Low Fuel Light Bulb Breva 750

[Wayne Orwig]

I don't think a new thermistor will fix it. It seems to me that shorting the wires to the dash should turn on the light.
I would think that the value of 2.5K would not matter as long as the resistance drops down low enough when the thermistor heats up when exposed to air.
The new thermistors I have that are rated as 2K test about 1.65K.

As long as the resistance is low enough to allow it to self heat. And of course low enough when hot to light the bulb.

I suspect that in cold weather, a 2k thermistor (which would be much more then 2k when cold) would have a hard time self heating to start the 'cycle'. I've always bee surprised that the 1.2k factory ones can get hot enough to work in cold weather.

Have you tried to get into the dash? Have you measured the voltage across the sensor with the key on? If no voltage, the issue is in the dash for sure.

[antmanbee]

As Roy suggested, I tested the current across the 2 wires to the dash.
This is getting interesting and confusing now for me.
When I tested the current the fuel light on the dash came on.
The current is 94mA. It takes about 5 seconds for the light to come on after the amp meter is put across the 2 wires.
A dead short still will not turn on the light.

[Wayne Orwig]

As Roy suggested, I tested the current across the 2 wires to the dash.
This is getting interesting and confusing now for me.
When I tested the current the fuel light on the dash came on.
The current is 94mA. It takes about 5 seconds for the light to come on after the amp meter is put across the 2 wires.
A dead short still will not turn on the light.

The ammeter lights the light, yet a short does not?
 

94ma is pretty much the current for a #74 bulb, like used in the other models.

[antmanbee]

Yes the ammeter lights the light.

I put a 162 ohm resistor across and it lights the light.
A 330 ohm resistor does not light the light.

So maybe the thermistor is bad and it just does not drop down to the proper resistance when heated up.  (out of contact with the fuel)

[antmanbee]

Another test with my new thermistor which is the one from the Ducati fix.
I had to leave it plugged in to the 2 wires to the dash for a minute or more and then it lit the dash light.
I had tried this before but only left it plugged in for maybe a half a minute or so and it did not lite. It felt warm but apparently it was not warm enough. I jumped to the wrong conclusion.
So it looks like my fix is to follow the Ducati thread fix.
People over there have had the fix working for several years now.
Wayne, you were right, I guess I was itching to replace the thermistor.

[Kiwi_Roy]

Thats great news.
I can't understand why your ammeter would make the light go but a short won't, pretty much the same thing, perhaps it has a cut-out for too much current.
I went and got the ribbon cable from a computer. I'm busy cutting it down to just 30 wires so I can test the dash and see what it does,
This is the sensor shown in your picture, I put one in a mate's Triumph Tiger, I think they are used in many cars as well
https://static6.arrow.com/aropdfconversion/599d112d4eefad38687211b895992cc0102dc8b9/aud0000ce2.pdf
I approached Panasonic about these last time and they denied all knowledge about them, I was able to find them on a Chinese website ----gate is all I can remember, I think $15 each
Any chance you could empty your tank to give the existing sensor a good test, 2.5 K sounds high but perhaps being in a can allows it to eventually take off, they do take a while.

I see you fixed the screen width, thanks for that.

[antmanbee]

I will empty the tank this coming weekend hopefully.
The sensor you linked would probably work. It does look a bit larger but that would probably not be an issue.
I think the key issue is what is the fully heated resistance. I think it probably needs to be in the range of 100-160 ohms. That specification is hard to come by.
I have the glass coated thermistors that I referenced from Digi-key in an earlier post that was also used to fix the Ducati.
If you look at post #41 and #42 in the Ducati thread there are some good pictures and description of removing the thermistor from the can and replacing it.
I will replace the fuel filter while the assembly is out of the bike and maybe the submersible hose too.
I got the bike with 23K miles and it now has 27K. I don't know if the filter has ever been replaced.

Thanks Roy and Wayne and everyone else!

[Muzz]

I believe so. On the V7, all the dash lights cycle when the ignition is first turned on. I shouldn't think the Breva would be different, same ECU as the early V7, though the dash is physically different.

Mine only lights upm at 160-170 miles, with about 5 lits left.

[antmanbee]

It should be fixed now. I just need to run it low enough on gas to see if it works.
The new thermistor worked before assembly by just putting it across the 2 wires to the dash and waiting a minute for it to heat up and the resistance to drop.

The failed thermistor,



I carefully pried open the folder over lip on the can and de- soldered the tip of the thermistor from the other end,



Soldered in the new thermistor,



The empty can with a tiny hole drilled in the end to accept the nre thermistor and solder,



The completed can with the lip bent back over with pliers and the other end of the thermistor soldered in, now ready to be attached to the pump assembly,



Pump assembly ready to go back into the tank,

[Kiwi_Roy]

One thing to be aware of, regular wire is not sealed from end to end gas could migrate up the cable between the strands. Hopefully you haven't compromised the tank seal where the cable enters the bottom of tank, just check once you have the tank full.

[antmanbee]

One thing to be aware of, regular wire is not sealed from end to end gas could migrate up the cable between the strands. Hopefully you haven't compromised the tank seal where the cable enters the bottom of tank, just check once you have the tank full.

I don't think this will be an issue. The thermistor/can assembly has it's own plug that plugs into the harness that comes through the fuel pump mounting plate.
There should be no compromising of that harness.

