This is the initial check to see if the problem is in the dash or the fuel tank.
1972 fuel gauge question
Thread Starter
Le Mans Master
Joined: Jul 2006
Posts: 8,117
Likes: 104
From: charlotte north carolina
1972 fuel gauge question
just finished reinstalling dash and wiring. fuel gauge appears inoperative. i'm reading 1100 ohms across the sending unit, and it doesn't change when i lift the float. i get 12v on the tan wire, and the ground is good. when i ground the tan wire the gauge moves slightly off the empty. when i apply 12v across the meter, it also only moves slightly off the empty. i'm thinking both meter and sending unit are bad. any thoughts? by the way, it's an ebay wreck i've rebuilt, so the parts are of unknown history.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,117
From: Crossville TN
The meter is easy to check: 'open' signal wire to meter should read full-scale one direction; short signal wire to ground should read full scale the other direction. If you get no movement on the gauge when you do that test, make sure there is not a 'break' in the signal wire before it gets tied into the rear-end wiring bundle. There could just be a 'flex-break' in that wire somewhere.
If the meter works as expected, just get a new sending unit. From your measurements, it is apparently "toast".
If the meter works as expected, just get a new sending unit. From your measurements, it is apparently "toast".
Racer
Joined: Nov 2009
Posts: 423
Likes: 7
From: Utah
Willcox has a good paper on it. I'm gonna have to tackle this soon because a full tank shows 3 o'clock on the gauge and an empty tank is about 11 o'clock. Right now I judge my fuel level on miles driven divided by 10 (BB w/ a heavy foot) and when I hit 15, I get gas.
http://willcoxcorvette.com/repairand...lp.php?hID=276
http://willcoxcorvette.com/repairand...lp.php?hID=276
Race Director
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From: Dayton, Ohio
Le Mans Master
Joined: Oct 2006
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From: Gladstone MO
C3 of Year Finalist (appearance mods) 2019
Thread Starter
Le Mans Master
Joined: Jul 2006
Posts: 8,117
Likes: 104
From: charlotte north carolina
i bought a new sender from advanced auto (looks just like the vette supplier's)and checked the resistance. it ranged from about 10 to 90 ohms-but here is the interesting part. when the float is up, the resistance is high. that means that the voltage drop at the sender will be higher relative to the drop at the meter. and when the float is low, the voltage drop will be higher at the meter. this means that the gauge is designed to read "backwards"-when the float is low there's high voltage at the meter, and when the float is high the voltage at the meter is low. i guess the engineer that designed this was standing on his head!
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
i bought a new sender from advanced auto (looks just like the vette supplier's)and checked the resistance. it ranged from about 10 to 90 ohms-but here is the interesting part. when the float is up, the resistance is high. that means that the voltage drop at the sender will be higher relative to the drop at the meter. and when the float is low, the voltage drop will be higher at the meter. this means that the gauge is designed to read "backwards"-when the float is low there's high voltage at the meter, and when the float is high the voltage at the meter is low. i guess the engineer that designed this was standing on his head!
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Former Vendor
Joined: Aug 2006
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From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
I agree, why check voltage... Do the quick test on the gauge as 7t1 suggested. Just pull the ohms wire off and the gauge should go full. If you ground out, it should go empty.
If the dash gauge is responsive but will only go to a quarter tank then there is a good chance the resistor on the back of the gauge is defective.
If you sending unit is reading between 0 (empty) and 90 (full) ohms then it is working properly.
Willcox
If the dash gauge is responsive but will only go to a quarter tank then there is a good chance the resistor on the back of the gauge is defective.
If you sending unit is reading between 0 (empty) and 90 (full) ohms then it is working properly.
Willcox
Former Vendor
Joined: Aug 2006
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From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
Roger to answer your question, the resistor on the back of the fuel gauge runs between the 12 volt pole and the ohms pole, so applying 12 volts to the gauge will not cook it.
A fuel gauge with the ohms wire off, broken or just plain missing that has power and ground should read right at 3 O'clock. When 12 volts is applied over the relay to the ohms pole the gauge read at 4 o'clock.
No smoking..
Last edited by Willcox Corvette; Sep 24, 2010 at 04:23 PM.
Thread Starter
Le Mans Master
Joined: Jul 2006
Posts: 8,117
Likes: 104
From: charlotte north carolina
thank you willcox!!!
i replaced the sender, and was getting 12V on the pink wire , and a proper resistance chage at the meter end( measured to ground, showing that the sender and the meter case was grounded, and that the tan wire was intact)-but stilll no dice!
but, boys and girls, i took a close look at the willcox trouble shooting info and PICTURES. i noticed that on my wire plug, the pink(12V) wire was on top, and in the pics from willcox the tan(ohms) was on top. the plug is polarized by the two little bumps on the lugs so the plug can only go on one way, so i instantly recognized( due to my "sherlock homes" like abilities) that something was amiss!
i reversed the leads as a test, and presto-we have a fuel gauge.
don't ask me how or when, but that plug was reversed.
i replaced the sender, and was getting 12V on the pink wire , and a proper resistance chage at the meter end( measured to ground, showing that the sender and the meter case was grounded, and that the tan wire was intact)-but stilll no dice!
but, boys and girls, i took a close look at the willcox trouble shooting info and PICTURES. i noticed that on my wire plug, the pink(12V) wire was on top, and in the pics from willcox the tan(ohms) was on top. the plug is polarized by the two little bumps on the lugs so the plug can only go on one way, so i instantly recognized( due to my "sherlock homes" like abilities) that something was amiss!
i reversed the leads as a test, and presto-we have a fuel gauge.
don't ask me how or when, but that plug was reversed.
Cruising
Joined: Jun 2012
Posts: 10
Likes: 0
Strange Problem
Here's a new problem I'm trying to figure out. On my 72 cpe the gauge reads full when full but reads empty when half full. I'm not sure if it's the gauge or sending unit? Has anyone encountered this problem?
