The V-max fuel gauge explained OR How and why it works and why it sometimes doesn’t.
Many V-max riders are puzzled by the workings of the V-max fuel warning light.
This Q&A is designed to answer some common questions and provide a technical explanation of how the system works and why it sometimes doesn’t work.
Q: “How big is the V-max fuel tank?”
A: The V-max fuel tank capacity is 15 litres, 3.3 Imperial gallons or 4.0 U.S. Gallons.
Q: “How big is my reserve tank?
A: The V-max does not have a reserve tank. The V-max has one fuel tank equipped with a low fuel level sensor and a low fuel warning light. The warning light will glow briefly when the ignition is turned on and turn off if the fuel level is higher in the tank than the low fuel sensor. If the fuel light stays on, the fuel level is below the level of the low fuel sensor.
Q: “If there is no reserve tank, why does the V-max have a ‘RES’ switch on the right hand side handlebar switch assembly next to the throttle grip?”
A: When the fuel level is low and the low fuel warning light is glowing continuously the fuel pump relay disables the fuel pump. In order to get at the fuel that is left in the tank the rider must move the switch to ‘RES’. This re-activates the fuel pump and you are on your way again.
Q: “Why does Yamaha turn off the fuel pump when the low fuel warning light comes on and then make you turn it back on again by using the reserve switch?”
A: This lets you know that it’s time to fill up. The V-max fuel tank is underneath the riders seat and is not a gravity fed system. Fuel has to be pumped up to the carbs so using a fuel cock to switch to the reserve supply is not an option. It’s a sure fire way to get your attention and not overlook the fact that your low fuel warning light is on. The fuel is pumped from underneath the seat so switching to reserve has to involve the fuel pump.
Q: “How much fuel is in the reserve fuel supply?”
A: On a new bike the reserve is approximately 3 to 4 litres but there will be individual variation between bikes. On an older bike the reserve may be considerably less or virtually nothing. Because of the way the sender unit works, the low fuel warning light can mean “As Good As Empty”.
Q: “How does the low fuel warning light work?”
A: The low fuel level sensor unit in the V-max fuel tank is a solid-state thermistor device. That means it has NO moving parts, N0 float and NO contact switch. Many other brands of bike use this type of sensor.
A thermistor is a semi-conducting material that allows current to flow when it is warm i.e. does not resist the flow of electrical current when it is warm but will resist the flow of electrical current when it is cool. A thermistor is best thought of as a slow switch.
The sensor uses the fuel in the tank as a heat sink. When the fuel level is over the sensor it is cooled by the fuel, becomes less conductive and prevents the input voltage from completing an efficient circuit to the electrical ground connection. This keeps the warning light off.
When the fuel level is below the sensor the surrounding air is not an efficient heat sink so the thermistor warms up and allows current to flow. This turns the warning light on.
So when the resistance in the circuit is high the fuel in the tank is above the low fuel level and when resistance in the circuit is low the fuel in the tank is below the low fuel level.
The resistance value for an unsubmerged fuel level sensor is 900 ohms +/- 200 ohms @ 20 degrees Celsius or 68 degrees Fahrenheit.
Q: “Why do some people seem to be able to go much further on their reserve supply and others just barely make it to the next fuel pump?”
A: The fuel level sensor is a temperature and voltage dependent resistor.
Variations in temperature, the quality of the electrical connections and the power available to the circuit will all affect its ability to complete a circuit to the warning light.
Here are some examples.
Example One – Temperature effects resistance.
It’s summer and your motor is hot, your aftermarket pipes have done away with the heat shield between the tank and the rear headers. This has warmed the fuel in the tank and so it is no longer an efficient heat sink for the thermistor in the fuel level sensor. Warm fuel will allow electrical current to flow through the circuit more readily than cold fuel so the low fuel warning light may come on early. In this case you would have (relatively) more fuel in the reserve supply. However, when a cold, wintery, wind chill is cooling the tank the low fuel light might come on at a relatively lower fuel level.
