The V-max fuel reserve (guage) explained.

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gleno

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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=&quot]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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Last edited:
Great post! I wondered how that worked. I haven't had my max long but I don't think that my light works. I put gas in it at 80-90 miles and have yet to see the light. I may try 100 miles just to test it. All of my riding is done on back roads and I figure that I get less than average mileage due to my riding style. I just don't want to end up stranded on one of those back roads.

The bike had been sitting for a while and the corrosion thing makes since.
 
Thanks for the info. I ran my bike to sputtering a total of twice (before switching to reserve). The first time I was a couple of miles to the nearest station. Nice.

The second time I switched when the bike began to sputter, it made no difference. Out of gas. That was a pisser. Now I sort of understand how it could happen.
 
I get nervous when I forget to flip the switch back and I have over 80 miles on the odo. I haven't ran out of gas yet so I'm good with the process. I seem to get 100 miles + before the light comes on no matter how I drive. The system works for me just fine, just a little different way to go about it, compared to a turn-a-valve gavity feed reserve system.
 
Hello all. just joined the forum and I had an issue maybe someone can help me understand a bit.

yesterday by accident i flipped my fuel switch to reserve and didn't notice. engine died a few moments later. i figured out what it was, as i was sitting idling in traffic. tried 10 minutes later again at the next stop and it stalled within a few moments again ( about 80 miles on the tripmeter so 80% empty tank). After that stall I had her running and flipped it to reserved while moving and ran the rest of the way to work fine, 5 miles, 10 minutes or so. Drove home and ran the tank almost empty, switching between on and reserve the whole time with no more issues. Light came on at about 102 miles and at home the tripmeter hit 107 miles.

Got home, looked in the tank, didn't see anything that woudl cause blockage or anything. filled her up with premium and ran her on "on" and "res" with some high RPMs and no hiccups at all.

Typically I would assume the fuel lines for the reserve were empty because I hadn't used it in probably a month or so, so it took a few times to pump the gas through, but the way you describe the fuel pump/reserve works that doesnt' really make sense as its all the same lines.

I also think I have run the vmax with the fuel light on, and switch to "On" for longer than i'd assume there was gas in the carbs and lines, so does it completely shut off the fuel pump when the light goes on?

Thanks in advance for your thoughts!

-garrett
 
So if gas is below sensor level the light should STAY on or is there a time limit? I just ran out of gas over the weekend, no light stayed on.
 
So if gas is below sensor level the light should STAY on or is there a time limit? I just ran out of gas over the weekend, no light stayed on.

Yeah, it should stay on until the fuel level is above the fuel sender unit.
 
Hello all. just joined the forum and I had an issue maybe someone can help me understand a bit.

yesterday by accident i flipped my fuel switch to reserve and didn't notice. engine died a few moments later. i figured out what it was, as i was sitting idling in traffic. tried 10 minutes later again at the next stop and it stalled within a few moments again ( about 80 miles on the tripmeter so 80% empty tank). After that stall I had her running and flipped it to reserved while moving and ran the rest of the way to work fine, 5 miles, 10 minutes or so. Drove home and ran the tank almost empty, switching between on and reserve the whole time with no more issues. Light came on at about 102 miles and at home the tripmeter hit 107 miles.

Got home, looked in the tank, didn't see anything that woudl cause blockage or anything. filled her up with premium and ran her on "on" and "res" with some high RPMs and no hiccups at all.

Typically I would assume the fuel lines for the reserve were empty because I hadn't used it in probably a month or so, so it took a few times to pump the gas through, but the way you describe the fuel pump/reserve works that doesnt' really make sense as its all the same lines.

I also think I have run the vmax with the fuel light on, and switch to "On" for longer than i'd assume there was gas in the carbs and lines, so does it completely shut off the fuel pump when the light goes on?

Thanks in advance for your thoughts!

-garrett

If you switch to reserve accidentally you should still get fuel. I would suggest that the electrical contact in the 'res' / 'on' switch mechanism may have had surface dirt or light corrosion on the contacts that prevented the 'res' setting being detected. By switching it multiple times you cleaned the contact and the circuit began to behave normally. (the relay may also suffer from this effect if it has not switched for some time especially on an older bike).

