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Fuel Level Switch

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Those of you who have burnt out fuel level sensors might be interested.

The sensor is actually a thermistor in series with the idiot light.When the sensor is covered in fuel it's kept cool and the resistance is quite high (about 1,000 Ohms). The current through the lamp is not enough to make it glow.

When the level drops the thermistor is no longer cooled by fuel so it warms up and it's resistance drops, the current goes up so it heats even faster until it's drawing all the current the idiot light can provide. It's a very fine balance between thermistor resistance and lamp resistance.

If the petcock plug is accidentally swapped with the sensor plug there is nothing to stop the current increasinfg so in a matter of seconds it can get red hot and burn out. The sensor is boult in a metal case so it won't ignite the fuel as it burns out.

These sensors are over $190 to replace.

 

Being cheap I sought an alternate. I found a fuel level sensor by Panasonic but I cannot find a source

The Panasonic part No is ERTLG12N7

This was listed on a Chinese site N Gate for $15, I ordered a pair but they never arrived so I suspect they are a bit dodgy.

 

What to do?

Reed switches are tiny magnetically operated switches in a glass capsule, they are very rugged and just need a tiny magnet to operate. The small ones are rated at 100 mA which is just perfect for the application.

 

I whipped up a prototype from a few inches of 1/4" copper tube, a cork and tiny magnet, total cost

The reed switch is hermetically sealed from the fuel tank contents by the copper tubing so there is zero chance of an explosion.

 

I don't intend to install this prototype, it's a bit crude looking, I will make a better unit out of small brass tubing. Any Non ferrous tubing will do but Brass and Copper are easy to solder, Steel is no good because it will prevent the magnet from operating the switch.

 

CIMG7114_zpssunbymj2.jpg

You can see the prototype at the top. This one is intended for a metal tank so I simply grounded the switch, For a plastic tank you would need two wires. The burnt out OEM unit is below.

 

The 1/4" tube is really too large, it takes a lot of room that would be better for flotation, 3/16 tubing would be much better.

The cork has to be small enough to enter the hole in the tank, this rough one floats about half submerged.

The cork is lined with aluminum foil to make it slide easier.

 

In theory you could let the float slide all the way down the tube that way you could add the correct amount of fuel then slide the switch up inside until it activates but as the level drops even further the light would go out again, so it's best to have a mechanical stop at the low level point.

The operating point of the OEM sensor is about 120mm from the flange.

 

Corks are not so common these days, I got mine from a Craft store, Michael's

 

BTW there are lots of industrial level switches that operate like this, some have multiple switches to give several alarm points.

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Do most / all level switches work like this ? We had one fail on a coolant level switch and shut down the powerplant . We had to run the engine on override until we figured out the problem . . 

 On the sensor you describe , is the reed switch N.O. ( normally open ) or N.C. ( normally closed ) ?  You can blab on this all you want because I am interested in the theory of operation of these level switches.

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No, not all switches work like that but the ones where a magnetic float slide up a stainless or brass tube often do, the contacts are generally low Voltage, low current.

Most reed switch contacts are N.O. but there are others that are N.C. and a few form C (N.O. Common N.C. type).

The two contacts are a tiny way apart, the magnetic field sucks them together.

Although the switch itself is Normally Open the magnet can be arranged to hold the contacts closed then open on High or Low level

They are used a lot in security systems as door and window switches also

 

Heres the one I used/

http://www.digikey.com/product-search/en?x=17&y=18&lang=en&site=us&keywords=374-1202-ND

Note the switching Voltage is quite high for such a tiny device, they rate most in Billions of operations before failure

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First - the fact that this level of ingenuity and conversation exists on this forum is so cool. 

 

Second - I haven't had the problem yet, but thought I'd give a try at searching for the part. I found something on a Honda ST forum that referenced the same part number - so there could be a cross-compatible Honda (aka cheaper) part. The same site also referenced a US$0.64 replacement thermistor from Digi-Key (from whom I just purchased my relays).  I do not know how to determine if this thermistor could be used to repair the stock MG part - but maybe you guys would know. Is $0.64 cheap enough?  :grin:

 

Third - please blab on. I am also interested in how things work - even if it does not address a specific issue that I am currently facing.

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That Digikey part may be the same thermistor but it would be risky to use without the metal case, it prevents the gas fumes exploding if it burns out.

The Honda part sounds interesting, can you post a link. My friends Tiger uses a similar one also. I'm sure it's used on lots of cars.

 

Make sure you never cross the cables over and your sensor will last forever, my V11 Sport one is keyed to prevent a screw up but I have 2 EV ones burnt out, their plugs were colour coded but interchangable.

 

 

BTW I do have some Digikey thermistors to replace the Oil and Air temperature sensors if anyone needs one.

part No BC2730-ND

 

Scud, I sent you a PM

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They are isolated from the vibration in the tank by rubber insulators mounting the tank to the frame. Level switches are mounted in engine oil pans , radiators and fuel tanks too. Vibration does not affect them either .

 The same ( different style ) level sensors work in dry solids such as ash , dust or other materials...

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Reed switches have such a small mass, there is no weight to make it bounce, the magnetic attraction keeps the contacts together, besides, does it matter?

Here are the specs

http://www.meder.com/fileadmin/meder/pdf/en/Products/OKI_Switches-En/OKI_Switch_ORD325.pdf

I have one triggering my Speedhut spedometer.

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The Panasonic part No is ERTLG12N7

 

This was listed on a Chinese site N Gate for $15, I ordered a pair but they never arrived so I suspect they are a bit dodgy.

 

 

Update:

After about 3 months waiting I was surprised to receive a pair of the Panasonic parts in the mail from N-gate

They are not a direct replacement you would need to modify the holder.

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So, I have a different sensor.  And the fuse I just blew shows that it needs more than a short to light up.  Do you have any idea what the "light on" resistance was?  That way i can put one in series with my reed switch.

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I'm guessing you have a short in the lamp holder to now a fuse. The lamps need to be a single sided contact type, the double sided ones short.

 

 

Sent from my shoe phone!

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The light on resistance would be 50 - 100. Ohms. The lamps draw ~100 milliamperes so the reed switch will work well. With a short you may have welded the contacts together. I highly recommend throwing the lamp holders as far as possible and soldering in 12 Volt LEDs

 

 

Sent from my shoe phone!

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I'm guessing you have a short in the lamp holder to now a fuse. The lamps need to be a single sided contact type, the double sided ones short.

 

Thanks,  I will check that tonight. 

 

The light on resistance would be 50 - 100. Ohms. The lamps draw ~100 milliamperes so the reed switch will work well. With a short you may have welded the contacts together. I highly recommend throwing the lamp holders as far as possible and soldering in 12 Volt LEDs

 

I was just at my favorite ELX shop this afternoon.  Wish I had thought of this...

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I'm guessing you have a short in the lamp holder to now a fuse. The lamps need to be a single sided contact type, the double sided ones short.

 

Nice catch!  It was the right bulb, before it got smashed and shorted out the contacts.  Once I dug all the debris out of the socket everything was fine.  Now I just have to replace the thermistor.

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