67-72 Chevy Truck fuel gauge diagnostics 101

[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

[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

[jjzepplin]

I get 66 ohms left to right pole when big harness plug is removed.
87 ohms right to middle and
74 ohms left to middle.

[jjzepplin]

Is this the resistor?

[TBONE1964]

Yes, that is the resistor. There should be a brown colored insulator that goes behind it.

The ohm reading you posted don't look right to me. I am not at my shop or I would test a few that I have in my inventory. As it sets, the resistor should show a reading of around 100 ohms. That is if it is loose from the gauge.

[jjzepplin]

It shows 88 ohms when removed. This is the test gauge though.
I have three fuel gauges and all test with different values-boo!
The closest to your original post is the test unit this resistor is from.
I am thinking my gauge is screwed up and my sender is bum as well.

[Justintime67]

Great write up!! Could have used this a couple months back.

[TBONE1964]

Originally Posted by jjzepplin

It shows 88 ohms when removed. This is the test gauge though.
I have three fuel gauges and all test with different values-boo!
The closest to your original post is the test unit this resistor is from.
I am thinking my gauge is screwed up and my sender is bum as well.

If you can, isolate the sender first with the instructions I gave. Then you will know for sure if you have a problem with one or both components

Keep us posted,
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.

[jjzepplin]

Well I guess I am going to have to pull the monster 35 gallon tank out. Sum b--ch! I will pump out the remaining fuel with an electric pump and test the sender. I think it is an Aero tech tank. Anyone know where to find a sender if I need one?

[TBONE1964]

Originally Posted by jjzepplin

Is this the resistor?

I just checked 43 of these resistors I have in my inventory and I get between 96 and 108 ohms on all of them. These resistors will/should have the same ohm value as the porcelain later style resistors. Also, the color code does not seem to have any difference in the value.

Hope you all are having a good holiday weekend.
Take care,
Tom

[TBONE1964]

Originally Posted by jjzepplin

Well I guess I am going to have to pull the monster 35 gallon tank out. Sum b--ch! I will pump out the remaining fuel with an electric pump and test the sender. I think it is an Aero tech tank. Anyone know where to find a sender if I need one?

Hey JJ,

Any word? Were you unable to check the sender in the tank by filling it up at the gas station while watching the meter? I am curious on how this turns out for you.

No clue on where to get a sender for that style of tank but make sure it is a 0 to 90 ohm range.

Have a good holiday,
Tom

[jjzepplin]

I haven't got the balls to do it yet, and won't have time off til Friday.
I will also check the gauge resistor on the truck a the same time.
I will also check the cluster going into "No Daggum Money" this way BEFORE I install it.

[TBONE1964]

Originally Posted by jjzepplin

I haven't got the balls to do it yet, and won't have time off til Friday.
I will also check the gauge resistor on the truck a the same time.
I will also check the cluster going into "No Daggum Money" this way BEFORE I install it.

I know that feeling. You just got to jump in with both feet and go for it. When you have the time that is. Remember, I am here to help you if you need it.

Have a good holiday
Tom

[jjzepplin]

You have already been a fantastic resource and I appreciate all you do. I WILL jump in and go for it and get this sucker fixed. It's only been 10 years of no gauge. It was a long drive from Cali to Fl. wondering if I was gonna run out of gas.

[jjzepplin]

OK. I installed my 4x4 front sway bar. WHEW- What a mutha! I am about to get back on this gauge. If I need a replacement sender for the Areotach tank.Where would I find one? I think it is the 5 screw type IIRC.

[TBONE1964]

Originally Posted by jjzepplin

OK. I installed my 4x4 front sway bar. WHEW- What a mutha! I am about to get back on this gauge. If I need a replacement sender for the Areotach tank.Where would I find one? I think it is the 5 screw type IIRC.

Hey JJ,

I am not sure about the sender for the aftermarket tanks. You may want to contact the company of the tank. Maybe someone else will chime in or you can post the question in a new thread on the general forum. I have absolutely no experience with these style tanks but I do know that it will have to have a sender with a 0 - 90 ohm range for the factory gauge to work.

Hope this helps and good luck.

Take care,
Tom

[jjzepplin]

This is the sender I found. What do you think?
http://www.summitracing.com/parts/sw...P4EaAjJX8P8HAQ

[TBONE1964]

Originally Posted by jjzepplin

This is the sender I found. What do you think?
http://www.summitracing.com/parts/sw...P4EaAjJX8P8HAQ

Looks good and it is a Stewart Warner, which I like. As long as the "10-15 inches" for tank depth is good for you and it fits, I say good to go. Good price also. Maybe get another opinion on this as well.

Keep me posted,

Tom

[67 Burb]

And the resistance is correct (0 - 90 Ohms). That is VERY important. Looks like it will do the job.

[jjzepplin]

Ordered it up. Stay tuned. I will test the gauge prior to installing the sender. Mostly to stall off from pulling the 38 gallon tank. BWAHAHAH! Meanwhile I got my Blazer tub on the frame - UGH. I will use your info to re-pin the harness to a gauge type dash. Hoping to get a tach in that one soon but oh well. I have the outer tin and printed circuit. Kinda like like my Caddy-one piece at a time.

[jjzepplin]

OK so I go after this mutha with a vengeance and tested the new sender with my old gauge and boom she works fine. Drop the empty tank and pull the old unit. Then it hits me-new one does not have a pickup for the fuel! Sumbugger! So I modify the old pickup to use the new sender. Took a pencil torch and soldered it on. Tested and boom still working good. Then it hits me-again this whole apparatus needs to fit thru a small arse hole. Aint happenin the way I have it. So rebend the pickup and float and bend this and that and finally it goes in. I think It works as I put 2.5 gallons from a can in it and the needle moved up. I think It may register full for a couple gallons before it registers any movement but it will tell me when it's empty. Full should be at 30 gallons.

[TBONE1964]

Originally Posted by jjzepplin

OK so I go after this mutha with a vengeance and tested the new sender with my old gauge and boom she works fine. Drop the empty tank and pull the old unit. Then it hits me-new one does not have a pickup for the fuel! Sumbugger! So I modify the old pickup to use the new sender. Took a pencil torch and soldered it on. Tested and boom still working good. Then it hits me-again this whole apparatus needs to fit thru a small arse hole. Aint happenin the way I have it. So rebend the pickup and float and bend this and that and finally it goes in. I think It works as I put 2.5 gallons from a can in it and the needle moved up. I think It may register full for a couple gallons before it registers any movement but it will tell me when it's empty. Full should be at 30 gallons.

Sounds like you got it. I will be curious to know how it works when you fill it and as it runs back down to empty.

Good work JJ,

Take care,
Tom