67-72 Chevy Truck fuel gauge diagnostics 101

[wayno]

fantastic write-up TBone.. now if I can get you to do a similar write up for TACHs lol

[jjzepplin]

Oh yeah! I'm in for that too!

[TBONE1964]

I am slowly working on getting the tools together to help me diagnose the factory tachs and figure out a way to repair them. I can currently fine tune the calibration and check function but no repair yet.

I have heard about some aftermarket kits that replace all of the insides to a modern style tachometer but allow the tach to look original.

I am always looking for ways to help people out with these things but limited to what I can invest.

I will always keep everyone updated with new developements.

Take care and have a good weekend,
Tom

[jjzepplin]

So I went and filled it up today. Bad news. Reads just over 1/4 tank. More than before but dagnabit! And as an added bonus now it burps gas upon fillup again for some stupid reason. SHAZAM! On the other hand the new front sway bar makes it ride fantastic. Think a rear is on the way.
I did not do any testing yet but will keep you all posted.

[TBONE1964]

Originally Posted by jjzepplin

So I went and filled it up today. Bad news. Reads just over 1/4 tank. More than before but dagnabit! And as an added bonus now it burps gas upon fillup again for some stupid reason. SHAZAM! On the other hand the new front sway bar makes it ride fantastic. Think a rear is on the way.
I did not do any testing yet but will keep you all posted.

Man, you sure are having the luck with this. I would bet the fill hose is kinked somewhere, maybe?? That would cause burping problem.

Do you have another fuel or temp gauge laying around? If so, change the resistor with your current one on the back of the fuel gauge. If you do not have one, PM me your address and I will ship you one. A bad resistor can cause the 1/4 tank issue. If you can, ohm check the resistor removed from the fuel gauge. It should be around 100 ohms give or take a few. Don't worry about the color code on them. I have all different colors of these, red, orange, yellow and blue and they all ohm check around 100 ohms.

If I don't hear back from you, have a great holiday. Keep me posted,

Take care,
Tom

[jjzepplin]

Have a great holiday yourself sir. And I will as well. I checked the gauge against the sender before installing and all checked out. I am thinking there is some sort of baffle in the tank that is interfering with the float or something. ARGH! AHJKVP*OTUDYHVBO{U"FJLGBULJF!!!!!!

[undertaker72]

tom, excellent tutorial thanks for your time in posting , and the others that have chimed in , this is what make this site better than the books super job

[TBONE1964]

Originally Posted by undertaker72

tom, excellent tutorial thanks for your time in posting , and the others that have chimed in , this is what make this side better than the books super job

Thank you very much Kevin. It is my pleasure to be able to help someone diagnose there issues. Sometimes these things can throw us for a loop and we need to break it down and diagnose it, piece by piece. Sometimes it takes a lot of patients but I believe most of us have the ability to fix there gauge issues with some pointers.

I also appreciate all who chime in with there own experience and yes, it is not always reading the books that we get the most help from. I learn a lot myself from the members on this forum and that is one of the things I love about being a member.

Take care and have a great holiday,
Tom

[jjzepplin]

Originally Posted by jjzepplin

So I went and filled it up today. Bad news. Reads just over 1/4 tank. More than before but dagnabit! And as an added bonus now it burps gas upon fillup again for some stupid reason. SHAZAM! On the other hand the new front sway bar makes it ride fantastic. Think a rear is on the way.
I did not do any testing yet but will keep you all posted.

Allrigty. Went out and tested the sender today to find it is reading 90 ohms-hmm. Went inside the cab and pull the sender wire and it reads 90 ohms- hmm. Pulled what I thought was the ground clip for the gauges and nothing happened to the gauge.
Is there more than one ground clip?
This gauge and sender tested fine out of the tank. Why would it be any different inside the tank?
The only difference I can find is the tank sender ground was a jumper wire.
I have a ground wire on the tank that previously was not hooked up but it is now.
Sound like a ground issue?

[TBONE1964]

Originally Posted by jjzepplin

Allrigty. Went out and tested the sender today to find it is reading 90 ohms-hmm. Went inside the cab and pull the sender wire and it reads 90 ohms- hmm. Pulled what I thought was the ground clip for the gauges and nothing happened to the gauge.
Is there more than one ground clip?
This gauge and sender tested fine out of the tank. Why would it be any different inside the tank?
The only difference I can find is the tank sender ground was a jumper wire.
I have a ground wire on the tank that previously was not hooked up but it is now.
Sound like a ground issue?

JJ,

The tank is full? If you are reading 90 ohms testing the sender, that would tell me the tank is full and the float is up all the way. If you were reading 22.5 ohms, then I would say the float is stuck at 1/4 tank or the sender is not working correctly

The little ground clip for the cluster is just an extra ground as a back up. The ground for the cluster is in the connector I believe with out looking it up is in the number 7 location. It is a black wire.

If you have a ground issue at the cluster, the temp gauge and/or lights would not work correctly. They all share a common ground.

What will happen now if you hook your jumper ground back up? Try it and see what happens. Are you still reading 1/4 tank? That would indicate to me that your ground is good. If you pull ground, the gauge should roll over to 3 o clock.

Keep us posted and have a good holiday,
Tom

[jjzepplin]

The tank is indeed full.
The ground clip removed does not change the gauge reading.
Maybe pull the big cluster plug and jiggle around?
I am going to try the jumper wire at the tank.
Is that what you mean?
I tried the ground clip to a known ground and nothing.