The low fuel warning light on the V-max will vary with temperature of the fuel heat sink.
Example Two – Corrosion increases resistance.
Corroded electrical connections (power and ground wires) add more resistance to the warning light circuit and make it difficult to complete an efficient electrical circuit. Your V-max will see this high resistance as a full tank. If the connection is severely corroded, the fuel sensor may never detect a low fuel reading.
Example Three – Fuel pump relay failure
A faulty relay may not be obvious. A faulty relay can have various types of fault each with it’s own consequences.
Here are a four relay faults and their consequences.
1) The green wire from the fuel pump relay powers the low fuel level sensor. If the voltage drops on this line, the thermistor will be way out of spec and may not activate the low fuel warning at all. The relay may still switch the fuel pump on and off and make all the satisfactory clicking noises when you power up the ignition and flip the ‘RES’ switch and still fail to deliver the required voltage to the fuel sensor.
2) If the relay is faulty and no voltage is supplied to the fuel pump when the fuel level is high but it will supply voltage to the pump when the fuel level is low, you are going to have to ride with the ‘RES’ switch flipped on all the time.
3) If the reverse is true then you can use the upper fuel level but you won’t be able to access the reserve fuel supply at all.
4) A total failure means the fuel pump won’t run. In either the ‘RES’ or ‘ON position.
Example Four – Changing OEM globes and instrumentation
[FONT="]You fit a ‘replacement’ globe to the low fuel warning lamp. Non-standard bulbs may change the electrical load on the thermistor circuit and cause false readings. Replacement bulbs must be rated the same as the original. Ditto replacement instrumentation.
Illustrations
1. Thermistor Sensor removed from can
2. Thermistor can (a Panasonic component)
3. Fuel level gauge
4. Fuel lamp 'off' circuit
5. Fuel lamp 'on' circuit
6. Thermistor data sheet
I hope you find this useful. As usual, comments and input are welcome.
[/FONT]
Many V-max riders are puzzled by the workings of the V-max fuel warning light.
This Q&A is designed to answer some common questions and provide a technical explanation of how the system works and why it sometimes doesn’t work.
Q: “How big is the V-max fuel tank?”
A: The V-max fuel tank capacity is 15 litres, 3.3 Imperial gallons or 4.0 U.S. Gallons.
Q: “How big is my reserve tank?
A: The V-max does not have a reserve tank. The V-max has one fuel tank equipped with a low fuel level sensor and a low fuel warning light. The warning light will glow briefly when the ignition is turned on and turn off if the fuel level is higher in the tank than the low fuel sensor. If the fuel light stays on, the fuel level is below the level of the low fuel sensor.
Q: “If there is no reserve tank, why does the V-max have a ‘RES’ switch on the right hand side handlebar switch assembly next to the throttle grip?”
A: When the fuel level is low and the low fuel warning light is glowing continuously the fuel pump relay disables the fuel pump. In order to get at the fuel that is left in the tank the rider must move the switch to ‘RES’. This re-activates the fuel pump and you are on your way again.
Q: “Why does Yamaha turn off the fuel pump when the low fuel warning light comes on and then make you turn it back on again by using the reserve switch?”
A: This lets you know that it’s time to fill up. The V-max fuel tank is underneath the riders seat and is not a gravity fed system. Fuel has to be pumped up to the carbs so using a fuel cock to switch to the reserve supply is not an option. It’s a sure fire way to get your attention and not overlook the fact that your low fuel warning light is on. The fuel is pumped from underneath the seat so switching to reserve has to involve the fuel pump.
Q: “How much fuel is in the reserve fuel supply?”
A: On a new bike the reserve is approximately 3 to 4 litres but there will be individual variation between bikes. On an older bike the reserve may be considerably less or virtually nothing. Because of the way the sender unit works, the low fuel warning light can mean “As Good As Empty”.
