11 replies to this topic

[mostly_harmless]

Hi,

This probably sounds like a really stupid question, but from searching I have found no answer.

I've been troubleshooting an issue with my fuel gauge ('92 British Open Classic), which shows full ok but starts floating up and down once the tank runs down a bit and eventually drops to empty at just over half full. Figured form other threads this is either a break in the resistance coil in the sender or a dodgy voltage stabiliser in the (Nippon-Seiki) gauge, or possibly even a combination of the two. Oddly the gauge seems to rise a bit at higher revs...

Eitherway, from reading about senders it occurs to me that they are basically a circuit passing current through a coil of some kind that is sitting in the fuel tank. So my question is; how is this safe? It clearly is, as this seems to be how most fuel senders work. But I assume that the same wear to the wire that can eventually cause a break could also cause a spark? I'm thinking particularly of when the area of wear or break is passed. Do these senders ever cause fire or explosions as a result? I can't find any reference to this, but it is suggested as a theoretical risk in the wikipedia article on fuel gauges.

It makes me wonder if I should disconnect the fuel sender until I have had it replaced.

Dave.

[jaydee]

I've never heard of such issue with the mini fuel tank sender. Most of the fire hazard comes from the postive battery cable and the fact that in some badly fitted aftermarket stage 1 kits the fuel line to the carb passes very close to the exhaust manifold.

[dklawson]

Your question is a good one and the answer is fairly simple.

For about 1/2 the tank the sending unit resistance wires are below the level of the fuel. With no oxygen there is no possibility of ignition. So... what about when the tank is below 1/2 full and the windings are out of the gasoline? This time the answer is the fuel/air ratio. The fumes inside the sending unit are very, very, heavily rich. The mix is so rich in fact that it is too rich to burn. There is a corollary in food and drink. If you take enough water out of something sweet, it will not spoil or ferment. The ancient example is honey. The more recent example would be bulk juice concentrates. Basically, the sender has too much gas for the mixture to accidentally ignite when a sending unit winding fails.

I agree with your diagnosis concerning the tank's "low" reading behavior. The next time the car does this and the gauge suddenly drops to empty, try the traditional paper-clip between the sending unit terminals test. With the ignition on, use a paperclip to bridge the two sender terminals on the tank while watching the gauge through the back window. If the gauge goes from empty to full (be sure to give it a few seconds to climb) then the sender has a winding fault.

[Ethel]

Rising at revs suggests the voltage regulator - alternator output is higher & that's what the stabiliser is supposed to stabilise. The temp gauge would do the same, but you might not notice it moving slower.

Another great tip from DK. Once the pickup gets the wrong side of the break in the winding the gauge will drop to empty. If fuel sloshing around causes the pick up to alternate from above and below the break the gauge would waver.

[mostly_harmless]

Thanks all for the useful and interesting replies, especially the info from DK regarding the role of the fuel/air ratio. Figured there must be a reason people's fuel tanks aren't exploding left right and centre, but its good to know that reason!

[tiger99]

I had to design something once upon a time which had to work in a potentially explosive atmosphere, and had to be "intrinsically safe". I can't remember the exact safety limits, maybe about 10 volts and 10mA, but there are current, voltage and energy limits below which ignition will not occur. The fuel sender, as powered via tha gauge, is well below these limits, so even in the brief time when the air/fuel ratio may be right for an explosion, the normal sparking at the wiper, or at a wire break, will not set it off. But don't put acetylene or hydrogen in the tank, they will ignite.

However, once upon a time, I was coming down the M40 towards junction 3, in Mini number 2, flat out downhill of course, and spotted Plod on the hard shoulder looking helplessly at an 1100, also on the hard shoulder, which was spewing a jet of flame and burning fuel out of the filler cap, and up the embankment. I often wonder what ignited it. My suspicion is that the fuel gauge wiring shorted to something carrying the full 12 volts, with lots of current available, as what other ignition source could there be? The fire was not (yet?) inside the car, nor had it any crash damage.

I can tell you that the induced field due to a mobile phone can create sparks well above the intrinsic safety limit between petrol pump nozzle and filler neck, especially if the phone is in your pocket while you are refuelling, so always switch off. An incoming call may trigger an explosion, so just not making a call is not sufficient. Some such incidents have been wrongly attributed by the fire brigade to static electricity, which is rather less of a hazard because it is more easily mitigated.

[alonzomerrill]

I've never heard of such issue with the mini fuel tank sender. Most of the fire hazard comes from the postive battery cable and the fact that in some badly fitted aftermarket stage 1 kits the fuel line to the carb passes very close to the exhaust manifold.

Your information is right about mini fuel tank sender but i have no experience for big fuel tank sender. I have know idea about how most of fire hazard comes by positive battery cable, so can you give more explanation no this topic.

[jaydee]

Its very simple, all minis built from aroun 69-70 (i think) are negative earth. The body shell is negative wired. The positive cable runs through car, if it splits, like at the holes where it passes through the shell, it shorts out, hence the fire hazard.

[dklawson]

The battery cable passes through the shell on those cars? Apart from the boot floor hole with grommet I thought on all Minis (positive or negative earth) the battery power cable ran underneath the car in the trough spot welded to the bottom of the floorpan. It is fairly common that the cable can short to the exhaust system at the front of the car where the cable exits the trough.

[spiyda]

I design stuff for fuel gauges and I can assure you there is no risk..

As others have said, the resistance winding on the sender is probably broken..

or the contact end of the moving arm is worn out

in any case, just get a new or used sender..

Chris

[dklawson]

Chris... do tell more. What do you design for fuel gauges? Do you work on other instruments also? If you don't want to discuss it here, send me a PM as I would like to know more about what you do.

[jaydee]

The battery cable passes through the shell on those cars? Apart from the boot floor hole with grommet I thought on all Minis (positive or negative earth) the battery power cable ran underneath the car in the trough spot welded to the bottom of the floorpan. It is fairly common that the cable can short to the exhaust system at the front of the car where the cable exits the trough.

Actually it runs through the shell because on the later minis positive cable passes through the firewall between boot and cabin and exit about at the rear seat panel IIRC. I'm pretty sure it passes below rear seat into the sound deadening on the inside of the car.
Then yes, on standard mini it runs outside the floor, which is even worse as it can be snapped/damaged, indeed in rally minis we have to run any cable/pipe inside the car to avoid such issues.