Could you blow yourself up this way?

[davepl]

[Please move this to electrical if it's too specific, but it's kind of a general safety question, so putting here for now]

It used to worry me that the fuel tank sender operated on 12V, but then I eventually figured out that there's a ton of resistance in the circuit, so there's pretty much no way it could ever arc or spark inside the fuel tank.

But as I was about to start troubleshooting a gauge issue with my power probe, I had to wonder:

What if I used it to inject a FULL 12V into the sender wire? Couldn't that spark/arc internally at the fuel tank sender?

If it were submerged, you'd still likely be OK, but if the fuel level was low enough that the sender mechanism was above the fuel line, it'd be sitting there in vapor, and... boom?

I'll definitely put it on my "don't try it" list, but I'm surprised I've never heard of anyone accidentally hooked it up wrong or otherwise sending a full non-resistance 12V into it by mistake.

Or maybe it happens all the time and it's still safe for some reason I haven't thought of?

Dave

[dave`12]

There is a reason that the wire going to the sender completes the ground, rather than being positive

[Gmc.guy]

I think it would have to depend on the upper and lower explosive limit. Upper would be a concentration of vapours too high to ignite (rich) and lower not enough (too lean) . It takes a sweet spot to blow stuff up. We deal with it in natural gas but I'm not sure of the % or ppm for gasoline

[CaptRMW]

Another mystery we will never solve. When someone finds the answer they probably won't be able to share it with us.

[Gmc.guy]

(Faint boom in the distance) and a pile of birds fly by

[davepl]

Originally Posted by dave`12

There is a reason that the wire going to the sender completes the ground, rather than being positive

Can you explain? It completes ground so that current can flow from a very restricted source at the gauge to the ground drain in the sensor, right? There's only one wire.

What I'm talking about is when you replace that highly resisted source with a straight-to-battery source, with no resistance. The ground circuit is completed through the send, and a full "whatever the circuit can carry" current path is established.

The resistance could be in the actual sending unit, but it's only 90 ohms, not a ton of resistance so far as I'm aware.

I'm a software guy, not a hardware guy, so type slowly so I can follow!

[72_K10_Vegas]

Google "Intrinsically Safe" and fuel.

[davepl]

Ok, now explain why you believe this circuit to be intrinsically safe if 12V is injected into the sender wire...

[71BUCKET]

One of my other cars had 3 450gph pumps sittings in fuel with a lot of voltage going to them. Running both 93 octane and e85, no issues

[brad_man_72]

I have a walbro 255 fuel pump in my tank. No problems so far...

[franken]

The ground side or positive side has nothing to do with anything. The same current flows through a circuit and the same energy is used regardless. Current flows to and from a source. Nothing is lost. Its sort of like if you walk out your front door, turn left or right and eventually walk back in. Same number of steps.
Lots of people who know nothing about electricity will give you lots of BS info. Learn Ohm's Law.

[rob32472]

Liquid fuel is difficult to ignite as long as everything stays submerged there is no chance it would ignite and explode. If you have any fuel in the tank, the vapor concentration is far past what you can ignite. (gasoline doesn't burn at all. gasoline vapors burn, and it has to be between 1 and 7% vapor to air. Any more or less and it won't do a thing) The only time you would get that low of a concentration of vapor in the tank is if it is absolutely empty, in which case you aren't moving the sender anyway.

[palallin]

Originally Posted by davepl

What if I used it to inject a FULL 12V into the sender wire? Couldn't that spark/arc internally at the fuel tank sender?

If it were submerged, you'd still likely be OK, but if the fuel level was low enough that the sender mechanism was above the fuel line, it'd be sitting there in vapor, and... boom?

I'll definitely put it on my "don't try it" list, but I'm surprised I've never heard of anyone accidentally hooked it up wrong or otherwise sending a full non-resistance 12V into it by mistake.

Or maybe it happens all the time and it's still safe for some reason I haven't thought of?

Dave

It depends upon which network news show is recording the story. "Film at 11."

[James the III]

Originally Posted by davepl

Can you explain? It completes ground so that current can flow from a very restricted source at the gauge to the ground drain in the sensor, right? There's only one wire.

What I'm talking about is when you replace that highly resisted source with a straight-to-battery source, with no resistance. The ground circuit is completed through the send, and a full "whatever the circuit can carry" current path is established.

The resistance could be in the actual sending unit, but it's only 90 ohms, not a ton of resistance so far as I'm aware.

I'm a software guy, not a hardware guy, so type slowly so I can follow!

Cus, most don't probe a ground wire with 12v+
if they need to check that wire it's for electrical continuity , it's with micro amps of power.. that not ark.. looking for a break..

[VetteVet]

Originally Posted by davepl

[Please move this to electrical if it's too specific, but it's kind of a general safety question, so putting here for now]

It used to worry me that the fuel tank sender operated on 12V, but then I eventually figured out that there's a ton of resistance in the circuit, so there's pretty much no way it could ever arc or spark inside the fuel tank.

But as I was about to start troubleshooting a gauge issue with my power probe, I had to wonder:

What if I used it to inject a FULL 12V into the sender wire? Couldn't that spark/arc internally at the fuel tank sender?

If it were submerged, you'd still likely be OK, but if the fuel level was low enough that the sender mechanism was above the fuel line, it'd be sitting there in vapor, and... boom?

I'll definitely put it on my "don't try it" list, but I'm surprised I've never heard of anyone accidentally hooked it up wrong or otherwise sending a full non-resistance 12V into it by mistake.

