fuel sending unit - Impala Tech
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post #1 of 10 (permalink) Old 06-06-2008, 03:55 PM Thread Starter
 
 
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Location: buffalo,ny
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Unhappy fuel sending unit

Installed 2 fuel sending units (with clean gas tank )in 4weeks.Interior dash
gage only shows 3/4 full when tank is full and at times does not show below 1/4 full when tank is almost empty. Is this the fault of the sending unit or the dash fuel gage?
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post #2 of 10 (permalink) Old 06-07-2008, 10:04 PM
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What year, model, and engine is the car?

Sending Units, Fuel Lines, Pick-ups, and gauges all differed from early to later generation full size Chevrolets.
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post #3 of 10 (permalink) Old 06-08-2008, 05:59 PM Thread Starter
 
 
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65 impala 350ci / 325hp - your help is appricated.
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post #4 of 10 (permalink) Old 06-09-2008, 01:35 PM
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There were two seperate Sending Units used for `65 & `66 applications, and that will be dependant on the engine originally installed in the car. Since you don't have the original engine in your `65, you'll first need find out what the car originally came from the factory with if you don't already know. If you don't, there are many ways to find out even if you don't have any original documentation for the car. `65's could have had anything from an I6 up to a 396/409, and everything in between.

The Fuel Tank Sending Units in `65 will be different for I6 & Small Block V8's vs. those used for Big Block cars. The I6/Small Block Fuel Lines from the Sending Unit were 5/16". The Big Block lines will be 3/8". You indicate that you've gone through two Sending Units in a month, so I'm sure you already know what size the Fuel Line is at the tank, but take a look to ensure that a previous owner has not installed an adaptor anywhere in the line between the tank and Fuel Pump to step up or down in size from the original configuration. Believe it or not, this was a fairly common practice back in the day if someone was doing an engine swap from a Small Block to a Big Block or visa versa. Assuming nothing has been altered here, you'll now be sure of the correct Sending Unit application for the car.

The reason I think you need to check all of these things out first is based on the information you've provided stating that the "gauge only shows 3/4 full when tank is full and at times does not show below 1/4 full when tank is almost empty".
This sounds like there is a strong possibility that the Sending Unit Float is either hanging up in the tank at these fuel levels, or is at it's actual maximum/minimum range of travel. If that's the case, it sounds like these are the wrong units for your application. Secondly, are you using OEM GM Sending Units, or cheap aftermarket assemblies made in God knows where? It would be to your advantage to use actual GM parts here when ever possible. (Just my two cents on that matter).

Once you are certain you have the correct Sending Unit installed, check all of the other obvious and non-invasive things like electrical continuity at every point between the Sending Unit and Fuel Gauge in the car. Use a good quality Fluke meter to do this if possible because you will be looking for accuracy down to the milli-ohm level here, and 43 year old wiring and connectors can get pretty crusty. The wiring schematics for this can be found in the `65 Chassis Service Manual.

If the Sending Unit is working properly, and you have solid electrical continuity throughout the circuit, you should read approximately 240 Ohms at the Fuel Gauge when empty, and approximately 33 Ohms when full. After you're certain that all else is correct, and you aren't seeing these numbers at the gauge, then the gauge itself is likely the culprit.

Hope this helps.

Good luck,

Bigblox

`68 L72 SS427 Impala
`73 SS454 El Camino
`97 Camaro SS Convertible
`07 SC Cobalt SS
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post #5 of 10 (permalink) Old 06-09-2008, 04:44 PM Thread Starter
 
 
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Sir; excellent advise -will try. ( original engine-283 small block )THANKS.
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post #6 of 10 (permalink) Old 03-27-2011, 04:07 PM
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Has anyone experienced a problem with the headlight switch causing your fuel gauge to move when activated?

After getting tired of running out of gas, I decided to troubleshoot and fix my fuel gauge. I dropped to gas tank threw in a brand new fuel sending. Everything was working fine, the gauge itself was was working properly.

Today I was driving the car and I noticed that when I pull the headlight switch the gas gauge drops from its current position to about a quarter tank. When I push the knob back the gauge will go back to whereever it was at before.

This is happening in my 64 Impala SS HT, 327/350trans.

Semper Fi,

Hector

1964 Impala SS HT
2007 Tahoe LTZ
2006 Kawasaki ZX10
2003 Yamaha R1 (sold)
1995 Impala SS
1993 Tahoe GT (sold)
1978 Buick Regal (sold)
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post #7 of 10 (permalink) Old 03-27-2011, 04:21 PM
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The only thing I can think of is that your wire comming off the back of your gas gauge is being grounded somehow somewhere around the switch or its harness.
I'd look at the gas wire under the dash as a start.
BTW...that a good trick .
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post #8 of 10 (permalink) Old 03-27-2011, 06:39 PM
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I had a similar situation once with my '77 Blazer, it was a bad ground in the printed circuit for the gauge cluster. The extra load from the backlighting (with headlights on) would cause the bad ground to fail, in turn causing the fuel gauge to drop.

'69 Impala 4-door hardtop
'70 Impala Custom coupe
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post #9 of 10 (permalink) Old 03-27-2011, 07:54 PM
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I suspect your alternator is not out putting enough juice to keep everything running. Do you have a Voltmeter wired into your instruments or an Ampmeter? If your voltage or current drops; then the PIE formula corrects the Ohms that the gas gage reads to keep physicist's happy with all of their derived equations.

These old cars were not much better than six volt cars of a decade earlier, or those that burned hydrogen gas given off by limestone dropped into vinegar for head lights a few decades before that. Further your original wiring was the very first to use of PVC as an insulation (in the fifties they were still wrapping silver wire in silk to wire cars). That insulation has gotten very brittle, and could have a few shorts, or the wire itself could have increased resistance in the wiring from the individual strands that make up the wire breaking from flexing over the decades that these cars have been around.

I know some people think it is sacrilegious to change anything (except to mount semi size rims and Conestoga wagon tires on their car, after they saw the springs in half); but I strongly recommend rewiring the car with heavier gage wire, and installing a new CS (one wire style) 140 Amp alternator to replace the original 36 Amp SI series that required a mechanical external voltage regulator. With today's audio equipment just the radio alone draws more than the 4 to 7 Amps the SI alternator can put out at idle.

Big Dave
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post #10 of 10 (permalink) Old 03-28-2011, 12:27 AM
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Alot of good info for me to process. I will start by checking the ground around the headlight switch area. Today I replaced the brittle wires coming off the alternator, tomorrow I will
check how many amps the alternator is putting out. One thing I have noticed is that when I drive the car with the headlights on and I step on the brakes the instrument cluster blinks. I think this has something to do with the alternator since the battery is brand new. In the next couple of days i will post my findings thanks to all.

Semper Fi,

Hector

1964 Impala SS HT
2007 Tahoe LTZ
2006 Kawasaki ZX10
2003 Yamaha R1 (sold)
1995 Impala SS
1993 Tahoe GT (sold)
1978 Buick Regal (sold)
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