67-72 Chevy Truck fuel gauge diagnostics 101

[67swb72klb]

great write up thanks for taking the time to do it pics and all

[jjzepplin]

10-4. Will test the gauge as above.
As for the tank...I am not sure how much is in it so maybe I can test ohm reading after trips as I run the level down. I carry a Jerry can with fuel just for kicks and if I see stranded people.
I am almost a road ranger of sorts. I suspect the sender as it is most likely aftermarket.
I do have a known good unit that I have repaired in my blazer tank that is easily accessible.
Wasn't looking to get into this today but saw this thread and thought what the heck, everything else is working and this has been this way for 10 or so years. May as well do some checking.

[TBONE1964]

Originally Posted by jjzepplin

10-4. Will test the gauge as above.
As for the tank...I am not sure how much is in it so maybe I can test ohm reading after trips as I run the level down. I carry a Jerry can with fuel just for kicks and if I see stranded people.
I am almost a road ranger of sorts. I suspect the sender as it is most likely aftermarket.
I do have a known good unit that I have repaired in my blazer tank that is easily accessible.
Wasn't looking to get into this today but saw this thread and thought what the heck, everything else is working and this has been this way for 10 or so years. May as well do some checking.

Sounds good JJ,

keep us posted on what you find out.

Take care,
Tom

[jjzepplin]

I went and tested the gauge.When I pull the brown wire from the fuse block the gauge reads just past the last "L" in full. Not 3 oclock.
When I ground the fuse block connection it goes to empty. When I turn the key off it slowly creeps back up.
Sender is reading 10 ohms.
I did test a spare gauge and it reads 38 ohms left pole to right pole. (just for reference)
So would the next step be to measure the gauge in the truck?
Think I will try that while waiting for response.
Thank You fellas for all you do.

[TBONE1964]

Originally Posted by jjzepplin

I went and tested the gauge.When I pull the brown wire from the fuse block the gauge reads just past the last "L" in full. Not 3 oclock.
When I ground the fuse block connection it goes to empty. When I turn the key off it slowly creeps back up.
Sender is reading 10 ohms.
I did test a spare gauge and it reads 38 ohms left pole to right pole. (just for reference)
So would the next step be to measure the gauge in the truck?
Think I will try that while waiting for response.
thank You fellas for all you do.

"I went and tested the gauge.When I pull the brown wire from the fuse block the gauge reads just past the last "L" in full. Not 3 oclock."

That is strange and would make me think there is something wrong wiith the gauge resistor or the gauge itself.

"Sender is reading 10 ohms."

You are referring to sending unit in the tank, correct? If it is accurate, I would believe you have or should have almost an 1/8 of a tank of fuel.

"I did test a spare gauge and it reads 38 ohms left pole to right pole. (just for reference)"

Did you do this to the gauge in the dash as well? If not, try it but unplug the connector from the cluster so there is no back feed.

Sorry but I have to log off and get to the Airport for El Paso. I will check back on this soon.

We are going to figure this out and get your gauge working properly

Take care,
Tom

[Fasteddieb]

Best fuel gauge explanation anywhere. Just got a 69 c10 and it had fuel gauge issues. Turn the key on and the needle went straight to full - and past! Bed mounted fuel cell made things a LOT easier. Took out the sender and it read way over the ohm range. problem is, there was an aftermarket fuel gauge in the dash. Did the PO get the 0 to 90 ohm version? I dunno!
Then I had a brain flash. Went to Radio Shack and purchased a 47 ohm resistor. Hooked it up between the sending wire and ground and turned on the key. Needle went right to 1/2 tank!
Ordered a 0 to 90 ohm sender yesterday. I'll have a working fuel gauge by the weekend!
Thanks guys!
Now I need to work on my wiper motor. It's always something!!!

[TBONE1964]

Originally Posted by 67swb72klb

great write up thanks for taking the time to do it pics and all

Thank you. We are just trying to help these guys/gals find and isolate a problem with the fuel gauge/sender to prevent just changing parts and not getting an expected result.

My hope is that this (with the help of Vette) will help diagnose problems that are often very simple to repair as long as you know how to test them for function.

If I have never said it, Vette, I appreciate you sir for adding your knowledge to this thread. I will soon do a separate thread for temp gauge diagnostics as they are often brought up in discussion on this forum.

Have a good Sunday,
Tom

[67 Burb]

Originally Posted by TBONE1964

I will soon do a separate thread for temp gauge diagnostics as they are often brought up in discussion on this forum.

Tom

Sent you a PM.