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fuel gauge

 
Retired Red Retired Red
User | Posts: 93 | Joined: 07/10
Posted: 10/15/10
05:39 AM

The fuel gauge in my 68 gto doesn't work.  New fuel tank, good ground on tank.  My alternater lite is on dimly also.  I have other gauges so I know my voltage is good.  I didnt know if there might be a connection so I mentioned it.  When I start the car the gauge moves from full to off the dial full and never moves.  Kinda sounds like a ground somewhere.  Can anyone be of help?  Specifics are very welcome.  Thanks Bill  
68 GTO  455/462
498.9 HP 558 TQ

Retired Red Retired Red
User | Posts: 93 | Joined: 07/10
Posted: 10/15/10
05:48 AM

I forgot, I believe I have a solid chasis ground.  Should I couple it with the engine ground?  Thanks all, Bill  
68 GTO  455/462
498.9 HP 558 TQ

My71 My71
Guru | Posts: 1232 | Joined: 02/10
Posted: 10/15/10
08:12 AM

Sounds like you may have an open circuit. TO have a completed circuit, you have power going through the instrument resistive elements then to ground. If that is broken anywhere along the way your guage will read all the way to the top since it sees infinite resistance or no voltage feed.
Your ground needs to get back to the battery, so if your battery negative is connected to the motor (as is the usual case) then you should have a ground strap from your body and your frame going to your motor also.  
Jim,

sjg3401 sjg3401
User | Posts: 74 | Joined: 12/07
Posted: 10/15/10
09:00 AM

I installed a new gas tank and sending unit in my 69 GTO earlier this year.  I ended up going back and forth with Ames… on my third sending unit I had one that worked.  My symptoms were the same as yours… Fuel gauge pegged out full when key on.   I was able to find the problem by doing step “A” below.  I know it’s a long story here but it might help you.  Remember that gas is flammable so don’t burn yourself down.  Take proper safety measures and use common sense.

I did receive some helpful hints on how to sort this out.  They are as follows and I’m not sure of the source:

A.  Testing the sending unit

1. Inspect all the connections (easy and silly but, you’ll be surprised)

2. Key on (easy)

3. Disconnect the fuel sending wire from the sending unit and ground it against the tank.  The fuel Gauge should go to Empty. (not so easy because you may need to drop the tank unless you have a connector in the trunk you can pull apart.)  If it does then your gauge and wiring to the tank are good.

4. If the gauge does go empty from step 3 then you may have a bad ground.  To check, hook your sending wire back up and connect a temporary jumper wire between the metal fuel feed line from the sender and a chassis ground.  If this works then make a permanent ground and your good.  If this doesn’t work then the sender is bad.

5. To prove the sending unit is bad, pull it from the tank and allow to dry.  Connect a multi meter leads to the ground wire and the other to the sending pole.  Adjust the float an you should see resistance change.

B. Testing the fuel Gauge wire

1.The Gauge will have two wires.  One will be power.  Check that it reads 12 Volts with the key on.

2. The other wire goes to the sending unit.  Disconnect the wire at the sending unit and ensure that it doesn’t touch any electrical ground.  At the gauge end disconnect the sending wire and check for a ground between the gauge end of the wire and the chassis.  If yes then you have a ground fault and the gauge would be reading empty not full in your case.  If no then ok.

3. Next reconnect the sending wire at the tank.  Check for resistance between the gauge wire and the chassis.  There should be some resistance.  If yes then ok.  If no then you have a brake in the sending wire between the sending unit and the gauge and the gauge would read full as is your case.

C. Testing the fuel Gauge itself

1. Assuming you do “A” and “B” above with no faults then…

2. Test for Bad gauge ground.  Connect a jumper wire between a chassis ground and the gauge case.  If the gauge starts working then fix the ground.  If not then it gets more difficult…

3. Remove the gauge from the dash.  Supply 12V from some source to the gauge, connect a ground to the case of the gauge, and ground out the sending unit connection.  The gauge should go to Empty.  Remove the sending gauge ground and the gauge should go full.  If not replace the fuel gauge.  

Retired Red Retired Red
User | Posts: 93 | Joined: 07/10
Posted: 10/18/10
05:14 PM

WOW  thank you I think...  I will print this out but it may have to wait till spring, or if I start now I'll be done by spring... LOL  
68 GTO  455/462
498.9 HP 558 TQ

OldGoat67 OldGoat67
New User | Posts: 2 | Joined: 08/07
Posted: 02/14/11
04:09 AM

90% of the time, it's caused by a grounging issue. If you are sure the sender has a good ground, check the ground straps from the engine to the firewall first (they are commonly removed) then check to see if the gauge itself has a good ground as mentioned above. If in doubt, run a temporary wire from the neg battery term to the gauge case just to see what happens.  

69GTOby 69GTOby
Guru | Posts: 1109 | Joined: 02/09
Posted: 02/14/11
07:25 AM

My fuel gauge has not worked in a long time.  Last summer I was doing all kinds of tests...  Come to think of it, I do not recall reinstalling the ground strap when I swapped the engines.  I think it looked so corrroded that I just tossed it.  I bet that's what my problem is.

Chances are it'll be something stupid and simple like a ground strap or corroded connection.  So take a quick look at this stuff and you might be done in 10 minutes Smile  
****
1969 GTO resto in progress. '76 350+.060, #13 heads, 9.2:1 CR, Lunati cam, Edelbrock Performer, 750 Holley, HEI, Ram Air manifolds, 2 1/2" exhaust. TH400 trans w/shift kit and 2400 stall.  235/60/15 front, 275/60/15 rear on Rally II 15X7s.  GM 12 bolt posi with 3.73's.