Q: “How does the low fuel warning light work?”
A: The low fuel level sensor unit in the V-max fuel tank is a solid-state thermistor device. That means it has NO moving parts, N0 float and NO contact switch. Many other brands of bike use this type of sensor.
A thermistor is a semi-conducting material that allows current to flow when it is warm i.e. does not resist the flow of electrical current when it is warm but will resist the flow of electrical current when it is cool. A thermistor is best thought of as a slow switch.
The sensor uses the fuel in the tank as a heat sink. When the fuel level is over the sensor it is cooled by the fuel, becomes less conductive and prevents the input voltage from completing an efficient circuit to the electrical ground connection. This keeps the warning light off.
When the fuel level is below the sensor the surrounding air is not an efficient heat sink so the thermistor warms up and allows current to flow. This turns the warning light on.
So when the resistance in the circuit is high the fuel in the tank is above the low fuel level and when resistance in the circuit is low the fuel in the tank is below the low fuel level.
The resistance value for an unsubmerged fuel level sensor is 900 ohms +/- 200 ohms @ 20 degrees Celsius or 68 degrees Fahrenheit.
Q: “Why do some people seem to be able to go much further on their reserve supply and others just barely make it to the next fuel pump?”
A: The fuel level sensor is a temperature and voltage dependent resistor.
Variations in temperature, the quality of the electrical connections and the power available to the circuit will all affect its ability to complete a circuit to the warning light.
Here are some examples.
Example One – Temperature effects resistance.
It’s summer and your motor is hot, your aftermarket pipes have done away with the heat shield between the tank and the rear headers. This has warmed the fuel in the tank and so it is no longer an efficient heat sink for the thermistor in the fuel level sensor. Warm fuel will allow electrical current to flow through the circuit more readily than cold fuel so the low fuel warning light may come on early. In this case you would have (relatively) more fuel in the reserve supply. However, when a cold, wintery, wind chill is cooling the tank the low fuel light might come on at a relatively lower fuel level.
The low fuel warning light on the V-max will vary with temperature of the fuel heat sink.
Example Two – Corrosion increases resistance.
Corroded electrical connections (power and ground wires) add more resistance to the warning light circuit and make it difficult to complete an efficient electrical circuit. Your V-max will see this high resistance as a full tank. If the connection is severely corroded, the fuel sensor may never detect a low fuel reading.
Example Three – Fuel pump relay failure
A faulty relay may not be obvious. A faulty relay can have various types of fault each with it’s own consequences.
Here are a four relay faults and their consequences.
1) The green wire from the fuel pump relay powers the low fuel level sensor. If the voltage drops on this line, the thermistor will be way out of spec and may not activate the low fuel warning at all. The relay may still switch the fuel pump on and off and make all the satisfactory clicking noises when you power up the ignition and flip the ‘RES’ switch and still fail to deliver the required voltage to the fuel sensor.
2) If the relay is faulty and no voltage is supplied to the fuel pump when the fuel level is high but it will supply voltage to the pump when the fuel level is low, you are going to have to ride with the ‘RES’ switch flipped on all the time.
3) If the reverse is true then you can use the upper fuel level but you won’t be able to access the reserve fuel supply at all.
4) A total failure means the fuel pump won’t run. In either the ‘RES’ or ‘ON position.
Example Four – Changing OEM globes and instrumentation
[FONT="]You fit a ‘replacement’ globe to the low fuel warning lamp. Non-standard bulbs may change the electrical load on the thermistor circuit and cause false readings. Replacement bulbs must be rated the same as the original. Ditto replacement instrumentation.
Illustrations
1. Thermistor Sensor removed from can
2. Thermistor can (a Panasonic component)
3. Fuel level gauge
4. Fuel lamp 'off' circuit
5. Fuel lamp 'on' circuit
6. Thermistor data sheet
I hope you find this useful. As usual, comments and input are welcome.
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