67-72 Chevy Truck fuel gauge diagnostics 101

[CarpBlazer]

Originally Posted by TBONE1964

If you want to add a ground, you can use the center lower terminal on the fuel gauge itself. Do you have any issues with any other gauges/lights? Also check the ground screws on the circuit board, one by the fuel gauge and one by the speedo. The cluster gets its ground through the connector and through the circuit board. very common burn spot on the circuit board is near the connector plug in. Make sure the circuit board does not have any burn spots and the grounds are tight. Also if the cluster tins have been painted, it can cause a ground issue between the fuel gauge tin and the main tin.

Check all the 5/16 nuts on the back and make sure all are tight. It sounds like a loose ground or bad connection to me.

Share what you find here please.

Thank you,
Tom

Curious about the common burn spot near the connector on the printed circuit, What could cause that spot to burn out?
Posted via Mobile Device

Edit: reason I ask is that my fuel gauge (71 Blazer) is stuck below empty, I noticed printed Circuit was burned out on ground circuit next to plug in. Replaced circuit and it burnt out again.
Turn signals are also permanently lit..
Grateful for any advice I’m stumped.
Ben

[TBONE1964]

Originally Posted by CarpBlazer

Curious about the common burn spot near the connector on the printed circuit, What could cause that spot to burn out?
Posted via Mobile Device

I am also a bit curious but my thought is it is the closest to the connector but weaker than the wiring in the connector. If any circuit gets shorted to ground, that spot seems to be the weak link in the flow.

While doing my testing, I have fried a few by shorts and that is the spot the burn through.

Maybe some of the others have some ideas. I know it is hard to keep good ones around for long

[Lancialonnie]

Thank you everyone for your input! It turns out I too have a burned out circuit board. When I reached up to feel what looked like a bad spot, I heard an arc so the battery is disconnected until the new circuit board is installed.
Posted via Mobile Device

[CarpBlazer]

Ok, so I figured out my electrical gremlin causing stuck fuel gauge below empty. I was having issues with all dash lights (turn signals solid, high beam indicator lit, everything that could light up on dash)to go on when ignition on and as I said above, fuel gauge was stuck below zero.
After checking the whole harness, replacing circuit board (only to fry the ground connection again) and hours of scratching my head..I realized the factory tach hot wire was running from fuse box ‘ignition unfused’ port to the upper center post on the tach. Problem is, that upper post is the ground post..Face palm...I attached it to ground and boom, everything works.
Fuel gauge is now working perfectly. I’m now on a mission to find correct wiring for factory tach.
This thread really helped me tackle all this and get it solved, Thanks!
Ben

[TBONE1964]

Originally Posted by CarpBlazer

Ok, so I figured out my electrical gremlin causing stuck fuel gauge below empty. I was having issues with all dash lights (turn signals solid, high beam indicator lit, everything that could light up on dash)to go on when ignition on and as I said above, fuel gauge was stuck below zero.
After checking the whole harness, replacing circuit board (only to fry the ground connection again) and hours of scratching my head..I realized the factory tach hot wire was running from fuse box ‘ignition unfused’ port to the upper center post on the tach. Problem is, that upper post is the ground post..Face palm...I attached it to ground and boom, everything works.
Fuel gauge is now working perfectly. I’m now on a mission to find correct wiring for factory tach.
This thread really helped me tackle all this and get it solved, Thanks!
Ben

Hi Ben,

Did you get the tach figured out? Pretty simple to wire up. Let me know if you need help with it.

With face in palm, looking at the back, 12v is on the right, coil on the left and ground is the center stud which would only be used if the tach is not getting a good ground through the tin(s).

Tom

[CarpBlazer]

Yes, from the previous tach wiring debacle, I fried one of the grounds in the printed circuit so decided to add a ground from the center stud at the tach to dash and all works great. Thanks for your help!

[Keystone_Hate]

Just read this entire thing, plus a few other threads on fuel gauge stuff. Followed the steps for troubleshooting and narrowed it down to a sunk float on my sending unit. Everything is working as it should now. Thank you everyone for the awesome information. Saved me a lot of time and headache.

[TBONE1964]

Originally Posted by Keystone_Hate

Just read this entire thing, plus a few other threads on fuel gauge stuff. Followed the steps for troubleshooting and narrowed it down to a sunk float on my sending unit. Everything is working as it should now. Thank you everyone for the awesome information. Saved me a lot of time and headache.

That is awesome and if this helps just one person it was worth the time.

Yes, thank you everyone who adds there experience here.

