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Joined: May 2003
Posts: 107
J
james S Offline OP
Member
Hi all,
just a quicky..(think this is the right area)

I have recently had a problem with my car fuel pump not working,on investigation i found that the fuel pump was submersed in petrol inside the fuel tank,
together with push on crimped connectors for the supply to the pump and another pair for the fuel float switch (for fuel gauge)sender.
The fault on the circuit was the connections to the pump situated inside where burnt out and short circuiting.

Now i know the pump is submersed in the fuel to keep it cool because its always working.

I know that you need an ignition source , oxygen and fuel form to create fire or spark

My question is ; how come there is burn marks at the loose connection and a build up of carbon? is it because its heated up and not arked do you think?

Also is it safe to have these push on crimped connectors in such a location because of the lack of air (i.e. is in a vacuum)and not able to make a spark?

lastly, even if the above is true could the 12volts potential difference not track and short circuit through the fuel instead of going through the pump?

This whole set up scares the s**t out of me to be honest,just does not seem right!!!

james

Joined: Nov 2006
Posts: 23
W
Member
Calm down James. Fuel pumps have been in car fuel tanks for over 20 years now, they have been proven safe. As far as the arc goes, if it happens it is not a big deal, liquid gasoline will not burn, not enough oxygen in it. When gas burns like in a can or puddle, only the top of the pool is on fire because it is in contact with the air.
As far as shorting thru the fuel, the conductivity of gasoline is too low for it to cause a short.

Joined: Jul 2004
Posts: 9,667
Likes: 6
G
Member
It should be noted, they started putting the fuel pump in the tank just about the same time they stopped venting them so there is not really that much oxygen in a fuel tank. It is mostly just fuel vapor. If I would be afraid, it would be a cool tank that was almost empty. As the tank cooled and the fumes condensed it would pull in more air.
Just about the time you are totally convinced his is safe, you think about TWA 800.


Greg Fretwell
Joined: May 2003
Posts: 107
J
james S Offline OP
Member
Yes I suppose you are right and I thought as much regarding the conductivity through the petrol! it's not as if this practice has not been tried and proven!
It just seems to go against everything you get told not to do.....electricity and near liquids...petrol and potential ignition forms.

Thanks guys, I will sleep like a log tonight ha.

James

Joined: Jul 2004
Posts: 9,667
Likes: 6
G
Member
If you want a good ignition source, how about a bolted fault 12v to the gas gauge sending unit wink


Greg Fretwell
Joined: Nov 2006
Posts: 23
W
Member
Greg the fuel gauge in American cars is nothing but a potentiometer, the voltage is cut down via a resistor in the gauge module, if I remember it is only like 1 or 2 volts which the wires and sending unit can carry without heating or arcing. Not to say that Murphy does not know his way around an automobile, and could be looking to cause trouble at any time.

Joined: Jul 2004
Posts: 9,667
Likes: 6
G
Member
The sending unit is more correctly a wire wound rheostat (single ended pot) usually 230-30 ohms to ground. The fault I was describing would be if you hooked the 12v directly to the sending unit. It will blow open BTDT.


Greg Fretwell
Joined: Nov 2006
Posts: 23
W
Member
Ok I got you now. Hope I am not there if that happens.

Joined: Jan 2005
Posts: 5,316
Cat Servant
Member
First a tiny bit of history: Way back, in the Roaring 20's, UL had a falling out with Henry Ford. Since then, the auto industry hasn't had the slightest interest in what UL has to say; Henry was confident that his guys knew more about building cars than anyone else.

Looking back, I think Henry has been proven correct. The auto industry -naysayers like Ralph Nader notwithstanding- have done a monumental job in perfecting the automobile. It IS pretty arrogant for someone to assert that 'they' don't have any method behind their madness.

Without knowing the intimate details of the design, there are a few points to consider.

The first is an engineering call as to the classification of the location. Gasoline has a pretty narrow range where there's the right amount of fuel and air for an explosion to happen.

Second, there are very likely two approaches taken by the designers that further reduce any risk. The very 'weakness' of the components may result in the quenching of any explosion by venting the pressures, and the circuit itself just might qualify as intrinsically safe, lacking enough current to be able to ignite the fuel.

As if to illustrate this point, I remember some clever soul claiming that an effective 'car bomb' was to break a bulb and stick a tail light in the tank. Step on the brake, and BOOM! Yea. right. Without getting into the dirty details, I once saw this attempted under laboratory conditions, and the most that was ever accomplished was popping the gas cap off. This wasn't the first time I've seen Hollywood debunked.

Joined: Jul 2004
Posts: 9,667
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G
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This is very much akin to the lab demonstration where they put have a container full of a combustible gas and light a flame off of a jet of air introduced into the container.


Greg Fretwell

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