Fuel Sender Check (Negative Earth)

[oestlarsen]

Just quick question as I put together my tank after painting it etc. I had issues prior to removal with getting a fuel read:
1. Guage pegged at FULL - even when wires disconnected (suspect short in the guage housing)
2. Not sure the sender is working

First, looks like our gracious host does not sell positive earth senders. Secondly, what can really go wrong in these - they look incredibly simple - does anybody know what happens when they go bad, and/if they can be reconditioned/repaired? As far as I can tell - it is basically a coil of thin wire with a connection that slides up and down, ultimately controlling the juice going to the gauge.

So far, I have not been able to successfully test it.

Thanks!!
Carl

[oestlarsen]

Correction in Subject - my Series is Positive earth...sorry for confusion

[Jeff Aronson]

My understanding was that fuel gauges that read full all the time usually have a ground issue. You're a positive earth Rover, like one of mine, so check the ground wire that comes out of the top of the tank. Do you have a clean connection and can you get a test light to illuminate if you run power to it and use its existing ground.

Jeff

[bmohan55]

My negtive earth truck came with a postive earth sender, the gauge always read backwards. I switched it out with the correct one so now I have an un-needed postive earth sender...I'll give it to you if you think it would help.

[oestlarsen]

PM sent - thank you very much, this will be a great help, as I wish to keep the truck original and positive earth.

[TeriAnn]

Fuel gauges & senders are very simple things and seldom go wrong. But sometimes either the gauge or sending unit dies. When a gauge dies it either doesn't do anything or the needle swings all the way to the other side with 12V applied and no wire to the sending unit (uses the gauge case as a ground connection). Sending units tend to go bad more often than gauges, usually caused by moisture getting into the tank causing rust on the sender units workings.

To trouble shoot a fuel level indicator circuit it helps to know what each part does so here is a beginner's primer:

The gauge is basically a double coil of dissimilar metals with a needle soldered to the middle end of the coil. The metals used have different expansion rates. When electricity is applied to the metal coil the resistance causes the metal to heat up and expand which moves the coil & the needle soldered on.

How much the needle moved depends upon how much electricity is going through the metal coil. The gauge movement doesn't care about polarity, just the amount of current that flows through the metal coil heating up the metal, causing it to expand. You can use the same gauge in either Neg. or Pos. circuits. The gauge can be built so that it reads either full scale with no current heating the metal coil or zero with no current heating the metal coil.

The sender unit is just a big variable resister. The float valve arm adjusts the resistance of the sender unit which determines the amount of current that flows through the gauge to heat up the metal coil. The variable resister can be built so that it has minimum resistance and allows the greatest amount of current to flow when the tank is empty or it can be built so that it has maximum resistance and allows the least amount of current to flow when the tank is empty.

The gauge needs to be matched to the sending unit in order to work properly. A gauge that reads full when there is no current flowing through it to heat the metal coil needs to be matched to a sending unit that has its maximum resistance when the tank is full. Otherwise the gauge reads backwards to the fuel level.

Land Rover used one style on its positive earth vehicles and the other style on their negative earth vehicles. The gauge & sender unit themselves don't care about polarity as long as the styles of each match.

When Land Rover switched to negative earth they started using a voltage stabilizer to achieve a more stable voltage to the gauges to help them read more accurately.

Positive earth sender units have a little metal tower on top and negative earth sender units have a flat top, often with three spade connectors on top.

If there is just one spade connector insulated from the sender unit body it goes to the gauge via a green/black wire.

If there are two insulated spade connectors, one goes to the gauge and the other goes to a low fuel warning light via a green/orange wire.

An uninsulated spade connector soldered directly to the top of the sender unit is a ground connector and a black wire connects that connector to frame ground.

The overall circuit is: Switched 12V to gauge (pos earth) via a dark green wire or to the voltage regulator via a dark green wire then to the gauge via a light green wire. Then from the gauge to the sending unit via a green with black stripe wire. Then from the sending unit to ground either through a sending unit grounded spade connector, black wire to frame ground or through the fuel tank fixings to the frame.

This should be enough to give you a good idea of what you are looking at.

Good luck!

[weatherm]

Make sure your wires aren't backwards! Thats what happened to me.

[Jeff Aronson]

TeriAnn,

What a terrific description and documentation! I'm keeping your information for the next time I run into that problem.

Jeff

[oestlarsen]

TeriAnn,
Great help, thank you! I have a spare (OLD) reserve Fuel gauge, and will try that one too since my current one is indeed pegged at "FULL" w/ no wire to sending unit connected.
After all, it IS kinda nice to know how much gas you have left :-)
Thanks again!
Carl

[KingSlug]

I can remember how its done but you can use the old style (the one with the pointy indicator) gauge and a newer style sender. Ike Goss wrote about it before, I seem to think it involved moving the float.

Jared

[TeriAnn]

After all, it IS kinda nice to know how much gas you have left :-)

Fuel gauges aren't really needed if you have a reserve tank to switch to. As soon as the engine starts hesitating you just reach over and turn the fuel valve. Early VWs kept part of their tank as a reserve. Land Rovers with multiple fuel tanks can do the same.

Funny thing. When I just had a single front functional fuel tank I was happy with a full tank and started looking for a station at a quarter tank on the gauge or if i was in a town the the needle hit the top of the "E".

Now that I have 3 functional fuel tanks, 42 US gallon capacity, I start that low on gas insecurity feeling when I switch to my last full tank. Instead of thinking I still have a full tank I get into "I'm on my reserve" mentality. sigh.

[oestlarsen]

Short update - and apologies for the "geekness" of this.
I managed to examine, very closely, the current sender. The incredibly thin wire ends up going from the roll/coiled up position to the bottom of the tab assembly. You could barely see it - but it was broken. Naturally it won't work then.

I am now working with a bleach/paint remover solution to shrink my fingers to the size of GI Joe Doll and may attempt to solder thereafter.

Alternatively - the tab assembly looks like it can be drilled out to provide better access to the micro wires.
Carl