Quote:
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
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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