Take care,
Tom

[Hoodogie]

I know this is an old post but I hope someone will direct me a little bit. Not being great at electrical work I have read the previous threads and checked some things out with my gas gauge. Let me start and say that the gauged worked fine before I removed the cluster to change the printed circuit on the back in order to get my dash lights working. When I reinstalled it the gauge was reading at three o'clock. I then reached up behind the dash and found out that I haven't tighten the nuts down on the gauge terminals. I don't know if I had the key on or not but as I was tightening the nuts the gauge went from 3 o'clock to 3/4 full and hasn't moved since. I checked the resistance at the tank and got 68 which is what I think is correct for the gas I think is in the tank. When I remove the tan wire from the fuse box nothing happens to the gauge. When I grounded the terminal nothing happens with the gauge. Did I blow the resistor when I was tightening the nuts up I'm stumped

[TBONE1964]

Originally Posted by Hoodogie

I know this is an old post but I hope someone will direct me a little bit. Not being great at electrical work I have read the previous threads and checked some things out with my gas gauge. Let me start and say that the gauged worked fine before I removed the cluster to change the printed circuit on the back in order to get my dash lights working. When I reinstalled it the gauge was reading at three o'clock. I then reached up behind the dash and found out that I haven't tighten the nuts down on the gauge terminals. I don't know if I had the key on or not but as I was tightening the nuts the gauge went from 3 o'clock to 3/4 full and hasn't moved since. I checked the resistance at the tank and got 68 which is what I think is correct for the gas I think is in the tank. When I remove the tan wire from the fuse box nothing happens to the gauge. When I grounded the terminal nothing happens with the gauge. Did I blow the resistor when I was tightening the nuts up I'm stumped

It sounds like you have lost the power to the gauge. My first question is does the temp gauge work? Make sure the temp gauge works before anything else is done.

If the temp gauge does not work, check for battery voltage at the #3 terminal of the cluster connector. You should have voltage with key on. If not, check the gauge fuse.

Also it is critical that the circuit board tangs where the connector plugs in have not moved and caused a short. Over years they do come loose. Yu can repair with a dab of super glue if needed.

If you suspect the resistor is bad, remove it from the gauge and test it with an ohm meter from one side to the other. You should have approx 85 ohms.

The gauge will set at the 3 oclock poistion when it looses ground or the tan wire is not connected, which is the signal wire from the sender.

The gauge will stay where ever it stopped after ign is shut off or the gauge looses battery voltage.

Hope this helps,
Tom

[Hoodogie]

Originally Posted by TBONE1964

It sounds like you have lost the power to the gauge. My first question is does the temp gauge work? Make sure the temp gauge works before anything else is done.

If the temp gauge does not work, check for battery voltage at the #3 terminal of the cluster connector. You should have voltage with key on. If not, check the gauge fuse.

Also it is critical that the circuit board tangs where the connector plugs in have not moved and caused a short. Over years they do come loose. Yu can repair with a dab of super glue if needed.

If you suspect the resistor is bad, remove it from the gauge and test it with an ohm meter from one side to the other. You should have approx 85 ohms.

The gauge will set at the 3 oclock poistion when it looses ground or the tan wire is not connected, which is the signal wire from the sender.

The gauge will stay where ever it stopped after ign is shut off or the gauge looses battery voltage.

Hope this helps,
Tom

I do have power at the gauge and the temp gauge works. What I did was run a ground wire to the bottom terminal and went down and put some gas in it the gauge is now reading a bit over full not at the 3oclock position. not sure if the ground fixed it or not yet I'll have to take it for a ride and see if it moves before I move to the next step. thanks you are a great help I'll let you know if the ground worked thanks again

[TBONE1964]

Originally Posted by Hoodogie

I do have power at the gauge and the temp gauge works. What I did was run a ground wire to the bottom terminal and went down and put some gas in it the gauge is now reading a bit over full not at the 3oclock position. not sure if the ground fixed it or not yet I'll have to take it for a ride and see if it moves before I move to the next step. thanks you are a great help I'll let you know if the ground worked thanks again

[Shotgunn]

I may need therapy before this task is accomplished...
Gauge is at 3 o'clock position..
new tank/sending unit/ wires go all they way into the cab..proper ground.(LMC under bed tank)
pull the brown wire clip out of the fuse block and it reads 92 ohms with a full tank of gas. plug the clip back into the fuse block an check it while plugged in..32 ohms..back feed? I have 12.6 volts at the #3 on the plug with the key on. the battery and temperature gauge is disconnected. after market gauges installed oil/volts/temp.

[VetteVet]

The wire from the tank sending unit terminal goes to the FUEL terminal on the fuse panel. It is located on the middle terminal on the end of the panel.



