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MG Midget and Sprite Technical - Petrol sender mystery question

On discovering why I was getting a constant reading on the fuel gauge (A35)I found the sender body including the variable resistor was submerged in petrol. Looking at the design of the sender there is no provision to keep petrol from entering the body as there are no seals on the pivot shaft, and consequently if you fill the tank to the neck the sender will be below petrol level, and can fill up. Or should it? Is it supposed to work on the principal of a diving bell where the pressure of petrol keeps the existing trapped air inside the sender body thereby keeping petrol out? But even so the variable resistor must work in a petrol mist environment and if and when petrol does enter the sender ( as it did in my case) what are the safety and electrical implications? I should add that although the sender body was full of petrol the tank itself only had 2 gallons in,and had last been filled over 18 months ago - so clearly was unable to drain. Obviously millions of cars don't explode every day so what's going on?
f pollock

Petrol will only burn when mixed with air (more specifically oxygen) in the correct ("stiochiometric") ratio. That's why the engine takes in fuel and air, and the carb mixes the two in explosive proportions (air / fuel ratio = 14.7 or thereabouts). In the fuel tank there isn't much air so there's no oxygen to burn with the hydrocarbons so even if the sender unit sparks it won't catch fire. I've heard of this being demonstrated by dropping a lighted match into the filler neck and it just goes out. Personally I wouldn't try that (there will be a point in the neck where the fuel / air mixture is about right!), but I understand the theory.

Petrol in the sender unit isn't a problem.

Petrol in the float will mean it doesn't float properly and therefore won't work. Also, whilst it's off the car, put a ohm meter across it from the body to the terminal and move the arm up and down. The resistance should change with the arm's position. If it doesn't it could just need the tab bending to touch the resistive track, or the track might need cleaning, or possibly the connector is broken somewhere.

Ant
Ant Allen

Thanks Ant, I understand the combustion part but are these senders supposed to fill with petrol and what happens to the resistance,ie the signal to the gauge if the variable resistor is flooded every time you fill up? - does it just default to full till the sender drains out again? Remember mine was still full of petrol 18 months later.
In my case there were 2 issues other than the one mentioned. The float was perforated like a sieve so I just made up a new one and put it on, and there was no earth on the sender body. Once these were done the gauge read normally.
f pollock

Any current flowing through the petrol will be insignificant compared to that flowing in the metal parts, so whether it's full of petrol or not doesn't really matter. I'm surprised it didn't drain out, but it's not a problem. The perforated float and lack of earth are good reasons for it not to have been working.

Glad it's fixed.
Ant Allen

all midget senders I've seen are open to "atmosphere" never seen one blow up yet, because of the over-rich atmosphere they sit in.

Ant's description comprehensively covers it

My newest sender was wildly inaccurate and I spent most of a warm sunday a few years ago, taking off the tank, draining and de-fuming it so I could calibrate the sender to the gauge

It took a while to persuade the sender "arm" to give a full-at-the-top-empty-at-the-bottom reading

I think it was worth every lost minute when now, if the gauge reads empty I know its telling me I have 25 miles of range left before the boot can comes out

I wouldnt have had it so accurate that empty shows an empty tank though, these cars deserve their little quirks
Bill sdgpm

I have always wondered why the gauge-sender doesn't present an explosion risk inside the petrol tank. I agree with Ant Allen's thoughts - but an 'empty' tank (or near empty)is full of air/petrol vapour. One would have thought that a crude wire-wound variable resistor with 10 or 12 volts hanging off it would give a lovely spark from the resistor's wiper contact.

Anybody else got any thoughts why the gauge-sender system is safe inside a petrol tank?

Cheers,
Andy H.

A Hock

Simply because empty or not the atmosphere is always "super-rich" inside

and the resistor track is always in contact with the wiper so no reason for sparks, there's no time between wire edges that the wiper is out of touch across the winding

Lucas wasn't such a 'orrible ogre after all
Bill sdgpm

This thread was discussed on 27/03/2009

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