fuel sending unit.

Shenango

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My sending unit is toast in my 64 Tempest Custom convertible. Originally the car came with a 326 auto but now has a 400 auto in it. I have owned the car for about a month so I don't know too much else about it. Rock Auto lists a different sending unit for 2bbl and 4bbl carbs and a 30 ohm unit or 90 ohm unit. The tank was jury rigged with the internals of the original unit cut off and it was just used as fuel outlet. The gauge sending unit was cut into the top of the tank and I have no idea what it was out of. The resistance of the rheostat varies wildly from 18 ohms to 100 ohms without moving the float. Can someone please steer me in the correct direction for the new sender.
Thank you; Jerry
 

melsg5

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If you read the full description it states one is for a 5/16" line and the other is for a 3/8" line.
Measure the size of the line.
 

Shenango

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If you read the full description it states one is for a 5/16" line and the other is for a 3/8" line.
Measure the size of the line.
Thank you, Melsg for your response. I had read the various descriptions several times. The fuel outlet of my sender is 5/16 steel tubing but it has a butt flair to mate with a 3/8 ID rubber hose. I would be able to deal with a different hose size but the wrong rheostat resistance would give me a bad gauge reading.
Jerry
 

melsg5

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The 5/16" is a single line.
The 3/8" states a 1/4" return line and shows two lines.
Do you have a return line?
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Shenango

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The 5/16" is a single line.
The 3/8" states a 1/4" return line and shows two lines.
Do you have a return line?
There is a single line available in 5/16 and 3/8. The difference in the two is in the resistance rating of the rheostat. I do not have a return line but that is irrelevant at this point.
 

melsg5

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Since you have an ohmmeter you should be able to determine roughly whether a 35 ohm resistance or a 90 ohm resistance causes the gauge to peg on full. If need be buy a 100 ohm resistor and three 10 ohm resistors and determine which causes the gage to peg on full.
 

Shenango

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Since you have an ohmmeter you should be able to determine roughly whether a 35 ohm resistance or a 90 ohm resistance causes the gauge to peg on full. If need be buy a 100 ohm resistor and three 10 ohm resistors and determine which causes the gage to peg on full.
At this point the gauge and wiring et-all is suspect. This car was reconditioned by a complete hack. All I wanted is the correct unit to put in my gas tank. Thank you for your attempt at helping.
Jerry
 

melsg5

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If you look at sites that cater to the restoration market such as OPGI, they make no mention of two different resistance gauges. Perhaps you should be looking somewhere else than Rockauto.
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melsg5

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Ames Performance lists two senders, 5/16" and 3/8" they are both 30 ohm units.
 

Shenango

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Ames Performance lists two senders, 5/16" and 3/8" they are both 30 ohm units.
Thank you. I have looked at the Ames catalog and with all the other research I have done in the past 2 days it seems the 30 ohm unit belongs on my 64. The sender in GM A-body vehicles was 30 ohms from 53 to 64 and changed to 90 ohms in 65 and stayed there until 73. I also have learned quite a lot on the operation and troubleshooting of the system. My first step has to be verifying what gauge is in the dash. Since my car was reconditioned a 65-67 gauge could have been used in the cluster to match the sender that was cut into the top of the tank.
 

melsg5

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You have been using the term " cut into the tank", you mean the tank has been modified and the current sender does not just fit into the normal hole found on the top?
 

Shenango

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You have been using the term " cut into the tank", you mean the tank has been modified and the current sender does not just fit into the normal hole found on the top?
The original A-body sending unit installed in the front of the fuel tank closest to the differential. The unit consisted of a fuel outlet with filter sock, a rheostat and a float. On my car an extra hole was cut into the top of the tank with what looks like a jigsaw and a separate float and rheostat was installed and sealed to the tank. The original assembly was cut off at the filter sock and everything was removed except the tube to let the fuel be sucked out. An external filter was added to the outlet to prevent any debris from getting to the inline filter at the carburetor. Remember in post 7 I stated a complete hack had worked on this car. I am suffering the consequences trying to figure things out.
Although this is my first Pontiac, I have been working on dirt track racers and or classic cars for about 40 years. I can usually figure most things out but some of the garbage this guy did is just baffling. Thank you for your understanding.
Jerry
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Shenango

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Any interest in a new tank and matching sender unit?
At this point I am still committed to fixing this mess. A new system may be in the works down the road some day.

I tested the wire that runs from the tank to the gauge today and found a problem there also. The wire should have about 7.5 VDC that would vary to ground through the rheostat. Instead of the DC voltage I have 28.5 ohms to ground and 0 voltage. There is evidently a problem with the wire, ignition switch or gauge also. SIGH.
 
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Shenango

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This pic is what I am dealing with. IMG_20201002_153341061.jpg I have a Dorman sender on order and hope to have the wiring sorted out in a couple days.
 

melsg5

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I believe the phrase 'Bubba was here' is appropriate.
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Shenango

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Finally got my new sending unit installed. I dropped the tank and give the inside a good cleaning and put the new unit in. The resistance readings were a little wonky with empty being 6 ohms instead of 0 and full at 27 ohms instead of 30. Ran down to the station and got 10 gal of ethanol free and poured it in giving me what should have been a half tank. When I turned the key to run the gauge still was pegged well beyond the full mark. I got under the dash, which is not an easy feat for my 69 year old bones, and pulled the connector from the back of the gauge. I isolated the sending unit wire and had continuity to the tank but there wasn't any DC voltage to the other side of the connector. I found a, run position, voltage source and connected it to the gauge feed side and the needle came down to very near where I think it should be. YAY GAS GAUGE. When my back quits hurting I will find out why my gas gauge feed wire is dead.
 
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