[walkerjay]

I am about near complete with all wiring and lights except for my fuel gauge. On the back of the gauge are the two screw terminals that accept the circuit board. Then there is also a lead on the center of the gauge. Does this get wired to anything?

Currently, sender is new, good ground, and getting continuity throughout the circuit. Gauge never budged, sitting on "full". Suggestions appreciated. Thanks.

 

[Bigwhitechap]

fuel Gauge issue

Many aftermarket gauges are OHM specific for example a 74 Nova takes a gauge that is 0-90 Ohms to work with the OEM sender. is your Gauge and sender original or aftermarket . I would start there.

 

[walkerjay]

i have two original fuel gauges. i took the better looking of the two, this thing is immaculate, however I did not bench test. I am thinking it would be fun to do that as custom Jim as outlined the procedure. Sender is new aftermarket that I bought with the Canadian Spectre tank.

 

[siege]

check your ground in the trunk. had the same problem with mine. it was simply unplugged. you can find it right next to your license plate light.

 

[Bigwhitechap]

aftermarket sender?

If you got an after market sender it is most likely wrong for your fuel gauge unless you ordered it car specific...we are Talking OHMS here people. Possible ur ground or when you shoved your tank back up the Ground wire came off.

 

[Custom Jim]

I am about near complete with all wiring and lights except for my fuel gauge. On the back of the gauge are the two screw terminals that accept the circuit board. Then there is also a lead on the center of the gauge. Does this get wired to anything?

Currently, sender is new, good ground, and getting continuity throughout the circuit. Gauge never budged, sitting on "full". Suggestions appreciated. Thanks.

I'm guessing it is a factory gauge in that you said that accept the circuit board (as in flexible circuit board) ?.

This is off of memory as I cannot get to my gauges but there should be two connections off of the flexible circuit board to the back of the gauge. I'm thinking these connections are at the 3:00 and 9:00 position. One of these connections if you trace it through the flexible circuit board to where the main harness plugs into the instrument panel it will go just to one pin in this plug. The other connection in the flexible circuit board will also go to a different single pin in the instrument panels main harness plug but also will go another direction in the flexible circuit board to power up other things. This connection with it branching off would be powered with an ignition powered 12 volts while the other unbranched one would go to the tan wire in the loom which then goes back to the fuel tanks sending unit.

On vehicles that have a factory indash tach they use the same flexible circuit board but in the wiring harness instead of the sending unit wire from the fuel gauge going to the sender the wiring harness is different and the wire in the harness goes to the negative terminal on the ignition coil.

Since the flexible circuit board becomes more fragile as it ages there may be a hairline crack in it keeping the fuel gauge from getting power. If things are not all the way wired and the instrument panel is not grounded this too could keep the fuel gauge from working. Since the fuel gauge and tach are grounded through their mounting bolts and rely on the instrument panel having a ground going to it from behind the dash, this too could be your problem.

For testing you need to know which terminal connection on the fuel gauge should be getting an ignition 12 volts. Let's call this terminal connection "A". By grounding a test light or meter to a metal spot behind the dash, turn the ignition switch to the run position and see if the test light comes on or the meter reads about 12 volts on when the test light or meters other lead touches this spot (spot "A"). If it does, then you are getting power to the gauge. Now you need to check for ground. Take the test light and connect it to a source of power. It can be a battery 12 volts or ignition 12 volts. It doesn't matter. Now take the test lights other end and touch it to the housing of the fuel gauge and it should light up verifying the gauge is grounded.

Now all that is left is to see if the gauges needle can be made to move. Normally if nothing is connected to the sending unit terminal connection and the gauge has power and ground then the needle should swing to full or above. Let's say it's already above full and you can't see it move. The problem may be in the gauge so another test needs to be done. By grounding the sending unit terminal connection on the gauge the needle should swing down to empty or below. You still need to have power and ground being applied to the gauge or this test will not work.
Now if the grounded sending unit terminal connection on the gauge is ungrounded the needle should swing to full or above.

If the gauge is working as above then the problem is from the sending unit terminal connection rearward to and through the sending unit. If the gauges needle does not move or move fully to empty or full then the gauge has a problem. I'm not saying there could not also be a gauge and sending unit problem but further testing will reveal other things either working or not.

