if it behaves when still but bounces while in motion it is most likely the gauge.  Fill tank and watch gauge movement when you turn on the ignition.  It should move slowly and smoothly from E to F.  If it slams against the F peg replace gauge, it has lost its dampening function.

Thank you for your answer. Mine is while moving, When keying on with a full tank it goes to full slowly.

[Ironpig9]

smoserx1   I checked my gauge too. Mine slammed like a bug hitting the windshield. New gauge for the Ultra next time I go into the cowling. Thanks again for your help. - Ironpig

[JerseyT]

if it behaves when still but bounces while in motion it is most likely the gauge.  Fill tank and watch gauge movement when you turn on the ignition.  It should move slowly and smoothly from E to F.  If it slams against the F peg replace gauge, it has lost its dampening function.

Love the Search feature on this forum!  This old thread and response above answered my question re. whether the fuel gauge or the sending unit on my 2009 Ultra was bad.  Time for a new fuel gauge.

[bump]

Rode my 02 FLHTC out west last summer. About 100 miles from home gas gauge started bouncing around a little. About 1000 miles later it was bouncing about 1/4 tank or more. You could still tell about how much gas was left by trip meter. Got home after 4700 mile trip and replaced gauge, working good now.

[Deye76]

Do the older gauges act the same? The gauge on my 1992 pegs, and bounces but is still accurate.

[smoserx1]

Do the older gauges act the same? The gauge on my 1992 pegs, and bounces but is still accurate.

This has been my experience.  The sending unit in the tank acts like a variable resistor with low resistance when the tank is full and higher resistance when the tank is empty.  This resistance causes the gauge to receive various voltages corresponding to the fuel level; for instance 12 volts when full and mush less when near empty.  In fact you can test just the gauge by disconnecting the sending unit and jumping the two pins of the connector going to the bike.  With the ignition on the gauge should go from empty to full when the pins are jumped and this should be a fairly slow smooth sweep unless the gauge has lost its damping.  Anyway the resistance from the float changes instantly as gas sloshes.  A damper wire in the gauge itself keeps the gauge from reacting rapidly to these instantaneous changes in resistance (unless the wire breaks).  With a severed damper wire even when the tank is chock full the gas surface will press the float against the top of the tank for a while and the gauge will show full and not bounce till the level of gas recedes.  And with a partial tank when the bike is absolutely still the gauge will be accurate and not bounce, but going down the road the gauge will bounce according to the float excursions caused by the gas sloshing.  Hopes this helps explain it, at least to my theory of operation.

[Karl H.]

The needle damping is realized by a disc on the needle axis and silicone grease (see pic). At the Touring bikes where the needle axis is oriented horizontally the grease can flow down and away over time when getting warm in hot climate.

[attach=0,msg1322742]

[Deye76]

smoserx1, thanks.

[JerseyT]

The needle damping is realized by a disc on the needle axis and silicone grease (see pic). At the Touring bikes where the needle axis is oriented horizontally the grease can flow down and away over time when getting warm in hot climate.

[attach=0,msg1322742]

Karl, very interesting.  Have you been able to successfully repair a fuel gauge by cleaning off the old silicone grease and reapplying fresh grease? Can the gauge be disassembled and reassembled without using any new parts?  Thanks!

[Karl H.]

... Have you been able to successfully repair a fuel gauge by cleaning off the old silicone grease and reapplying fresh grease? Can the gauge be disassembled and reassembled without using any new parts?  Thanks!

No and no! 
To get access to the damping disc you have to unsolder and completely remove the stator windings.

[attach=0]

[JerseyT]

Karl ... Damn!  You're not kidding.  No way I'm tearing into that.  Now that I see what's inside that little gauge I feel better about ordering a replacement gauge for $75.  Thanks for posting!