The fuel pump will switch off once the fuel light glows continuously for more than x seconds. I do not know what the value of x is but at startup the fuel level sensor takes about 30 seconds to stabilize.

The fuel level sensor is a 'slow' reacting switch and will take time to reach a stable switched on state i.e. low fuel light on.

If it were instantaneous the fuel relay would switch on and off every time you leaned the bike or hit a bump and the fuel level changed. You also have a limited supply of fuel in the carb float bowls that may get you and extra 100 feet down the road before the engine splutters.
 
Many times the sensor wire is broken and can be soldered back in place.
Sean
 
Don't listen to Morley....he sends sick pictures via picture mail. Hehe.
 
OK, so somebody set me straight. I have a 90 V-Max, have never run out of gas, have never seen the fuel light on. Usually get gas round 90 miles or so before I refill..........I'm going on a "longer" trip tomorrow, probably 400 miles or so, may have to get 100 miles if we're not close enough to a gas station. So, here's the 64K dollar question. On the handlebar switch, what is the "normal" positional, or position I want to have the switch on when full of gas? To the left, pointing at RES, or to the right, point to ON?
 
Boy, the search function is a handy, dandy thing! Nevermind!

I found the fuel switch confusing, this is what it looks like

Fuel
Res. On

You should ride with the switch to the right after a full tank. when the light comes on after burning 3.2 gallons, push the switch to the left (under the "Res." text). This turns the fuel pump back on as noted by others. Fill up the tank within 10- 15 miles and push the switch to the right (back under the "On" text).

At first I read the second line as "Res. On" as if putting the switch under the "On" text would turn the Reserve On, but that's not right. This stupid wording thing gets a lot of people, I wonder what it says in Japan. Maybe it is clearer.
 
For those who have never seen the light, I suggest you take a small container of gas (1 liter or quart plastic oil bottle for example) and go ride till either the light comes one, or you actually run out.
This way you'll know for sure if your reserve sender / fuel light / fuel pump switch all work as expected, and if you do actually run out of fuel, that container will get you to the next filling station!

Great post btw Gleno, explains everything, apart from the fact that I've still never seen more than about 90 miles before my red light starts glowing! And that's doing constant speed on the highway and with a Venture diff fitted too...

Gonna have to switch my v-boost off to see if it makes a difference, I think my problem is this twitching right wrist disease I've developed since riding Maxine..
 
Here it is simply put..

First I will explain left and right..
Hold both of your hands up in front of you arms extended palms away 4 fingers closed together and thumbs horizontal..
Which ever hand your thumb and index finger make the L thats going to be your left for future reference..

Fuel Switch Right
Low Fuel Indicator Comes on
Move fuel switch to your left, if you get confused and have to refer back to the hand trick I taught you, please pull over cause chances are you cant ride with no hands.
Once you established left and right set the fuel switch to the left. and look for gas within 30 miles.

Good Luck
Fuel switch reads 2 selections
ON
RES
I dont see how this confuses anyone but hopefully this will help..

Lefty Loosey
Righty Tighty
:clapping:

Actually it gets even simpler than that, it doesn't matter what position the switch is in. Switching it to the opposite position will re energize the fuel pump and you will be on your way. This isn't set up a as a "normal" reserve.
 
I would clean the switch first. Next, the fuel pump relay.
 
Here's my version of a fuel reserve problem that's slowly been getting worse.......

When my light comes on and I switch to "reserve" it finishes stumbling for a minute or so then starts to stumble again. I switch to "on" then back to "reserve". It clears again then soon starts to stumble again. I did it a 3rd time then all was fine for 5 or 6 miles to the gas station. No problems while "on".

It's been getting a little worse since it started happening last season.

I'm going to hit the switch with some contact cleaner for now. Do you guys think I may have a bigger problem on my hands? I will probably get out my multimeter and test the switch.

Also, this bike doesn't sit much (except for winter) and is garaged full time.

Any other ideas? Relay screwing up?? TIA. Mike.


I had this problem a while back. Now I switch the switch over to reserve when I first see the red light come on. I stopped waiting for the motor to start stumbling before I move the switch. And since doing that I haven't had any problems.
 
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