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Electrical Questions and Answers This is the place to ask all your question about electrical problems and instalation issues.

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Old 04-29-2013, 12:05 AM   #1
Len57Belair
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Lightbulb Fix - Fuel Gauge 30 Ohm with 90 Ohm sender

The fix is to add 33 ohm resistor to the sending unit.

I just wanted to post this fix. After three guages and three sending units I ended up with the wrong sending unit. Long story and you know how it goes. So before I take the tank out again I used this fix.

Fuel Guage 0-30 ohms and a 0-90 ohm sending unit.

The 0-30 ohms is the stock range.
The 0-90 ohms is often used with after market guages.
Both read empty at 0 ohms and full at the high ohm reading.

Add a 33 ohm resister in parallel to the sending unit. Connect a 33 ohm resister between the sending unit stud and ground. You can connect the resister at any point along the wire from the sending unit to any good ground.

With this fix you guage will read a litte low from full to 1/2 and a little high 1/2 to empty.

Here are the calculations
The 30 ohm sending unit measured 36 Ohms when lever is full up positon.
To calculate resistance the formula is R1xR2 devided by R1+R2
R1=33 ohms Fixed resistor
R2= Tank sending unit


Adjusted--------Correct
Ohms ------------Ohms
24.1--------------36.0---- -Full
22.2--------------27.0----Three quarter
19.0--------------18.0----One Half
13.4-------------- 9.0-----One Quarter
0.0----------------0.0-----Empty


Look at the Actual Ohms collum and compare it to the Correct Ohms collum
At full the gauge will read about 7/8 full
the gauge will be seeing 24.1 ohms instead of 36 ohms
At 1/2 the guage read 1/2 because 19.0 ohms instead of 18.0 ohms is very close
At 1/4 tank the guage will read a little high
The guage will see 13.4 instead of 9.0
At empty 0 is 0 and the guage will go to the empty stop set on the guage.

Remember when your guage reads 1/4 you only have 1/8 left so it is time to fill up.

You can use a 20 ohm resister and the guage will read 1/2 when full but be very close below 1/4

If you do not use a resistor your guage will read
full when you have 1/4 tank
1/2 tank at 1/8 about 2 gal
1/4 is about 1 gal in the tank.

The correct sending unit for the guage you have is the best option but adding the resistor my work until you get it changed.

Last edited by Len57Belair; 04-29-2013 at 12:27 AM..
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Old 04-29-2013, 06:31 AM   #2
Yerny
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Nice work...now we just need to figure out how to inverse the resistance. I have a F150 fuel tank in my Nova - fuel gauge works but reads backwards.

The few options I can think of:

1) place the float 180 off( pointing in the opposite direction), if feasible

2) the electrical part of the sender consists of a resistance strip with a connection at one end, and a sliding contact. If it has provisions to do so, you could move the wire from one end of the strip to the other.

3) try to build a circuit yourself (Op Amp?) - though I did find this supplier, which is what I'm probably going to go with: https://www.spiyda.com/magento/index...izard-mk2.html

Anyone got a better solution or non-UK vendor?
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Old 04-29-2013, 06:39 AM   #3
angs1957
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You sure have gone to alot of trouble to work it all out
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