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 Fuel Gauge Mystery

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Cosmic_Jumper
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PostSubject: Fuel Gauge Mystery   Fri Aug 21, 2015 6:08 pm

scratch I’m putting this puzzle out to The Wisdom of the List:

I’ve been having an on-going problem with my fuel gauge. When the ignition is switched ON the fuel gauge sweeps to full and then normally it drops to whatever the fuel level is. However over the past few months, if the fuel gauge reads one bar above half, or less, and I park the scoot for the night (on the center stand, in an attached garage) and return the following morning the gauge sweeps to full then drops to reserve flashing. This will continue to happen until I refill the tank, in which case the gauge will sweep to full and then drop to whatever level the fuel is above that “one bar above half” level.

Now, when I take the scoot out for a ride and go approximately 2-3 miles the gauge will gradually begin to rise; first one bar, then another, etc. until after about 6-8 miles the gauge is up where it belongs. If I continue on my way the gauge will drop normally and a about 130 miles I refill the tank. During that initial 1-2-3 bar rise if I shut the bike off for a few minutes the gauge will revert to reserve flashing again, but soon correct after I start riding.

This problem began in late spring and I started adding Techron when filling the tank and also added Seafoam. After several tank treatments I switch to a premium fuel (Shell) thinking that it’s claimed detergent properties would help.

The scoot sat, unused, in the garage for a month with a full tank of Shell along with Seafoam while I was recovering from a medical issue.

Last weekend I decided to reacquaint myself with my riding skills and went for a day long 120 mile ride. The gauge performed completely normally. It never missed a beat, even after stopping several hours for an extended brunch. With 120 miles on the odo I refilled with Shell before stopping for the night. The next morning the gauge swept to full and remained there.

Two day later I went for another ride and returned with about 80 miles on the odo. The next morning the gauge swept to full then dropped to reserve flash. So much for my detergent properties of Shell fuel theory.

After much consultation with the wiring diagram and checking several internet posts I suspected that my problem was due to sulfur and crud deposits on the fuel gauge sending unit, but given all the Seafoam, Techron and premium fuel treatments, that remains to be seen.

After that 80 mile ride, and the gauge going to “one bar above” before parking for the night, I plugged in the battery charger thinking that something is partially draining the battery and that a full charge will fix the problem. I let that charger work its magic for over 24 hours, and when I turned the ignition on the gauge swept to full then dropped to reserve.

So without jostling or moving the scoot I fired it up and let it idle for a full 20 minutes, all the while watching the fuel gauge and the temperature gauge. After 20 minutes run time the temperature gauge was well into the operating range and the fuel gauge was merrily flashing away like a grannie on the freeway with her turn signal still on.

So the gauge will read full with a full tank; it will read normally throughout the full tank range if a full tank is used in one ride; it will drop to reserve overnight; it will gradually resume working over, maybe, 8 miles; the problem will reoccur if the ignition is turned OFF too soon; the battery is at full charge; and it doesn’t appear to be a function of run time; the fuel sending unit float is nothing that can get filled with fuel and drop to the bottom, or otherwise bind.

What is it that happens overnight which causes this issue?

It seems to me that if the problem were a failing component on the Combination Meter circuit board / fuel gauge that the 24 hour charge and/or the 20 minute idle (far longer that it would take to ride those 8 miles) would have been adequate time for that component to restore itself.

Any ideas, anyone?

Tim
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PostSubject: Fuel Gauge Mystery   Sat Aug 22, 2015 12:01 am

My (limited) educated guess would be a defective sending unit in the tank or a bad ground.The tank unit works on a very low resistance and can be very fickle. Howard
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Cookie
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PostSubject: Re: Fuel Gauge Mystery   Sat Aug 22, 2015 3:32 am

My guess is that your fuel sensor is faulty and the wiper in it is losing contact overnight. When you ride off the movement of the fuel jiggles the float and the wiper makes contact again.
The gauge is probably heavily damped which would explain why it increases slowly to the correct position - they do this to stop the gauge flicking up and down as the fuel sloshes around in normal use.
When you just left it idling there would have been no movement in the tank to jiggle the float around and make the wiper re establish contact.
It might get better with use since the bike has been stood up for a while - if not you could need a new sensor.

I had exactly the same happen on a car once.
These are just my thoughts obviously. Good luck with it.
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PostSubject: Re: Fuel Gauge Mystery   Sat Aug 22, 2015 10:53 am

Thanks Cookie. Thanks Howard.

I guess my next 'test' will be to repeat the idle test until the temperature gauge comes up. Then, after resetting the shocks to the softest position, start hard bouncing the rear end and see what happens to the fuel gauge.

I should probably rig up some form of jumper harness for the fuel pump / sending unit connection too so that I can initially test the input voltage to the sending unit to ground (probably 5 volts), then check any voltage from the output to ground. And again check those connections after the gauge starts displaying properly.

The only factor that throwing me off at the moment is that IIRC both of those sender unit wires run thru the Combination Meter board --understandably given the 5VDC power supply-- but the return signal ground) wire ultimately may not be the same as negative at the battery.

Maybe I'm over thinking this and creating too much anxiety.

Thanks again for the suggestions guys

Tim

Oops: I checked the FSM and wiring diagram. Of the two wires feeding the sending unit the Green-Black goes directly to a common Ground terminal, while the Gray-Black comes from the Meter ECU --and is, presumably, 5 volts. So if my digital test meter reads ~5VDC from Gr/Bl to Ground when the fuel gauge is malfunctioning that would confirm that the problem is in the sending unit wiper as Cookie suggests.


Last edited by Cosmic_Jumper on Sat Aug 22, 2015 1:13 pm; edited 1 time in total
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PostSubject: Fuel Gauge Mystery    Sat Aug 22, 2015 11:40 am

Unfortunately the Honda service manual (21-18) does not give a great deal of help on this matter. I wonder if a person could have another persons S/W and disconnect the wiring at the fuel sensor and use there sending unite to give a different reading to your gauge. Howard
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