94 VFR750 fuel light issues

So today on my way home on the 4th gen, I guess I watched the fuel gauge get a little lower than I should have. For the past 50k, the FUEL light comes on as it should with about a gallon left. Today, the light only came on as I was coasting to a stop on the side of a CA freeway @ rush hour. I was watching for it, then I was gonna fill up as I usually do when it comes on. Today, I got about a 1 second warning when the light came on, then I was sputttering. Bone dry tank. Good times pushing it UP the wrong way on a nearby on-ramp in the dark.

Anyhow, was wondering where to start the search for why the warning was so late with this tank. Any ideas guys?

 

OK...so here's what I can't figure out so far.

The fuel light on the dash works, as it comes on when you turn the key, stays on for a second, then goes out. So it's not the bulb.

And the fuel level contraption in the tank works, as when the tank was bone dry the float was resting on the bottom of the tank (that I could see with a light) and the fuel gauge was well past E (as it should be). When I just now filled it to the brim, it naturally swung and pegged to well above F, as it should. So I know the fuel float/gauge works.

Had a lot of readers on this...anyone out there have an idea of what else to check?

 

The low level sensor is seperate from, but still part of the fuel gauge sensor. Meaning that if the sensor is bad, you'll probably have to change the sending unit, as a whole. It is on the same connector, but uses the brown/blue wire and green/blue is the ground.

It is just a basic thermistor, that conducts when warm (due to lack of fuel to cool it).

Hopefully, but not likely, it is just a wire snag that can be easily fixed. Basically, there should be 12V to the light when dry and 0V when wet.

Good Luck.
:crazy:

 

OK. You're talking about the little black clip connector that runs from the bottom of the tank. There are 3 wires going into it. Thanks for the color help...brown/blue is the positive/power wire, and the green/blue is the ground. Got it.



[/QUOTE] It is just a basic thermistor, that conducts when warm (due to lack of fuel to cool it). [/QUOTE]

Oh. I get it. It comes on when the fuel level drops below it, it gets hotter due to not having fuel around it to cool it, and the light comes on. Neat design.


[/QUOTE] Hopefully, but not likely, it is just a wire snag that can be easily fixed. Basically, there should be 12V to the light when dry and 0V when wet. [/QUOTE]

OK. I guess I'll check the length of the wire to...the dash light? I know it runs under the tank too, but that part looks like it's OK with no nicks, etc.

Guess I'll have to wait till the tank gets really low and see if the light comes on. If not, then I'll check it for 12V. Or I guess I could check it now and see it anything registers. Would you just check it @ the connector, or pull the dash apart and check it @ the light somehow, or does it make a difference?

And thanks for the tips.

 

There are several things integrated with this circuit, which kind of helps with the troubleshooting. The fact that the fuel guage still works and that the light comes on at start up tells you the grounds are good and that the 12V supply is present. The fact that the light came on at the end, but too late, suggests that the thermistor is getting weak and just took too long to conduct. This would probably be caused by your constant use as a time to fill up signal. Although the 12V is always present, it is normally cooled by the fuel and it gets to live a long, happy life.

There is a chance that it might just need a cleaning, due to it's constant use. The remnant fuel may have been baked, which gummed it up over time, causing it to work slower. This is probably the best place to start.

:crazy:

 

Thanks for the tips/advice. So your thinking is to wait till the tank gets low again, pull the tank, pull the entire thing out of the bottom of the tank & clean it with....anything special I'd need besides maybe kerosene and a stiff brush? From what I saw, it's all metal parts and can take a good scrubbing, i.e. nothing delicate to worry about?

I usually wait for the light, then find the next gas station. Never thought that it would impact the life of the unit. I'll go from the gauge from now on then.

 

It may just be the 3 small holes in the end of the canister, that contains the thermistor. They are what allows the fuel to come and go. They are quite small and probably easy to clog up and clean up.

I always chicken out, before the light comes on. This is not a bike I wish to push any distance.

