Low fuel warning light

My low fuel warning light isn't working all of a sudden on my '85 XJ700N. I ran the bike to shut -off before switching to reserve just to see if it would come on. All the wiring looks good. Any suggestions on the process of tracing the problem? Where is the best place to start, etc?

 

Start by checking the bulb.

 

If it's not an external wiring issue it could be this...

 

It's been awhile but I finally checked the bulb and it seems to be good. At least it tested good with a ohm meter. I didn't try 12 volts because the bulb is so small. Assuming the bulb is good, can I test it by simply jumping the wires at the sending unit without damaging anything?

 

You can't test it that way. The resistance of the sending wiper increases as the float drops. Jumping the sending unit harness will tell the bike that you have a full tank of fuel.
Stick a 90 ohm resistor in there instead. Better yet, hook up a 120 ohm pot and turn the resistance up until the light comes on.


Even better....go for a ride. It's the weekend forchristsakes.

 

That's the best idea.

On the issue of "low fuel warning light" though the resistance will go down as the tank empties, which allows enough current to light the bulb. With a fuel gauge the resistance is usually lowest at full, and goes up as the tank empties.

The only info I could find on that fuel sending unit was the resistance for a full tank, which is specified at 1.1K +/- .2K.

Also, per the manual,

Check the bulb - OK / Check 12V at fuel sender connector (green) - OK / Check fuel sender

And per the specifications all the indicator lights are 12V 3w so shorting across the two pins should illuminate the low fuel indicator bulb.

 

Hmm... my memory (which is increasingly fallible) says that the fuel sending unit works the way that I described because of how the systems monitor works. Of course his XJ is not the same beast as the 750. I do know (or remember thinking that I know) that when the harness is left unplugged (infinite resistance) the low fuel warning in the 750 is triggered.

 

I think your memory is spot on for the 750 Seca with the CMS. However, Tabaka45's 85 XJ700 I believe is a much simpler setup - simply a bulb and a sending unit to warn the operator of a low fuel condition. Since the bulb and sending unit are a simple series circuit, then the resistance has to decrease in order to light the low fuel indicator lamp.

I can only speculate that the sender is a negative temperature coefficient thermistor, and when submerged in gas is cooled giving the higher reading. When the fuel level drops the thermistor would heat lowering the resistance sufficiently to light the bulb. That theory is somewhat supported by the manual which specs the resistance at 1.1K +/- .2K at 68° F.



Br wire 12V from signal fuse
10 - low fuel indicator bulb
11 - fuel sender
12 - tachometer

 

Rooster nails it (again)!

 

Rooster must have time to actualy read the FSM before replying

 

Yea, sometimes the new guys have to do that because we are still learning. I think this applies to you and quite a few other members on this site " you have probably forgotten more about these bikes than I will ever know."

 

Took the tank off today to check the petcock and went ahead and removed the low fuel sending unit. Sure enough the wire inside had come loose at the bottom. A little solder and it's back together again. Guess I'll know if that fixed it when I run low on gas.