When finding out what terminal on the gauge should be for the sending unit and which should have power, double check and make sure you are right. I've hurried more than once and gotten things backwards and things can be damaged if not tested correctly.

Jim

 

[walkerjay]

I've got's some homework to do.. Jim, your advice is invaluable. Printing this out now.......Hope to return the favor someday

...and this sender is in fact car specific so hopefully I am ok in that regard.

 

[Custom Jim]

Pictures might help also so here we go.

Backside of a 71 Nova instrument panel with a fuel gauge:

 

[Custom Jim]

Connections of the fuel gauge to the circuit board and the paths in the flexible circuit board to the main harness connector.
The added red line on the picture shows how at the main harness connector it branches off to power other things in the instrument panel.

 

[Custom Jim]

Connections of power, ground and the sending unit on the fuel gauge.

By applying only power and ground to the gauge the gauges needle should go to the full or above mark.

By applying power and ground to the gauge and then shorting the sending unit terminal to ground the gauges needle should go to the empty or below mark.

By inserting a 45 ohm resistor between the sending unit terminal and ground the needle on the gauge should go halfway between empty and full (half full/half empty mark).

 

[Custom Jim]

By inserting different value resistors between the sending unit terminal and ground the gauge can be made to read 1/4 full or 3/4 full.

 

[walkerjay]

Jim, great pics and explanations. A month later I finally found time to get into my wiring. One thing that was confusing me was that my car has an original wiring harness for console gauges. I was able to track down and confirm continuity of the sender wire to the gauge.

I have performed the steps you listed and am kind of at a stopping point.

For testing you need to know which terminal connection on the fuel gauge should be getting an ignition 12 volts. Let's call this terminal connection "A". By grounding a test light or meter to a metal spot behind the dash, turn the ignition switch to the run position and see if the test light comes on or the meter reads about 12 volts on when the test light or meters other lead touches this spot (spot "A"). If it does, then you are getting power to the gauge.

Check. I am getting power to the gauge.

Now you need to check for ground. Take the test light and connect it to a source of power. It can be a battery 12 volts or ignition 12 volts. It doesn't matter. Now take the test lights other end and touch it to the housing of the fuel gauge and it should light up verifying the gauge is grounded.

Check. Ground is good. btw, gauge reading just above half tank. (I lost track of how much fuel I have as I have been driving around a bit)

Now all that is left is to see if the gauges needle can be made to move. Normally if nothing is connected to the sending unit terminal connection and the gauge has power and ground then the needle should swing to full or above.

I removed the circuit board so that nothing was connected to the gauge. Turned the key for power and needle moved smoothly to FULL or above, just as you noted.

Let's say it's already above full and you can't see it move. The problem may be in the gauge so another test needs to be done. By grounding the sending unit terminal connection on the gauge the needle should swing down to empty or below. You still need to have power and ground being applied to the gauge or this test will not work.

I grounded the sending unit terminal, got a small spark and the needle does not move, stays at full.

Now if the grounded sending unit terminal connection on the gauge is ungrounded the needle should swing to full or above.
Jim

Didn't get to this last test because when I ground the sending unit terminal the needle does not move so this test is moot.
I am going out now to see where the gauge is reading. Left the battery off so I am hoping things will kind of reset themselves and my gauge needle should drop down a bit because I know the tank is not full.

Comments on my last unsuccessful test are appreciated. Thanks Jim.

 

[walkerjay]

Update

Update:

Seems I performed the last test incorrectly. I replaced the gauges fuse. That one must have blew when I grounded out the sending unit terminal. Problem was, I left the circuit board attached.

This time, I kept power and ground to the gauge but isolated the circuit board connection to the sending unit terminal. Now when I ground out the sending unit terminal, the gauge goes to empty. Voila! Looks like I passed all the tests.

Now I am going back out there to ensure my connections are good and see where the fuel needle goes. At least now I know that my gauge is working properly and I have continuity to the sender so my wiring is correct. This is a brand new sending unit I got with the Spectre tank.

 

[walkerjay]

I double checked continuity from the sending unit terminal to the sending unit and all looked good except I noticed I was not 100% where the circuit board meets the gauge. 40 year old and a little brittle. So I decided to bypass the circuit board and go from the sending unit wire below my dash directly to the terminal with a round hold solderless connector. I am now positive I have continuity.

