67-72 Chevy Truck fuel gauge diagnostics 101

[TBONE1964]

Originally Posted by Fasteddieb

i got mine at Radio Shack. I may have one lying around. If you want it let me know. It'll cost you a stamp!

I believe he is referring to the stock porcelain type resistors.

You can use a 100 ohm resistor from Radio Shack but the stock style are much easier to install and there are plenty out there available.

Thanks for your in put on the subject,

Take care,
Tom

[RWHSVAR]

This is great information. I have been fighting with the fuel gauge on my 72 C10 for 2 months and getting nowhere. I replaced the tank sending unit which did not help. I just did the diagnostics in your post and got the following readings.

Picture 1 - 44.4 ohms I read 23.4 ohms
Picture 2 - 98.2 ohms I read 23.3 ohms
Picture 3 - 142.8 ohms I read 2.2 ohms

My best guess is a bad resistor. Do you agree? If so, where can I buy a new resistor. If I'm wrong, please advise next steps.

Thanks so much.

[vics stuff]

I also have found that time to time the resistors fail.
With what you indicate I would believe your problem is in the resistor.

.
They are available on this site.
Good luck.
Vic

[RWHSVAR]

Thanks Vic,
I'll see if I can find the resistor. You said they are available on this site. Are you talking about 67-72chevytrucks.com? I don't see anywhere to buy them.

Thanks again.
Ron

[RWHSVAR]

Does anyone have a source for the fuel gauge resistor on a 72 C`10?
Thanks,
Ron

[TBONE1964]

Originally Posted by RWHSVAR

Does anyone have a source for the fuel gauge resistor on a 72 C`10?
Thanks,
Ron

Hi Ron,

Send me a PM with your address information and I will send you a good resistor.

Typically the main problem when the resistor goes bad is you will never register more than 1/4 tank no matter how much fuel you have.

I don't see what photos you are refering to but the stock fuel gauge resistors will read around 90-100 ohms. The 23.3 ohms you have would only get your gauge up to 1/4 tank based on 90 ohms at full, 45 ohms at half and 22.5 ohms at 1/4 tank.

Hope this helps, let me know,

Tom

[Sparkys66]

Hi guys, this is my first post and the fuel gauge in my blazer is driving me nuts.

I purchased a new gauge and sender for the blazer and did all the testing shown on page 1 of this post. Everything was spot on with the readings so I decided to go ahead and hook everything up while on my bench. at .8 ohm's the tank read empty. as the ohm's increased the needle on the gauge started to move, but once I raised the float arm to 1/4 or 22.5 ohm's the needle was already on full. If I lift the float arm anymore than 1/4 tank the needle continues past full. The sender reads true and so does the gauge. The other bummer is I had to install everything in the blazer so I could not only use the vehicle but know when to get gas. when the needle starts to drop below full, I find a gas station quick. any ideas would be great.

Mark

[67 Burb]

Originally Posted by Sparkys66

Hi guys, this is my first post and the fuel gauge in my blazer is driving me nuts.

I purchased a new gauge and sender for the blazer and did all the testing shown on page 1 of this post. Everything was spot on with the readings so I decided to go ahead and hook everything up while on my bench. at .8 ohm's the tank read empty. as the ohm's increased the needle on the gauge started to move, but once I raised the float arm to 1/4 or 22.5 ohm's the needle was already on full. If I lift the float arm anymore than 1/4 tank the needle continues past full. The sender reads true and so does the gauge. The other bummer is I had to install everything in the blazer so I could not only use the vehicle but know when to get gas. when the needle starts to drop below full, I find a gas station quick. any ideas would be great.

Mark

Hey, Mark, what is the resistance of the sender with the float all the way up? (As in full tank.) Is it around 90 Ohms?

[Sparkys66]

Hi Bob, yes float all the way up reads 90 ohm's.

[67 Burb]

That sender will work properly with a GM gauge. What gauge did you buy? It sounds like it is not a match with your sender.

[Sparkys66]

i purchased a stock replacement gauge for my 72 blazer.

Mark

[Sparkys66]

Tom, I purchased the new fuel gauge from Brothers. Part #GGE1001 nothing was removed to my knowledge. I will try removing the resistor and let you know.

Mark

[TBONE1964]

Originally Posted by Sparkys66

Tom, I purchased the new fuel gauge from Brothers. Part #GGE1001

2 thoughts (ok maybe more than 2),

Contact brothers and ask if it requires an external resistor or if it is built into the gauge.

I can sell you a decent used fuel gauge for $25.00 shipped that would be tested and working. Or I would check with Brothers and see if they can send you a new one. These are not complicated. They need battery voltage, ground and a signal from the fuel sender to work. It sounds like you have done your home work and possibly just got a bad gauge from Brothers.

Hope this helps.

[Sparkys66]

Thank you Tom, removed the resistor and the gauge now reads 1/4 with 5 gallons of gas in the tank. i will go top off the tank later today and see what happens.

Mark

[Sparkys66]

Thank you Bob for taking time out of your day to help with this issue.

Mark

[67 Burb]

Not a problem, Mark. Glad to help. Let us know how it turns out.

[Sparkys66]

Works perfect now. gauge read 1/4, added 17 gallons, now reads full.

Mark

[TBONE1964]

Good stuff here. Glad We are able to help each other get through these gremlins.

Take care,
Tom

[jjzepplin]

Sure helped me out! Thank You cluster king!

[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?

[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]

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