:crazy:

 

Well....went thru another tank, and the same problem exists. So the tank has to come off for sure now. Wondering if it might have anything to do with poor mileage on this last tank. I drove like a grandmother, and only got 150 miles from a full to the brim tank of fuel. That seems....low. Would a bad sending unit cause a leak where it goes in? I came o-so-damn close to running out again last night around midnight and having to walk. That last 1/8 tank dropped like a rocket...literally went from "I need to get gas" to "looks like I'm walking" within a half a mile or so. Stock engine and carbs, slip on 2BR, so no real mods to it.

 

Maybe just time for a tune-up. If it was leaking, you would smell the gas. I have never had a fuel gauge that didn't seem to jump from 1/4 tank to wtf.

If by driving like a gm, you meant lugging it, then poor mileage is what you'll get.

When you get it out, take note if the canister is full of gas or not. If it is dry, chances are it is probably just toast. You should be able to plug it back in to test it while it is out.

Good Luck

:crazy:

 

Well, I don't think it's a tune-up. I did the 50k valve adjustment, oil change about 1000 miles ago. I wasn't lugging it, just staying at the speed limit, which was hard as hell actually. I got up to 6th quickly on the freeway, and...stayed there, @ 65mph and 5k. 150 miles on this tank seemed low to me.

Thanks again for the tips. I'll see if I can burn up some fuel today and yank it out this weekend sometime and report back. Is there an o-ring on the sending unit? Does it come with a new sending unit if so?

 

I was curious about the "canister" you are talking about, so I found fotos here:

Gas gauge/light. This is what I get. - Third and Fourth Generation VFR's - VFR Discussion

Can you clue me in on the canister in these fotos?

 

Oh nevermind. The canister is the small C battery-sized part that the wire plugs into. Got it.

 

It's the little tin can with the wire coming out of it wrapped around the rod.

There is an O-ring for the sending unit. ($3-4) There is often a couple of sending units on ebay for $10-30.

 

I don't mean to hijack this thread, but as it appears to have reached a conclusion, I would like to throw another issue into the discussion. This group obviously knows and understands this aspect of the bike.

I've got a '91 that I've had since '95. When I got the bike, everything worked fine. It developed an intermittent tendency for the right side of the dash - fuel gauge, low fuel light, neutral light, indicators - everything - to not work. Sometimes it would work at start then stop - sometimes the other way round. It eventually got to the point that it simply does not work - I've been riding the last few years relying on the odometer for a remaining fuel indication.

I've had the dash apart and checked the obvious things and not found a problem. By the wiring diagram, there appears to be a common connection from the affected gauges to the "A" 10 amp fuse - but this and the wire appear to be good. Anyone got any other suggestions for things to check?

Not a serious problem - but it does annoy me!

Jeff Rees

 

Look at you, lurking for 4 years and your 1st post is a hijack.

First you'll have to confirm that 12V is getting to the dash. If it is good, you'll need to check the ground. An easy way to do this, is to provide a new ground to the cct, with a jumper. If this fixes the problem, then you'll know that you need to fix or replace the ground.

 

OK - I deserve that!

Not familiar with the CCT acronym - and a search says it is too short a term to return results.

 

Sorry, cct=circuit

 

UPDATE: Well, I pulled the fuel sending unit just now. Hooked it back up, and the gauge moves with the float as normal. Still could not get the light to come on though, and I even tested it with the tank bone dry before I pulled it out. I can see light thru all of the 3 holes in the canister...sorta. They ain't easy to see thru. You kinda have to move it around a bit in order to see light thru them.

The wire and the soldering connection looks fine, no breaks, etc.

Anything else I should check before I just ASSUME I need to swap it out?

Oh...I looked for the o-ring you spoke of, and didn't see it...on the canister? Where the whole thing connects to the tank?

Also, I read your post about seeing if the canister was full of gas when I pulled it out. I drained the gas before I pulled it out, so I'm not sure how to check for this exactly.

 

Oh. Found it.

 

If the holes did not allow the fuel to escape the canister, it would have been full, until you cleaned them out.

I'm not sure how long it takes for the thermistor to warm up and conduct. It could take a few minutes.

You can try hooking up a meter to see if it opens and closes, with a heat source applied and removed.

:crazy: