Another fuel gauge question - Impala Tech
Electrical & Wiring Troubleshooting electrical problems

 
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post #1 of 5 (permalink) Old 03-13-2017, 10:23 PM Thread Starter
 
 
Join Date: Oct 2016
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Another fuel gauge question

I know this topic has been covered many times as I've seen from searching the forum. I'm needing some specific help with my gauge so bear with me please. Lol.

The fuel gauge on my 66 Biscayne has read about 1/4 tank since I got the car back about 9 months ago. It is not the factory gauge if that makes a difference. I replaced the sending unit and it still did not move after filling the tank with fuel. A friend who knows much more about wiring than I do ran new wire from the sending unit to the gauge and it still didn't work. Looking at the wire from the sender to the gauge I can see the signal wire and ground come off the unit and go to the front of the car. The signal goes to the gauge and the ground splits with one side grounding to the bottom of the metal dash and the other attaching to the rear of the gauge. There appears to be 4 wires attached to the gauge. I haven't pulled it out to see which wire is on which prongs though. The other morning I removed the ground from the sending unit and the gauge reads past full. I then re-attached the ground and removed signal wire from the sender and the gauge needle read past full as it did before. After reading a ton of threads about the gauge issues, I started thinking that the two grounds (on the gauge and the dash) was causing a problem. Does this seem possible? If not, I'm leaning towards the new sending unit being bad. Any ideas??


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post #2 of 5 (permalink) Old 03-13-2017, 11:20 PM
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All electrical gauges in your car (gas level, Amperage, Oil pressure, Temperature) are just volt meters. The power comes out of the fuse box to the gauge where it goes to the "sender" that is actually a receiver. The voltage goes through the "sending" unit to ground and the amount of voltage drop is what is displayed by the gauge.

The gauges work on a nominal 12 volt system so any resistance in the circuit will affect the reading on the gauge. In the case of a gas gauge it should read from 0 to 90 OHM resistance from the line going into the sender to where it comes out again (the ground wire). If that works you know you have a good sender.

If the gauge moves from when disconnected (isolated by near infinite resistance to a 12 volt system by a foot or so of air gap), and then moves when grounded you know the gauge works.

If you can read voltage in the feed wire to the guage and it reads the same at the back where it enters the sending unit then your wiring is ok; and the only thing left is a bad ground.

Somewhere in your car a bit of rust has built up under some part that is supposed to be part of the grounding circuit. This works for aftermarket gauges (which there are only gas gauges used in most brands of cars; either a 30 Ohm resistor used in older cars, and the 90 Ohm resistor used for cars made after .... I can not remember when.) But according to the Oracle of Google it was 1965 that GM switched over.

Big Dave
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post #3 of 5 (permalink) Old 03-14-2017, 10:44 AM
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1966 Fuel gauge and sending unit

I was having similar issues but with my factory parts. I created two videos that I though might help others:



Paul Winvoices

Paul

1966 Impala SS

Please Visit our new site:
http://www.winvoices.com/
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post #4 of 5 (permalink) Old 03-17-2017, 06:40 PM Thread Starter
 
 
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Thanks for the info Big Dave and the videos Paul. I'll go back to checking grounds. I'll go back to the question I asked in my original post though. Does the aftermarket gauge need to have a ground ran directly to it from the sending unit AND one ran to the gauge from the metal dash? This seems like it will cause a problem to me. But as I said I don't know wiring and avoid it normally at all costs. All else fails I'll pull the sending unit out and bench test it as Paul's video illustrated.


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post #5 of 5 (permalink) Old 03-17-2017, 09:49 PM
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I can not answer that question because I didn't make the gauge. But the manufacture could if you know who made it. There may even be an installation instruction sheet to be found on line. But since I don't even know what you have I can not answer that question.

Kind of like Gollum being asked what Bilbo Baggin's has in his pocket.

Big Dave
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