Low Fuel Indicator – Part 1: Debugging

In my first bike post, I mentioned several issues, one being that the low fuel indicator does not work. Here is my journey in figuring this issue out:

Possible Work Around for the cheap/lazy: Watch the KMs on your trip meter. Once it gets to 200km, get gas and reset.

To understand what is wrong with a system, you must first understand the system. The low fuel indicator system consists of several parts:

  1. A light that sits behind the plastics (the indicator)
  2. Cabling/wires that run from the light to the sensor (the wiring)
  3. A sensor of some sort that triggers the indicator (the sensor)
  4. Maybe a fuse

It also helps to understand what sort of sensor is involved with your particular vehicle. There are buoy sensors, like you see in the back of toilets (a floaty that raises with the fuel level). There are thermal sensors, like my bike has. There are other sensors too!

To test what the issue is we want to work in a logical order, but also prioritize by cost/laziness/likeliness of the problem. We can “test” by simply replacing parts until it works, or we can test that each individual component is working to save cash. Wires are cheaper than bulbs and sensors, but a pain to replace or troubleshoot at times. Sensors may be screw-in/bolt-on, but are more expensive than bulbs and require the tank to be near empty (to avoid spilling gas during the process). Bulbs might be cheap, but also might require removing a face plate to get at. Hmmm. So they are all kind of annoying.

Lets look at the different tests we can perform though to help us decide where to start:

  • We can turn on the bike to test the indicator, as most bikes light up all the indicators as a “test” for a few seconds when first turned on
  • We can take out the bulb and try it in another socket with a power source if we really wanted to
  • We can run test leads from the bulb socket to the sensor to verify minimal resistance in the wiring
  • Understanding how our sensor works (closed-circuit when the tank is low), we can “short” the leads to verify the wiring and bulb both work
  • We can remove the sensor, place it in and out of cold liquid, and test the leads with a multi-meter at each stage to verify the sensor is tripping correctly

We should start with testing the indicator. It is the simplest test as it just involves turning the key. Also, it eliminates 1/3 of the potential problematic parts.

Here are the basic steps to how we are going to test for the source of the problem today:

  1. Turn on the bike, watch to see if the indicator turns on. If it does not, try changing the bulb. If the indicator does turn on, move to step 2.
  2. Leave the bike on, remove the seat cover, short the pins of the sensor. If the indicator does not turn on after a few seconds (and you are sure you properly shorted the pins), investigate the wiring. If the indicator does turn on, move to step 3.
  3. When the gas tank is near empty, remove the sensor. Since it is the last piece of the puzzle, either assume it is busted and replace/repair it, or perform a test on it as listed above.

Okay, now that we are settled on what to do, lets see how it pans out for me!

First, I start the bike.


It lights up! That is good and bad. Good because the indicator works fine, and also any inline fuse does as well. Bad because now we have to do more debugging 😦

Next up is the wiring. Please be careful not to shock yourself during this step. Use a jumper cable (not the car kind, just a short cable with insulation and exposed ends) or an insulated tool if you feel daring I guess…


I like to live on the edge so I used an insulated knife… the low fuel indicator lit up right away! Phewf! The wiring is fine. I dread the thought of opening up sealed bundles of wires. That leaves one part: the sensor.

There are three options now: replace the sensor, fix the sensor (sometimes this can be done!), or live with the issue and develop a workaround. Research time!

  • Replacing the sensor is going to be hard, the sensor changed between the 1982-3 and 1984 models of the bike, and the 1984 model was not sold in the states. A similar model (VF700C) was sold but it did not have the sensor. I found some in Europe ranging from $120-180 plus shipping. Not really worth it to me.
  • The type of sensor I have is “easily” repairable, and several guides exist online. Even a youtube video! The thermistor (thermal resistor) only lasts about 20-25 years, and the bike is over 30 so that is likely the issue. A new one can be found online for $1-2 from newark, mouser, and (for the Canadians out there) digikey.ca. It is recommended that you shop around and consider shipping costs/exchange as well. Digikey has the part for $2 instead of $1, but the shipping is less than half the price as the other two for Canadians, plus no exchange rates/fees.

Now, I should probably take the sensor out and test it before moving on to the “part ordering” stage. I am not going to do so, because that requires having a near-empty tank. I will test it later, but at $2/part I really would rather take the sensor out just once instead of once to test and once to fix.

With the part on the way, it is just a matter of waiting until it arrives (and waiting until I have used most of my gas up) until I can make post #2!

5 thoughts on “Low Fuel Indicator – Part 1: Debugging

Add yours

  1. Yeah, seriously. You have a trip odometer, and you know the volume of your gas tank.

    I’ll bet you get about 13-16km/L, and assuming a 10L gas tank, you’ll get about 120-150km per tank. Reset your trip odometer every new tank of gas, and you have your fuel indicator.

    I mean, I get that you’re doing it as a hobby/project, but this is one where the time and stress aren’t even worth it to me.


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