Brazil Brazil
Addict | Posts: 3887 | Joined: 06/09
Posted: 02/14/11
07:44 AM

When we are so involved in the actual big stuff it's easy to overlook the simple things. It's the story of my life. Crazy  

Torqued400 Torqued400
Addict | Posts: 2479 | Joined: 10/08
Posted: 02/14/11
07:59 AM

That missing engine to body ground strap can cause all kinds of electical gremlins.  
"We build Excitement"

69GTOby 69GTOby
Guru | Posts: 1109 | Joined: 02/09
Posted: 02/14/11
12:13 PM

You're right Torqued!  The more I think about it the more I realize I need not blame my Painless system (although I still do not prefer Painless), but rather my own negligence.

That explains my electrical in dash gauges: oil press, temp, gen light, fuel, tach(?).  My elec choke, too.

Would this cause my in dash Rally tach to not work?  I hooked up a SunPro tach to the coil -, and the Rally tach to coil - also, but only the SunPro works...?  
****
1969 GTO resto in progress. '76 350+.060, #13 heads, 9.2:1 CR, Lunati cam, Edelbrock Performer, 750 Holley, HEI, Ram Air manifolds, 2 1/2" exhaust. TH400 trans w/shift kit and 2400 stall.  235/60/15 front, 275/60/15 rear on Rally II 15X7s.  GM 12 bolt posi with 3.73's.

wave1073 wave1073
New User | Posts: 14 | Joined: 05/13
Posted: 08/24/13
11:42 AM

Same kind of issue here, read the posts and tried a few things but not luck.  IF anyone has more input I'd be glad to hear it.

Facts:
-fuel gauge pins at full when the vehicle is running and about 3/4 when key is just on.
-took advise here, added ground strap from engine to firewall, and remounted the ground from the sending unit to the body above the tank strap...a good clean bare metal connections. Also battery ground goes to engine
-finished that and the same pinned at full, but empty when key is on or off..
-bought a/m gauge and connected it to the reverse light for power and sending unit wire, and clapped ground with vise grips, nice connection.
-and last took that remote gauge and tried it by 12v at fuse box,sending unit wire and again nice vise grip on good ground.  Still no luck,

Any ideas not listed here, i would like to avoid dropping the tank as i did that before and installed new sending unit a few years ago, vehicle mostly sat and i changed the sending unit because back then it didnt work ,, and never has since that time before .. thanks in advance, dave 72 lemans GTO clone  

waynep712222 waynep712222
Enthusiast | Posts: 310 | Joined: 03/13
Posted: 08/24/13
03:59 PM

dave... 72...

first thing... a 2 minute check...


engine running.. headlights on..   digital volt meter set to 20 volts DC..

1.   Negative battery post to the Positive battery post.. 14.1 to 14.8 volts DC.

2.   Negative battery post to the Engine block..  0.04 volts DC MAX.

3.   Negative battery post to the body.. 0.02 volts DC MAX.

4.   Engine block to the body.. 0.02 volts DC MAX..

if you get more than 0.04 or 0.02 volts on tests 2, 3, 4.. fix the grounds between those components..

with a full frame car or truck.. please  run test 3 again.. between Negative battery and the frame.. 0.02 volts DC max..

if your car passes these tests.. then we can move on to additional tests.

this is just the FIRST STEP...  

waynep712222 waynep712222
Enthusiast | Posts: 310 | Joined: 03/13
Posted: 08/24/13
04:14 PM

OHM the sending unit in the tank.. set the digital volt meter to 200 OHM scale.. usually the lowest on any meter ohm scale..  

do you get something between 2 and 90 ohms.???   while testing.. have somebody wiggle the car slightly..

Since 1965, all GMs use a system with a sender that works 0 to 90 ohms - empty to full. Most Dodges and Fords use a system that works 73 to 10 ohms empty to full
warning... and this is probably not an issue for this forum. some digital fuel gauges use a reversed ohm scale.

when you have the wire to the sending unit unplugged... and you turn the key on.. what does the gauge *** full or empty... one or the other..

while the key is on.. ground the wire that connects to the sending unit.. this should make the fuel gauge go all the way the other way from when you unplugged it..

this proves the gauge wiring is intact..

when you ohm the sending unit to the tank.. if you get more than 90 ohms..with the tank full.. it will PEG the gauge...

there is a trick to use a trimmer pot.. to adjust the resistance between the sending unit and the gauge...   the ideal thing is  if you need to .. adjust the resistance to get the tank to measure when the tank actually is empty.. use an air nozzle with the fuel line extended into a fuel container.  when the fuel starts to allow air past.. thats about the time the car is going to run out of gas...
the car has to be level for this...



do you have any pictures of the back of your gauge cluster... and the front..

i have a chevelle cluster marked out... its different but the way theory on how it works is the same...

again.. these are 72 chevelle gauges.. not a GTO

72 Chevelle Cluster Colorized


72 Chevelle Cluster Ground Connections  

waynep712222 waynep712222
Enthusiast | Posts: 310 | Joined: 03/13
Posted: 08/24/13
04:26 PM

in the images above..

the green circles around the screws in the printed circuit board.. ground different sections of the printed circuit board...  

the individual sections of the stamped steel cluster housings are only connected together with the printed circuit ..  

in the lower image.. you will note there is a ground strap shown on the lower right corner.. with a letter A and B..  the ground wire should actually be hooked up to where it is down on the headlight switch where the end of the wire is a big ring terminal and the mounting screw goes thru for the headlight switch ...

the other terminal should probably go under the screw head marked B...  

waynep712222 waynep712222
Enthusiast | Posts: 310 | Joined: 03/13
Posted: 08/24/13
07:38 PM

Dave... i had some time during the cup race...

72Pontiaclemansinstrumentcluster Zps814bea77 1  

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