Author Topic: Gas Guage Issue  (Read 2279 times)

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Offline dsags

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Gas Guage Issue
« on: November 04, 2012, 11:02:40 AM »
On the new convertible the gas gauge is reading incorrectly. New gas tank and sending unit. Checked the ground and its good. Finally put my test light on the connections. At the rear of the flat wire everything makes for a bright bulb in the tester EXCEPT the gas gauge line. Its a dim light. Then checked the voltage and the other connections are 12 volts while the gas gauge connection registers only 8 volts. Anybody know why this would be ? Thanks.
1965 Gran Sport 4 speed Convertible, BCA Sr Gold
1965 Gran Sport Hardtop

Offline Brian

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Re: Gas Guage Issue
« Reply #1 on: November 05, 2012, 07:42:32 AM »
The sending unit in the tank is a variable resistor, so the voltage on the wire to the tank from the dash is going to have varying voltages on it depending on the level of fuel in the tank.   Is your gage reading E or F all the time, or is the accuracy of it off? 
'64 Skylark 2dr ht 4 speed, 300-4
'65 GS ht, 4 speed,2-4s,AC,PS,PB,PW,Pseat,Tilt
'66 Skylark 2dr ht 300-2 automatic
'78 Yamaha DT 400 2 stroke
'88 Ford F-150 4x4 (used to be 4x2)
'89 Ford F-350 4x4
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Offline dsags

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Re: Gas Guage Issue
« Reply #2 on: November 05, 2012, 08:51:36 AM »
Brian,

Its just the accuracy. If the instrument cluster connection is removed the gauge will read full. To be clear the 8 volts is at the trunk end of the flat wire harness that is disconnected from the trunk and gas tank wiring.

1965 Gran Sport 4 speed Convertible, BCA Sr Gold
1965 Gran Sport Hardtop

Offline Brian

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Re: Gas Guage Issue
« Reply #3 on: November 05, 2012, 09:10:29 AM »
Tell me more about exactly what it is doing vs. what you think it should be doing?
'64 Skylark 2dr ht 4 speed, 300-4
'65 GS ht, 4 speed,2-4s,AC,PS,PB,PW,Pseat,Tilt
'66 Skylark 2dr ht 300-2 automatic
'78 Yamaha DT 400 2 stroke
'88 Ford F-150 4x4 (used to be 4x2)
'89 Ford F-350 4x4
'03 Honda Accord Coupe V6 6-speed manual (daily driver)

Offline dsags

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Re: Gas Guage Issue
« Reply #4 on: November 05, 2012, 10:23:08 AM »
With everything connected I filled the gas tank and the gauge only read about 3/8th to half full.
My limited understanding is that the ignition supplies 12 volts to the harness which in turn is sent to the tank via the flat wire. The sending unit will send back a voltage to the gauge which varies based on the amount of gas in the tank. At the end of the flat wire I expected 12 volts. The point on the flat wire I measured at corresponds with the tan wire on the harness.
1965 Gran Sport 4 speed Convertible, BCA Sr Gold
1965 Gran Sport Hardtop

Offline Brian

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Re: Gas Guage Issue
« Reply #5 on: November 05, 2012, 12:41:21 PM »
I think the way it works is the ignition sends 12v to the gauge in the dash, and the wire through the flat cable and to the tank is the ground wire for the gage.  The variable resistor on the sending unit changes the resistance between the wire coming from the gage and the ground wire that comes off the sending unit to the body, therefore varying the resistance on the ground wire to the gage to make it read the appropriate value. 
  First thing I would do is remove the sending unit and move it by hand and see if it works properly.  My experience is that new sending units do not always have the float arm bent properly.  If so, it may be hitting the top of the tank before it moves the variable resistor all the way to the full position. 
'64 Skylark 2dr ht 4 speed, 300-4
'65 GS ht, 4 speed,2-4s,AC,PS,PB,PW,Pseat,Tilt
'66 Skylark 2dr ht 300-2 automatic
'78 Yamaha DT 400 2 stroke
'88 Ford F-150 4x4 (used to be 4x2)
'89 Ford F-350 4x4
'03 Honda Accord Coupe V6 6-speed manual (daily driver)

Offline dsags

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Re: Gas Guage Issue
« Reply #6 on: November 05, 2012, 04:41:58 PM »
Thanks Brian. I will remove the sending unit and check it. Question - does the gas tank have to be drained to remove the sending unit ?
1965 Gran Sport 4 speed Convertible, BCA Sr Gold
1965 Gran Sport Hardtop

Offline Brian

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Re: Gas Guage Issue
« Reply #7 on: November 06, 2012, 05:33:32 AM »
Only if the fuel level in the tank is above the bottom edge of the flange on the sending unit where it mounts in the tank!
'64 Skylark 2dr ht 4 speed, 300-4
'65 GS ht, 4 speed,2-4s,AC,PS,PB,PW,Pseat,Tilt
'66 Skylark 2dr ht 300-2 automatic
'78 Yamaha DT 400 2 stroke
'88 Ford F-150 4x4 (used to be 4x2)
'89 Ford F-350 4x4
'03 Honda Accord Coupe V6 6-speed manual (daily driver)

Offline dsags

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Re: Gas Guage Issue
« Reply #8 on: November 07, 2012, 04:06:18 PM »
Removed the sending unit from the tank. With it still grounded, the tan power wire attached and the ignition turned on the float arm was moved from full to empty. The dash gauge would move but never went to full. When it moved it was slowly. Does this mean the gauge is bad ? Also, is the correct resistance for the sending unit 90 ohms ?
1965 Gran Sport 4 speed Convertible, BCA Sr Gold
1965 Gran Sport Hardtop

Offline Brian

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Re: Gas Guage Issue
« Reply #9 on: November 07, 2012, 05:24:57 PM »
The gauge should move slowly.  It is that way on purpose so you don't see it bouncing around as the gas sloshes in the tank when you accelerate/decelerate. 
 90 ohms sounds right--should be in the service manual.  If you connect the tan wire straight to ground, the gauge should go to full.  If not, then something is wrong with the gauge.  If you ground the tan wire and it does go to full, then something is wrong with the sending unit.  The sending unit should be around 90 ohms at empty and close to 0 ohms at full.   
'64 Skylark 2dr ht 4 speed, 300-4
'65 GS ht, 4 speed,2-4s,AC,PS,PB,PW,Pseat,Tilt
'66 Skylark 2dr ht 300-2 automatic
'78 Yamaha DT 400 2 stroke
'88 Ford F-150 4x4 (used to be 4x2)
'89 Ford F-350 4x4
'03 Honda Accord Coupe V6 6-speed manual (daily driver)

Offline dsags

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Re: Gas Guage Issue
« Reply #10 on: November 07, 2012, 07:57:43 PM »
Took a while to make some wires to connect the tan wire to the sending unit and a ground wire that were long enough so that I could hold the unit in hand and test resistance and watch the gauge. Resistance starts at 8 ohms and even a small movement sends the reading to infinity. Barely moving it goes to 100 ohms. Ames Performance has a 90 ohms resistance 5/16 ths line size for $50. Guess I'll order one tomorrow. Glad it wasn't the gauge.
Thanks for your help Brian.
1965 Gran Sport 4 speed Convertible, BCA Sr Gold
1965 Gran Sport Hardtop