67-72 Chevy Truck fuel gauge diagnostics 101

[67 Burb]

Originally Posted by Lancialonnie

This is a great write up however I am having a slightly different problem. I know my sender works as it is new and tested. My fuel gauge just moves erratically, it goes from being correct to way past full for a few minutes then back to correct and it may stay there or it may go down to empty then back to past full to correct again. There is no rhyme or reason to this, it just moves all over the place. is it a bad ground? Where should I ground the gauge to make sure I have a good ground?

How is the sender grounded? It sounds like it momentarily loses ground and the gauge goes past full, then it finds ground and reads correct. If it is using the tank for ground, that may be the problem. It needs a ground wire from the sender to the truck frame.

[Lancialonnie]

I have a good ground on the sender so I believe the problem is a bad ground on the gauge. I'm just not sure how the gauge should be grounded, from one of gauge terminals?

[VetteVet]

Originally Posted by Lancialonnie

I have a good ground on the sender so I believe the problem is a bad ground on the gauge. I'm just not sure how the gauge should be grounded, from one of gauge terminals?

The gauge is grounded to the cluster with the lower terminal on the gauge. If it was bad the gauge would give a non reading rather than what you are seeing. Concentrate on what Burb is saying and check the tan wire connection on the fuse panel marked FUEL. The other possibility is the no. 4 terminal on the cluster plug connection. It is the return path to ground for the gauge.

[TBONE1964]

Originally Posted by Lancialonnie

This is a great write up however I am having a slightly different problem. I know my sender works as it is new and tested. My fuel gauge just moves erratically, it goes from being correct to way past full for a few minutes then back to correct and it may stay there or it may go down to empty then back to past full to correct again. There is no rhyme or reason to this, it just moves all over the place. is it a bad ground? Where should I ground the gauge to make sure I have a good ground?

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

[Benjamin]

Subscribing

[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

[CarpBlazer]

Thanks Tom, looks like I’m on the hunt for a short to ground somewhere, I’ll report back once I find out where it is..hoping it will fix this fuel gauge stuck on empty situation.
Ben

[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.

[Hoodogie]

I pulled my gauge cluster out during my resto and I replaced the circuit strip thing on the back because my temp gauge wasn't working. When I put it back together the gas gauge was at 3'oclock and before that it worked fine. I found out by reaching up behind the dash that I forgot to tighten the two nuts that hold the circuit strip to the gas gauge. As I was tightening the nuts the gauge move to 3/4 full and did not move so I attached a ground wire to the bottom terminal of the gauge and went down and put some gas in it then the gauge move to a bit over full. I did drive it a little bit the gauge is now at the full mark so it did move just not sure if its accurate yet. going to have to drive it a little more and carry a gallon with me.

[TBONE1964]

Originally Posted by 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.

Lets try and make it simple.

First using a test light, check for power right at the fuel gauge with the key in the run position. Is there power at the gauge? Or prefered use a meter and see what the actual voltage is. It should be 12.6 or so with just key on and engine not running. if you have 12.6 that means the circuit board is OK.

If you do not have 12.6, check the gauge fuse. The #3 pink wire at the connector is the gauge power and it powers both the fuel gauge and temp gaughe. If the circuit board touchs ground when the key is on, that will blow the fuse. You need to protect that terminal on the circuit board or use a stock temp gauge and connect it as designed. Not sure this is an issue but you should check.

You can also run a wire from the sending unit to the the back of the gauge. This will by pass the path of the tan wire. If this makes the fuel gauge work, the is a problem between the signal terminal on the gauge and the terminal at the sender for the signal wire.

Let me know if this helps you. It is easy to check your tank and sending unit for ground. Run a test light from the positive side of your battery and touch it to the sending unit. If test light lights, you have good ground at sending unit.

Tom

[Shotgunn]

i have 11.8 volts on the top post of the gauge. seems low because my battery is 12.4 at the battery. all my lights in the dash work including the dimmer switch for the headlights and background light dimming.
If i have 92 ohms at the brown wire plug before i plug into the fuse block i shouldnt have to do any further checks on the sending unit..92 ohms is the correct reading for a fuel tank right?

[Shotgunn]

it must be a bad gauge. i ran a jumper wire from the brown wire plug ( the one that plugs into the fuse block ) to the signal post on the gauge and the needle came back up to the 3 o'clock position. i noticed when i unplugged the brown wire clip from the fuse block, the needle took a nose dive to the 530 position.

[Shotgunn]

Should I try a new gauge?

[Shotgunn]

I guess I’m being ignored