Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

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

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

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?

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

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.

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

Should I try a new gauge?

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

I guess I’m being ignored

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

Not ignoring you, just very busy and way behind.

Based on your diagnostics and what I can tell by reading your post, I would try a new gauge. Remember some new aftermarket gauges do not require a external resistor to work.

Hope this helps,
Tom

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

You may need a new gauge, but before doing that, I would test the gauge itself. Here's what I use for testing a 90 ohm fuel gauge.

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

Thank you both for responding.

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

Trying to finish a blazer tank swap on a truck I just bought. They left the tank swap about halfway done. My sending unit has the ground wire coming off it (I attached the other end of that to a tank bolt). I don’t have a wire connected to the center post/bolt on the sending unit. What wire should I be looking for? Tan wire that would have ran to the tank behind seat??

I haven’t looked yet, hopefully it’s just back there and I can add some wire to it to extend to the back.

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

Yes it will be the same tan wire and you can extend it or just run a new wire. It runs from a location on the fuse panel named "FUEL GA"

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

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.

Attachment 1778496

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.

Attachment 1778497

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

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.

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

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

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

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.

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

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

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

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

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

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.

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

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

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

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

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

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

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

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.

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

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.

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

This photo shows the voltage in the sender line. Because of resistance, you will never see 12 volts here. The photo shows 6.63 volts at full tank and will show about 1 volt when at 1/4 tank or so. Voltage drops with float movement.

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

Any questions on above? Please share your issues and lets try and help get you guys happy and not frustrated with fuel gauge issues.

A further note, If you have opportunity to compare your gauge with another used/new one, that helps you understand what the numbers should be good vs bad.

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

Here's what I use for a quick check of a fuel gauge. As a replacement for a sending unit, three 33 ohm resistors in series giving 33, 66 and 99 ohms for 1/3, 2/3 and full, depending where you connect the jumper. hooked up as in the photo, the gauge should read 2/3 of a tank just under 3/4 full.

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

Great read!!

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

Hey guys, I’m stumped with my fuel gauge. Here’s my story. The fuel gauge never really worked since I got my 72 so I decided to change the sender. After install it stayed at 1/4 tank. After searching here I determined it had to be the resistor, so I checked the ohms on the resistor and nothing. Ok, so now I order a resistor and new printed circuit from truck shop. I then installed the new printed circuit and the resistor. It read 3/4 tank so I drove to fill up and it stayed at 3/4. So I take the cluster out again and thought well maybe I put the resistor on wrong because I had the circuit under it. I changed it so the circuit is on top of the resistor. Now it reads 1/4 again, but I get 86 ohms on the resistor, so it should be good. The gauge ohms are very similar to the ones in this thread and I get 90 ohms on the #4 wire and 14v on the #3 wire (on the tender). I have already verified that the sender ohms moves from 0 empty to 90 full. Also, the gauge test with the wire off the sender goes to around 2 o’clock and grounded goes to empty. Is the copper on the resistor supposed to touch the copper on the circuit? Any ideas as to what may be my problem? Your help would be greatly appreciated. Thanks.

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

More info. With the gauge outside of the cluster and hooked up, it works. I’m thinking it must be something with how I have the resistor on or the wiring coming into the cluster. Does anyone know what the readings should be on each wire at the plug?

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

Seeing how you have already verified that the sender ohms moves from 0 empty to 90 full, test the gauge on the bench like I have done. I use three 33 Ohm resistors soldered together as in photo. Apply 12V + to the IGN post on the gauge and 12V - to the GROUND post of the gauge.

Attach one end of a wire to the SENDER post on the gauge and attach the other end to the resistors at point "A" on the resistors. Attach a wire to the gauge ground post and touch the other end to points A, 1, 2 and 3 on the resistor string and if the gauge is working properly, it should read A - empty, 1 - 1/3 tank, 2 - 2/3 tank, 3- full tank.

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

OK. I missed your last post as I was posting mine. Now you know that the sender is OK and the gauge works out of the dash unit, make sure the gauge cluster is well grounded.

Where did you test the sender from? At the sender or from under the dash?

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

Also, feed wire at plug is 90 ohm, but once plugged in it goes to 36 ohms.

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

I have tested the sender both in the tank and out. Ohms moved as it should while moving the float up and down. The tank is full and reading 90 ohms. At the dash plug and tank both read 90 before plugging up to the cluster

It’s weird. Seems to work until I connect the printed circuit

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

Yes, once you plug it in, you are introducing more circuitry into the ohm measurement.

Run a wire from the ground stud on the back of the gauge to a good ground.

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

The one I have the black jumper on.

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

Tried grounding stud with a wire and no change while harness plugged in

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

I'm stumped. All seems to be OK, but still it doesn't work. :banghead:

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

Here is a great YouTube explanation on fuel gauges I thought I would share. It covers the resistors and such. https://youtu.be/bVT2Jh_p02A

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

I just wanted to thank Vettevet and Tbone for this great thread. I had to replace my gauge resistor but thanks to this thread I was able to get my gauge working again. You guys are awesome.

Re: 67-72 Chevy Truck fuel gauge diagnostics 101
 

Glad the information provided has helped you. It's always a pleasure to help .