[antmanbee]

Went out for a spin.
The fuel level in the tank was low.
I was proceeding without any extra fuel in a can.
The light remained off.
I didn't know how far I would have to go to get the light to come on if at all.
Only just a mile down the road the light started to flicker.
After another mile it was on steady.
Pulled into a gas station and filled it up.
3.17 gallons.
Light off again.
Yeah! It works.
 

[malik]

So far, so good.

[Kiwi_Roy]

Congratulations on a successful project
I was curious so I looked your sensor up, although they don't actually spell it out its obvious they make sensors for measuring fuel.
https://sensing.honeywell.com/honeywell-sensing-temperature-sensors-line-guide-009033-4-en.pdf look at 135 series on page 6
Here is another thermistor that Guzzi have been using for engine temperature, Air temperature etc
https://www.digikey.ca/en/products/detail/vishay-bc-components/NTCLE203E3302SB0/2601763?s=N4IgTCBcDaIHIBUDCAZAomADAZjd7mYAygEKYgC6AvkA
The one they use is 3000 Ohms at 25 degrees Celsius
NTCLE203E3302SB on page 3 you will see the Ohms at different temperatures lines up with the Air and Oil temperature sensors in the guzzi manuals
I'm sure Honeywell have the same thing.

[antmanbee]

I was looking for a data sheet that gave the resistance chart like the one you linked for the vishay thermistor. I could not find one.
The chart is very useful but the bit of missing information that is still needed to determine if the thermistor will work is what temperature/resistance value is it with voltage across it when out of the fuel and in the air?
To work I know 320 ohms is too high and 0 ohms is too low. So the (out of the fuel/in the air) resistance needs to bee somewhere in between. I know 162 ohms works but I don't know what the actual range is
Testing with a potentiometer could determine this range.

I found some more of the glass coated thermistors that look to be the same on ebay for really cheap.
Search,    10pcs NTC Thermistor 2K ohm 1% B 3800 Diode Type Glass Encapsulated Thermistor
I didn't want to put in the super long hyperlink and have the text wrap issue again.

[Wayne Orwig]

I was looking for a data sheet that gave the resistance chart like the one you linked for the vishay thermistor. I could not find one.
The chart is very useful but the bit of missing information that is still needed to determine if the thermistor will work is what temperature/resistance value is it with voltage across it when out of the fuel and in the air?
To work I know 320 ohms is too high and 0 ohms is too low. So the (out of the fuel/in the air) resistance needs to bee somewhere in between. I know 162 ohms works but I don't know what the actual range is
Testing with a potentiometer could determine this range.

I found some more of the glass coated thermistors that look to be the same on ebay for really cheap.
Search,    10pcs NTC Thermistor 2K ohm 1% B 3800 Diode Type Glass Encapsulated Thermistor
I didn't want to put in the super long hyperlink and have the text wrap issue again.

Most thermistor specs include a chart to give the thermistor value at various temperatures. I put a thermistor on the microprocessor based battery charger I just designed and the thermistor included a chart with the resistance value at every 5C degree step. So it was easy in my software to determine the battery temperature within 5C. Then interpolate the actually temperature between each step. I was surprised at how accurate it really was.

In this case, since it is self heating, the length of the leads and a lot of such things will determine how fast the heat can escape the thermistor, so it will determine how hot it gets and thus the final resistance in open air. I'm still surprised that a 2k ohm thermistor can self heat enough to start the 'cycle'. That is only 0.072 watts at 12 volts. Not a lot. I still wonder if it will work in cold weather when the thermistor resistance is even higher.

You mentioned earlier that the light 'flickered'. On the EV style setup, the thermistor slowly heats up and passes more current to the bulb, as the thermistor slowly heats more and more. It is a very simple bulb and thermistor design. Eventually the bulb starts to glow and get brighter and brighter. It takes a few seconds to come on. Does the bulb on yours actually flicker on and off rapidly? If so, that would mean a more complicated design. Not sure why they would do that, other than to make the bulb light brighter. The EV type bulb never gets real bright.

You can use https://tinyurl.com/ or such to generate a short URL to add to a post.

[SoCalBreva750]

Just an FYI, a low fuel sensor from a 67-73 Mustang / Cougar Low Fuel Warning SENSOR - Fuel Sender Mounted $16.75(Ebay) works!!  Thought it was a fail until I was referred to this post and discovered I needed to leave the ignition on for longer than a minute..

[Yard Sale]

Necro thread

My B750 ran out of gas last week without the low fuel light ever coming on. It usually comes on with 1.5 (US) gal remaining.

[Muzz]

Necro thread

My B750 ran out of gas last week without the low fuel light ever coming on. It usually comes on with 1.5 (US) gal remaining.

I know how many miles mine will do before it comes on. It has never failed me yet; howver, if it went say 10 miles past where it shou8ld be on I would be most definitely stopping and give the tank a slosh. If it was sounding low I don't think I would risk it. I can assure you, the Breva might not be as heavy as a big block but pushing it a couple of kms up a slight rise is something I don't want to repeat.

[MotoGuzzi750]

Just spoke with Gonzo. On his Breva, the low fuel light does NOT cycle on initial ignition. Although it does come on sometimes, then fade out without fuel being low - and that's on an 8,000 km bike.

On my 2004 Breva 750 with ~22k miles, the fuel light is fairly accurate. As in, when the fuel level is low, it comes up. When I fill it up, the light goes out.
Basically, the light does a good enough job of warning me to fill the gas