Question:
By bypassing the circuit board with my sender wire, am I still fuse protected or is there a fire issue? Essentially, I have bypassed the entire circuit board wiring connector.

After I performed the last test where i got the gauge to read empty, I pulled off the sending terminal short at 1/4 tank. That is where the needle stayed. Now that everything is hooked back up, it is still at 1/4 tank. Does this gauge and sender need some time to "settle in" for accuracy or should it be immediately responsive to whatever level of fuel is in the tank?

I think my next test will be to either drain the tank or fill the tank. I'll probably go with draining it first, and see if the needle drops.

Hope I'm not ranting too much, just want to document this procedure for others to see and learn.

 

[Custom Jim]

You stated:

Seems I performed the last test incorrectly. I replaced the gauges fuse. That one must have blew when I grounded out the sending unit terminal. Problem was, I left the circuit board attached.

The reason I think the fuse blew was from having the ignition 12 volt terminal on the gauge or some other ignition powered circuit on the instrument panel touch a ground.
Grounding out the sending unit wire on the gauge would give the same results as if the fuel tank was empty. GM did not design the fuel level system to blow a fuse each time the gas tank was run dry. Something else happened OR there is an internal problem with the gauge.

I'm going to guess you have the instrument panel loose to where you can access the backside of the instrument panel wiring (flexible circuit board) and also the wiring on the back of the fuel gauge. If this is the case then here's what I would do next.

Get a volt meter to measure the voltage on the terminal of the fuel gauge that should be getting the ignition 12 volts. Ground the meter to a spot under the dash. Now take a reading. You should get, let's say 12 volts. Now move the test lead off of the meter that was grounded under the dash to the backside metal of the instrument panel housing. The voltage should be the same. This is to verify the instrument panel is grounded good. If you got something like a 6 volt reading or posibly zero volts then the instrument panel is not grounded or grounded well.

I've bench check many many things through the years and if it was my car and I think it might be the gauge itself that has a problem I would remove the gauge from the dash and to do a bench check.

If I had the fuel gauge out of the dash I could easily test it at work using a power supply and some jumper test leads but a make shift one can be made and I've done testing this way before but you just have to be careful.

To bench check a fuel gauge you will need a fused 12 volt wire from the vehicles battery, another seperate section of wire from the negative terminal of the same battery, and some jumper wires (preferably with alligator clips on the ends).

Start by taking a fuse holder and with no fuse in it and on one end of the fuse holder attach a length of wire to it to reach from the car's battery to a small table or something like it in front of the car. Hopefully you can find a wire that is colored red, white, blue, green or whatever but NOT black.

Now cut another section of wire the same length but have it black in color.

You want these two wires different colors so you can easily identify them as a positive wire and a negative wire.

Now connect the black wire to the housing of the fuel gauge. Since the gauge get's it's ground by being bolted into the instrument panel, you need to either crimp on a small ring terminal to this ground wire going to the battery negative terminal and then nut and bolt this to the fuel gauges housing or if not come up with a way to attach this wire solidly to the gauges housing.

Now take this length of ground wire and attach the other end to your car's battery negative terminal.

Now you need to know which of the two studs on the back of the gauge would have gotten an ignition 12 volts from the cars original flexible circuit board. Once this is identified properly and correctly, attach you other colored length of wire to this stud. Now route this colored wire towards the vehicles car battery and wire in a fuse holder close to the battery and after the connections are made, put in a 10A fuse. When the fuse is put into the fuse holder the gauge should now be getting 12 volts and be grounded.

With these two connections the gauge needle should go to the full mark or above if the gauge is working properly.

If not double check connections, check for a blown fuse and if you were then to take a volt meter and check the voltage and ground at the fuel gauge and it should read 12 volts (the same as what the battery in the car is at).

If after checking connections and all is good and the needle is anywhere else but the full mark or above then I would say the gauge has a problem. If for some reason it blows the fuse then the gauge has a problem.

