Fuel level sender.

PostPost by: gherlt » Tue Feb 24, 2015 6:11 am

If low resistance = full tank, maybe there is a short circuit in between ?
I would check the following: disconnect the end terminals of fuel tank sender cable on both ends (instrument - fuel tank sender) and check resistence between end terminals, should be zero (or near zero). If not cabling is broken.
If this is OK, I would check resistance of fuel tank sender and post it here.
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PostPost by: TroonSprint » Tue Feb 24, 2015 9:17 am

Chris, regarding which way up the sender goes, I have had a look at my Sprint and the earth terminal is above the sender terminal, which is the same as yours. I haven't bothered with a voltage regulator for mine and have it working on battery voltage. While the sender was out of the tank I calibrated the gauge to read properly at full and empty using the little adjusters on the back of the gauge. The gauge is heavily damped so I don't think fluctuations in battery voltage will be a problem with an unstable reading - I'll know for sure once the car is back on the road. Let's face it, these gauges are incredibly inaccurate anyway!

If yours is going full scale all the time, I think you must have a short circuit somewhere.

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PostPost by: ericbushby » Tue Feb 24, 2015 11:53 am

Chris, I have had a look at mine and the black earth wire goes to the top connection and to my surprise the plate is not stamped as to which way up to fit it.
Because you say that the guage reads high if you connect the wires together, then I suggest that you disconnect the sender to see if it then reads zero. This will show that the wiring is OK.
With a nearly empty tank the sender should read maximum. Something like 90 ohms.
I am a little surprised, as I thought the later models had a different system which worked the other way round but it seems yours is like mine. i.e. low resistance, high reading.
The guage is a meter measuring the current through the sender variable resistor.
Did it work OK before you fitted it to the tank. I wonder if there is a short circuit on the terminal. It is a fairly crude arrangement.
Keep going, but I fear it is going to have to come out again.
Eric in Burnley
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PostPost by: Grizzly » Tue Feb 24, 2015 12:40 pm

Unfortunately i'm thinking the Sender will have to come back out, i disconnected the two sender wires and the gauge dropped to zero, i also checked the wire from the Sender to the gauge was good and the negative was also good.

So with all things pointing towards the sender i checked the resistance and there was next to none as you would expect from a full tank but i know there is less than a gallon in there. So maybe some thing stuck or the arm of the sender catching a baffle?? Typical, i actually tested it was working before it went in too which makes it a bit more frustrating.
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PostPost by: AHM » Tue Feb 24, 2015 4:30 pm

Chris,

Don't assume that low resistance is a full tank. It depends on the sender and the gauge Low resistance can mean empty - there are different constructions of gauge.

see here
lotus-carbs-f40/fuel-gauge-operation-t25804.html

I think the sender terminals, although different sizes are bolted on. have you taken them off?

To test which is the earth - with both sender wires disconnected put a meter across the tank and each terminal in turn.
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PostPost by: RogerFrench » Tue Feb 24, 2015 4:53 pm

I understand yours is a later car, which means it should have the bimetallic type of fuel gauge, not the moving coil as fitted to S1 - S3. Your kind read full with low resistance, as you've found out. They also read full if you connect the signal wire to the earth connection on the sender and the sender also happens to be earthed anyway!
You should be able to tell easily which connection is which. The signal is insulated, earth is attached directly to the sender mount.
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PostPost by: Grizzly » Sat Mar 07, 2015 1:39 pm

Just to let you know how i got on, I did end up taking the tank out and tried turning it upside down etc to see if the resistance changed at all (which it didn't) so i removed the sender and took the cover off it to find the wire filament had failed and had made contact with the Sender body, So i just unwrapped enough to meet the spade connector terminal and now its working :)

I did play with it a bit to make sure the Min and max level was more or less correct and the amount i unwrapped didn't seem to make any difference so i put it all back together.

So thanks guys for the help, i got there in the end.
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PostPost by: ericbushby » Sat Mar 07, 2015 8:05 pm

Chris,
Thankyou for the update and well done.
Remember, they are all easy afterwards, when you have done all the work.
Eric in Burnley
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