[TBONE1964]

Originally Posted by jjzepplin

The tank is indeed full.
The ground clip removed does not change the gauge reading.
Maybe pull the big cluster plug and jiggle around?
I am going to try the jumper wire at the tank.
Is that what you mean?
I tried the ground clip to a known ground and nothing.

Are you talking about the ground clip near the cluster? It is always good to have it hooked up but I don't think it is related to your problem.

Yes, the jumper ground at the tank but like I said, If you have a bad ground there, that will make the gauge read past full.

It sounds like you have confirmed the sender is good and the test worked.

Do you have another loose fuel gauge with the resistor on it? If you do, try to give it 12v and a ground and unplug the tan wire from the fuse panel and hook it directly to your other gauge and see what that gauge does. This is going to tell you if the gauge resistor is keeping the gauge from reading past 1/4 tank. Let me know if you need help figuring out where to hook the jumpers on the back of the gauge.

I know this is a PITA but think you have/had multiple problems with your fuel gauge not reading correctly.

[jjzepplin]

I put a jumper on the tank ground and tested it with the meter to make sure it was grounded and still no change.
I am thinking it is a ground issue somewhere though.
Am I right?

[jjzepplin]

Tested two unknown gauges with jumpers and both read full.
Cluster in truck might be the issue.
I pulled the main cluster plug and put jumpers on the gauge in the cluster and same reading as before. Not full.
Must be the gauge even though it tested good right before I replaced the sender.
Sumbeotch.

[TBONE1964]

Originally Posted by jjzepplin

Tested two unknown gauges with jumpers and both read full.
Cluster in truck might be the issue.
I pulled the main cluster plug and put jumpers on the gauge in the cluster and same reading as before. Not full.
Must be the gauge even though it tested good right before I replaced the sender.
Sumbeotch.

I feel a celebration coming soon. Although you will need to pull the cluster again. While it is out, take a good close look at the circuit board as well and also where the connector plugs in. Make sure there are no burn spots and the tabs where the connector plug in are all in place. You can use super glue on those if they are loose.

I will be waiting for a big YEEEEEEEHA

[jjzepplin]

Ok so I pulled out the cluster and checked the gauge removed. Tested perfect. Right up to full and no further.
Swapped out the printed circuit and checked again.
Right to the full mark and no further.
Cleaned all the harness contacts and plugged in.
Checked out good.
Almost YEEHHAAA!
Installed completely and checked. Goes over full (further than ever before).
Not much maybe 1/8 tank or so.
I guess I will drive around with a gas can till I can confirm
the levels. I know it is way full right now and has maybe 3-5 gallons before it should start registering anything on the gauge.
But it is NOT at the lower end of the gauge which is where it has spent most of it's years.
If I disconnect the sender wire it acts as it should.
Thank You so much and I hope this helps someone.
Will keep you all posted with long term results.
Happy Thanksgiving!

[TBONE1964]

Originally Posted by jjzepplin

Ok so I pulled out the cluster and checked the gauge removed. Tested perfect. Right up to full and no further.
Swapped out the printed circuit and checked again.
Right to the full mark and no further.
Cleaned all the harness contacts and plugged in.
Checked out good.
Almost YEEHHAAA!
Installed completely and checked. Goes over full (further than ever before).
Not much maybe 1/8 tank or so.
I guess I will drive around with a gas can till I can confirm
the levels. I know it is way full right now and has maybe 3-5 gallons before it should start registering anything on the gauge.
But it is NOT at the lower end of the gauge which is where it has spent most of it's years.
If I disconnect the sender wire it acts as it should.
Thank You so much and I hope this helps someone.
Will keep you all posted with long term results.
Happy Thanksgiving!

Sounds like the chapter is going to end soon. Go have some Turkey and relax.

You have a good Thanksgiving as well JJ,

Take care,
Tom

[jfultz1971]

Where can you buy the resistor? My sons truck is a 72 c20. All new complete wire harness and fuel gauge and sending unit complete.

[TBONE1964]

Originally Posted by jfultz1971

Where can you buy the resistor? My sons truck is a 72 c20. All new complete wire harness and fuel gauge and sending unit complete.

I can help you with your needs. I will send you a PM (private message) after the holiday and we can discuss exactly what you need.

Have a good holiday,
Tom

[jfultz1971]

Ok, sounds good. Thanks, happy gobble gobble.

[jfultz1971]

Never got a pm from you, on resistors.

[TBONE1964]

Originally Posted by jfultz1971

Never got a pm from you, on resistors.

My apologies. PM coming right up.

Thank you,
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!!!

[jjzepplin]

Originally Posted by jjzepplin

Ok so I pulled out the cluster and checked the gauge removed. Tested perfect. Right up to full and no further.
Swapped out the printed circuit and checked again.
Right to the full mark and no further.
Cleaned all the harness contacts and plugged in.
Checked out good.
Almost YEEHHAAA!
Installed completely and checked. Goes over full (further than ever before).
Not much maybe 1/8 tank or so.
I guess I will drive around with a gas can till I can confirm
the levels. I know it is way full right now and has maybe 3-5 gallons before it should start registering anything on the gauge.
But it is NOT at the lower end of the gauge which is where it has spent most of it's years.
If I disconnect the sender wire it acts as it should.
Thank You so much and I hope this helps someone.
Will keep you all posted with long term results.
Happy Thanksgiving!

Update-so far gauge is functioning. Now on to that tachometer since you were so nice s to furnish some much needed parts. Thank You!

[zicc1835]

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