OK, so let's review. When the gauge has 12 volts going to the proper terminal on the gauge and it's metal casing is grounded the gauge needle should go to full or above. With just 12 volts and ground this is what the gauge should read if working properly. There has been no connection made on the gauges sending unit terminal so the gauge is seeing a resistance higher than 90 ohms.

Since one test will not show if all is good we need to do more tests. The second test requires the gauge to still have 12 volts and ground getting to the gauge like in the first test. By now grounding the sending unit terminal on the gauge to the case of the gauge, the needle should then go to the empty mark or below. The gauge now thinks it is seeing zero resistance on the sending unit terminal and as it was designed would have the gauges needle go to the empty mark.

If with these two tests the gauge does what I've descibed then I would have to say the gauge is good.

You can do optional further testing but this would be to see how well the gauge is calibrated when different resistance values are placed between the gauges sending unit's termianl and ground. By inserting different reistance values you can mimic the gauge to think it is seeing a gas tank mounted sending unit. Since the gauge needs a variable resistance between 0 and 90 ohms to make it read between empty and full you can on a test bench insert resistors to where when a 45 ohm reisistor is wired in the gauge should read 1/2 full and with different value resistors have the gauge rear 1/4 full, 3/4 full and read full.

OK, if with testing the gauge on the test bench you can now look for problems in the car.

I would then put the gauge back in the car and then verify it is getting 12 volts and ground and if so it will read anywhere from empty to full but I would think the gauge needle would move from where it sat prior to installing it back into the dash.

Now I would go back in the trunk area, identify the sending unit wire coming out of the rear wiring harnes and find a factory connector that you can unplug and unplug this connection. Now go back and turn the ignition switch on and see if the gauges needle goes to the full mark or above. It should if everything forward is working and making connections.

Now with the ignition still on take a small jumper wire and attach one end to a good ground point in the trunk. Take the other end of this jumper wire and on the connector end side that goes into the rear wiring harnes and goes up forward towards the indash fuel gauge put this wire inside the connector. The gauge needle should then go to the empty or below empty mark.

If you take this wire back out of the connector the gauge needle should go back up to full or above.

If it does these two things then guess what. The problem is further back through your new sending unit or where it is grounded to the car's body.

If the ground wire from the ending unit to the body of the car is original it may have a corroded crimped on termianl on the wire in that you might be able to cut an inch or two off of the wire and crimp on a new and and have things working properly but if thsi doesn't work and you have also cleaned and makde a solid connection of this wire to the cars body then the tank will have to be dropped for further troubleshooting.

I hope this wasn't too long but it has to be to track things down. You have to do multiple test and checks to find out what the problem or problems are and get things working again.

Jim

 

[walkerjay]

Jim, you have gone above and beyond the call of duty. This thread should go in best of under fuel gauge testing.

I will print out your guideline and go thru it step by step. I have pretty good access to the gauge, as it is mounted in the car, but I can easily remove it for the bench tests.

I am pretty sure that the reason the fuse blew is because I grounded the sending unit terminal with the circuit board still attached. Once I replaced the fuse and isolated the circuit board on the sending unit side (only leaving power and ground), the gauge moved to empty.

I have two gauges so pulling this one out to bench test each one is pretty easy. Just time.......

I'll keep at it. If I understand correctly, if I get everything working properly but the gauge does not read accurately, I can fine tune it with some resistors. I'll cross that bridge later.

Thank you so much Custom Jim. Next 2 hour slot I get I'll be back out there for more diagnostics.

Sincerely,
Jay Walker

 

[Custom Jim]

I drew this up a while back and maybe it will help:

http://rides.webshots.com/album/570235195atVBBV

These pictures below are from an El Camino I worked on with a fuel gauge problem (problems with the sending unit):

http://good-times.webshots.com/album/568852675wbgvwz?start=156

Jim

 

[walkerjay]

I drew this up a while back and maybe it will help:

http://rides.webshots.com/album/570235195atVBBV

These pictures below are from an El Camino I worked on with a fuel gauge problem (problems with the sending unit):

http://good-times.webshots.com/album/568852675wbgvwz?start=156

Jim

Those diagrams are awesome. At this point, I think I am pretty well educated in how everything works. I do not know for sure the ohms of my sending unit but will continue with my investigation...