It's on the right end center terminal of this panel. This is just a connector for the sender wire to the fuel gauge which exits out the rear of the fuse panel and goes to the dash cluster plug to pin 4.

[heyfish]

My fuel gauge doesn't move and it is below the empty mark. I pulled the resistor from the back of the fuel gauge and it doesn't give any readings with the multi-meter. I also checked the printed circuit and it looks like it is in pretty good shape. Would the resistor cause the gauge to not move at all? Could the fuel gauge just not be receiving voltage? The rest of the lights in the instrument panel work but the brake light only lights up when the engine is starting. The fuel gauge doesn't move though.

Any help would be greatly appreciated.

[TBONE1964]

Originally Posted by heyfish

My fuel gauge doesn't move and it is below the empty mark. I pulled the resistor from the back of the fuel gauge and it doesn't give any readings with the multi-meter. I also checked the printed circuit and it looks like it is in pretty good shape. Would the resistor cause the gauge to not move at all? Could the fuel gauge just not be receiving voltage? The rest of the lights in the instrument panel work but the brake light only lights up when the engine is starting. The fuel gauge doesn't move though.

Any help would be greatly appreciated.

Typically if the circuit board is good and the wiring to the circuit board, the temp gauge will work. The #3 wire on the cluster connector is a pink wire that feed battery voltage to the temp and fuel gauge. The power splits in the circuit board. If you have a good working temp gauge then your power is good up to the connector.

I would take a good look at the back of the circuit board and make sure the path for power is not burnt. You can use a test light and check for power right at the gauge itself.

The resistor should show some resistance. You should get around 85/90 ohms from a good working resistor. If not, replace it and see what happens.

Hope this helps,

Let me know,
Tom

[Threerun]

Glad I stumbled into this thread- my fuel gauge (previously working great) just went to full 3 O'clock and won't budge. Last night I started simple- pulling the tan wire from the sending unit- noticed as I moved it up or down on the sending unit post the fuel gauge bounced to just below 'F' and then back to 2-3 O'clock.

Appears my tank is grounded to frame.

Might try a jumper ground wire from sending unit to frame- see what happens.

Great thread, btw- appreciate the information very much.

[heyfish]

Does anyone know where you can get replacement resistors for the back of the fuel gauge?

[Tenncman]

I also would like to know where I can get the resistor needed for the gas gauge. Thank you in advance

[heyfish]

http://www.truckandcarshop.com/

The online catalog is a little slow but its in there. TBONE1964 hooked me up with this site.

I replaced mine and my fuel gauge started working again. Before I replaced the resistor, the gauge was way below empty and didn't move. It was a much easier option than pulling the sending unit to check it out.

[Tenncman]

Thank you heyfish. The part number at truckandcarshop is 67-04508.

[vitamin_c95]

Resurrecting a very nice thread as I have been searching for awhile, gave up, searched again yada yada. I was guessing my ground went bad at the tank, as my gauge is stuck at 3 o'clock spot. I just got the truck up and running (been sitting awhile) and decided to wait on messing with the gauge as I have active spider webs all over, so trying to get a good steam cleaning before I go reaching into dark areas.
Anyways, I was trying to install a lap belt so I can drive to a vehicle lift and steam clean
the underside. I accidently found a tan wire, taped all up and tucked in my cab corner. Is this my ground wire? Can I ground this side and check my gauge?

Edit-i have 2 saddle tanks. Is there a separate wire for the right side tank? I have no idea if it works. My selector valve is stuck/broke in the left tank position

[TBONE1964]

Hello guys and gals,

I am going to add some information from a bench test I did today. This is going to show how to wire a fuel gauge loose from the cluster and test.

I will try and make it basic and clear as to what is going on for each photo. This will take some tome so let me get everything loaded before responding please.

I try to keep up with this but very busy with other things. I appreciate anyone who chimes in with good info from their own experience.

Thank you,
Tom

[TBONE1964]

I am using a new AC Delco fuel gauge and a GM 0-90 ohm sending unit.

These photos show how the gauge is wired to the sender and I use a 10 amp batt charger for power.

The ground can be shared between the sender and the gauge.

I use a yellow jumper as the "tan" wire that runs from sender to fuse panel and then to cluster.

I use green lead from my batt charger for the pink power wire from the ignition switch through the fuse panel and to the circuit board in location number 3 on the connector.

It is really this simple. If you can do this, you can test your fuel gauge out of the cluster.

[TBONE1964]

This photo shows the ohm reading (NO POWER / IGN OFF) across the resistor when installed. 42.7 ohms. It will be in the area of 85 ohms when the resistor is loose from the gauge.