Or maybe it happens all the time and it's still safe for some reason I haven't thought of?

Dave

Do not ever apply a full 12 volts to the sending unit wire. At the very least you will damage the variable resistor on the sender, and at worst it may ignite the fuel vapors in the gas tank and cause an explosion and fire, and burn down the truck and possibly cause injury or death.

The fuel gauge receives a full 12 volts from the ignition switch via the fuse panel, and this voltage runs through a resistor on the back of the gauge. This voltage also powers the needle coils in the fuel gauge. This combination of resistances causes a voltage drop from the original 12 volts to about 8 volts, and then the current pushed by this lower voltage is routed back to ground through the tank sender, variable resistor.

The lower voltage is not enough to push enough current through the variable resistor to damage it, or cause an overheat or spark problem.

Now suppose you increase the voltage to a full, non- resistance, 12 volts. It would consist of a short to ground through the variable resistor, which would probably cause it to overheat or glow, and possible burn in two and even create a spark when the wires parted.

The chances of this happening are greatest when the fuel level in the tank is low or empty and the VR is exposed. This is when the resistance is lowest in the sender VR. It 's range is 0 to 90 ohms 0 being empty and 90 being full.

As an example, if you take a tail light bulb and put 8 volts on it, it will be very dim or barely glow, and then increase the voltage to 12, and it will burn your hand if you hold it. That's because at 8 volts, there is not enough current going through the filament to cause it to heat enough to glow, due to the lower voltage ( electromotive force) being insufficient to force the current through the resistance. So we add more voltage and the current flow increases and the bulb glows and gives off more light. It also gives off more heat.

All these situations are due to the relationship between voltage, current and resistance. Ohms law.

Another way of looking at it would be to picture a furnace igniter, or gas dryer, or stove, and even a glow plug in a diesel engine. If the variable resistor were to glow hot enough, it could ignite the fuel vapors in the tank. I believe 7 to 1 is the ideal air/fuel mixture for internal engine combustion, however I also believe it could take place at a lower or higher ratio.

The fuel pumps in the modern tanks are spark enclosed and isolated from the fuel so there is no worry of sparks or explosions with them.

Here is a website that is very good for checking fuel gauges and it has a diagram illustrating how the fuel gauge and sender are wired.

http://6066gmcguy.com/Electrical-02.html

[GASoline71]

Originally Posted by davepl

Ok, now explain why you believe this circuit to be intrinsically safe if 12V is injected into the sender wire...

You go first...



Gary

[davepl]

Originally Posted by franken

The ground side or positive side has nothing to do with anything. The same current flows through a circuit and the same energy is used regardless. Current flows to and from a source. Nothing is lost. Its sort of like if you walk out your front door, turn left or right and eventually walk back in. Same number of steps.
Lots of people who know nothing about electricity will give you lots of BS info. Learn Ohm's Law.

Maybe Ohm's Law is printed on the back of my engineering degree. Not sure, I'll have to check. Until then, I'm learning so much!

I think this thread proves that a little knowledge is a dangerous thing.

[davepl]

Originally Posted by VetteVet

Do not ever apply a full 12 volts to the sending unit wire. At the very least you will damage the variable resistor on the sender, and at worst it may ignite the fuel vapors in the gas tank and cause an explosion and fire, and burn down the truck and possibly cause injury or death.

Thanks! I'll take that as a "Yes" to my original post and confirmation that the scenario I'm describing (12V injected to sender wire) is a "bad" thing to do. You described it exactly as I envisioned it, complete with low fuel levels being the most dangerous time to make the mistake.

But I thought it was "intrinsically safe"

[Wrenchbender Ret]

I've often wondered about that. The hot wire to the dome light & the wire to the sending unit all run down the edge of the floor. I've pulled the threashhold plate up & found them all corroded & bare. It seems that if those two shorted with each other it could get exciting.

[suprachrgd82]

There's 3 components required to support combustion - 1) fuel 2) oxygen 3) heat. There's not ample oxygen in the fuel tank, only gasoline vapor.

[luckylogger09]

I understand a gallon of gas equals 22 sticks of dynamite. So it won't matter.

[garyd1961]

Just on a side note be careful probing wires on a car with airbags. A guy at work was probing seatbelt wires and set off all the airbags in the car, about six of them. Didn't hurt him but he was stunned for a few minutes.

[MagmaJct]

Originally Posted by franken

The ground side or positive side has nothing to do with anything. The same current flows through a circuit and the same energy is used regardless. Current flows to and from a source. Nothing is lost. Its sort of like if you walk out your front door, turn left or right and eventually walk back in. Same number of steps.
Lots of people who know nothing about electricity will give you lots of BS info. Learn Ohm's Law.

Yes, and No. You are correct, the same amount of current flows through the circuit whether the resistor is on the ground side, or the positive side of the circuit.

However, having the variable resistor (inside the gas tank) on the ground side means there is pretty much zero voltage potential at the tank. If the wire shorts at or near the tank, there will be pretty much no arc (due to the resistance at the gauge).

My `74 VW handles this very differently than our trucks. The gauge has two wires to it, one positive, and one negative. However, the positive side passes through an electrical vibrator (crude voltage or current regulator) that drops the voltage or current low enough to be safe.

[Willie Makeit]

Originally Posted by davepl

Maybe Ohm's Law is printed on the back of my engineering degree. Not sure, I'll have to check. Until then, I'm learning so much!

I think this thread proves that a little knowledge is a dangerous thing.

if you took ANY engineering classes then you were exposed to Ohm's Law.

retention of said exposure is a different topic all together.