 

[walkerjay]

Get a volt meter to measure the voltage on the terminal of the fuel gauge that should be getting the ignition 12 volts. Ground the meter to a spot under the dash. Now take a reading. You should get, let's say 12 volts. Now move the test lead off of the meter that was grounded under the dash to the backside metal of the instrument panel housing. The voltage should be the same. This is to verify the instrument panel is grounded good. If you got something like a 6 volt reading or posibly zero volts then the instrument panel is not grounded or grounded well.

I got thru this step Super Jim. 12 volts to gauge + terminal no problem. Turns out when I grounded it I dropped to 6.7 volts as you explained might happen! I then ran a new terminal from the instrument panel directly to a really good ground that I set up near my heater box. Voila! Getting around 12 volts when I ground the instrument case. Found something out here...

To bench check a fuel gauge you will need a fused 12 volt wire from the vehicles battery, another seperate section of wire from the negative terminal of the same battery, and some jumper wires (preferably with alligator clips on the ends).

Start by taking a fuse holder and with no fuse in it and on one end of the fuse holder attach a length of wire to it to reach from the car's battery to a small table or something like it in front of the car. Hopefully you can find a wire that is colored red, white, blue, green or whatever but NOT black.

Now cut another section of wire the same length but have it black in color.

Now connect the black wire to the housing of the fuel gauge. Since the gauge get's it's ground by being bolted into the instrument panel, you need to either crimp on a small ring terminal to this ground wire going to the battery negative terminal and then nut and bolt this to the fuel gauges housing or if not come up with a way to attach this wire solidly to the gauges housing.

Now take this length of ground wire and attach the other end to your car's battery negative terminal.

Now you need to know which of the two studs on the back of the gauge would have gotten an ignition 12 volts from the cars original flexible circuit board. Once this is identified properly and correctly, attach you other colored length of wire to this stud. Now route this colored wire towards the vehicles car battery and wire in a fuse holder close to the battery and after the connections are made, put in a 10A fuse. When the fuse is put into the fuse holder the gauge should now be getting 12 volts and be grounded.

With these two connections the gauge needle should go to the full mark or above if the gauge is working properly.

Done. I set up the bench test just as you described. The gauge I pulled out of the car did get to full and also back to E when I grounded out the sender terminal. Then I decided to test a second fuel gauge that I had. This one passed the test however I noticed with the second gauge that the needle moved a bit more swiftly....especially to the FULL mark. The first gauge was very slow to the full mark.[/QUOTE]

Since one test will not show if all is good we need to do more tests. The second test requires the gauge to still have 12 volts and ground getting to the gauge like in the first test. By now grounding the sending unit terminal on the gauge to the case of the gauge, the needle should then go to the empty mark or below. The gauge now thinks it is seeing zero resistance on the sending unit terminal and as it was designed would have the gauges needle go to the empty mark.

Per my note above, both fuel gauges that I have passed this test.

If with these two tests the gauge does what I've described then I would have to say the gauge is good.

They both passed but one had a quicker needle than the other. Is this a good thing?

Once I hear back I'll reinstall the gauge and continue on with the testing.

Thanks again Super Custom Jim!

 

[Custom Jim]

They both passed but one had a quicker needle than the other. Is this a good thing?

Once I hear back I'll reinstall the gauge and continue on with the testing.

Thanks again Super Custom Jim!

I guess a person could go either way with either a fast reacting gauge or a slower one to monitor the fuel level.

Just like anything else you may get the new gauge in the dash and have it not work right so then in this case at least you know what is right and what is not to help narrow down the problem(s) to troubleshoot things.

On my car the other day the fuel gauge was reading 30-35% on the Dakota Digital gauge and I filled the tank up and it took 14.3 gallons. I had worked on my original sending unit years ago with making sure the connections were good and I did a little math and found out that my sender is only off a few percent (1.5% to 7.5% off) and I think I'm doing good. I did notice right when I was pulling in to get gas it was reading 35% but once I parked with the nose of the car down a little bit at the pump it went down to 30%.

I wonder too that if the speed of the needle and how fast it reacts is due to the jumper resistor I've seen of the back of some fuel gauges. If one resistor is at 10 ohms or something and another is 100 ohms how that might affect things. I just don't have access to my fuel gauge to try different scenario's.

Jim