Fuel Gauge at Full

[The Rocknrod]

1964 C10 - So I just put power to the truck, installed a battery. Put some fuel in it and noticed the fuel gauge is pegged at full.
I have the sender wire at the normal sender connection. But I also placed a ground wire in one of the sender hold down bolts. Is that why it pegged?

[Dead Parrot]

A fuel gauge stuck on full or past full is normally from a break in the circuit somewhere. Think broken wire = full, short to ground = empty.

A lot of senders have a tab for the ground wire. Might make sure the other end of the ground is really grounded. A painted frame or buildup of dirt/rust can ruin a ground connection.

[The Rocknrod]

I read (on my Fluke meter):
18 ohms at the sender. Using a known ground.
17.5 ohms at the gauge input. Using a known ground.
Gauge pegged over full. Unplug the gauge = still pegged out.
12v gauge power is there.
I'm thinking my gauge is bad? Thoughts?

[The Rocknrod]

Just to make sure, i ran a single ground wire from the sender ground (in pic above) to the ground bus. No change.
I have continuity on my gauge panel to ground.

[VetteVet]

Have you checked the sender terminal to ground flange for 0 to 90 ohms ?

Then you should check the sender wire and the gauge reading, with the sender
wire grounded to a known good ground, you should see empty as was stated.

[The Rocknrod]

Originally Posted by VetteVet

Have you checked the sender terminal to ground flange for 0 to 90 ohms ?
Then you should check the sender wire and the gauge reading with the sender wire grounded to a known good ground you should see empty as was stated.

First question I did as in post #3, maybe I didnt word it right but yes.
I will do #2 right now.
So sender wire grounded... Not sure what that means but I have a known ground bus I'm using. I will take a test lead from the back of the gauge at the sender input and the other end to ground. Right?

[The Rocknrod]

Okay i did the sender wire to ground. No change. THEN I unplugged the v-shapped two prong gauge connector (stock 64) turnned it around so the power wire was still plugged in but the sender wire was not. Then ran a test lead from the sender terminal (back of gauge) to ground. No change.

[The Rocknrod]

From another post of vettevette - "To test the gauge you should first use an ohmmeter and read between the two posts looking for 40 or so ohms. Then run a positive 12 volt wire to the plus side of the gauge and then a ground to the negative side and you should get empty. You have to be sure the gauge body and the cluster are grounded as well. I can't tell from the pictures what is on the circuit board.

Here are the resistance values of the gauges on the 67/72 trucks."
Except I have a 64.
So tomorrow I will try this. and post what ohms I get.

[VetteVet]

Actually you just need to disconnect the sending unit wire at the sender and ground it then with the key on you should see empty.
If you disconnect it and don't ground it you should see full plus about a 1/4 tank.

Measuring ohms , with no wires connected between the tank sender terminal and the tank flange mount you will read the resistance of the sending unit variable resistor which should be between 0 and 90 ohms.

[The Rocknrod]

30 ohms? For a 64?
Okay that will be easy to do, in the AM I will repot.

[The Rocknrod]

Originally Posted by The Rocknrod

From another post of vettevette - "To test the gauge you should first use an ohmmeter and read between the two posts looking for 40 or so ohms (I got 37.4 ohms). Then run a positive 12 volt wire to the plus side of the gauge and then a ground to the negative side and you should get empty. You have to be sure the gauge body and the cluster are grounded as well (they are so very grounded). I can't tell from the pictures what is on the circuit board.

Here are the resistance values of the gauges on the 67/72 trucks."
Except I have a 64.
So tomorrow I will try this. and post what ohms I get.

Okay so made the gauge swing the opposite way using the gauge terminals, test leads to power and ground. So it now reads empty.
Next I ran a ground lead outside the truck from the sender ground to a ground buss (known ground) directly.
I ran a test lead from the sender to the gauge terminal directly. And a power lead to the 12V terminal to a known 12V source. This was all separate from the installed harness.
Still stuck on empty, tank is half full. Although the gauge jumps a bit (just slightly) like it wants to work when it gets 12V.

[Dead Parrot]

If you have a 30 ohm sender, the reading on your ohm meter of 18 ohms seems about perfect for a half tank of gas. Pretty sure senders are linear in response. 0 = empty, 30 or 90 depending = full. Open circuit on a 73-87 gauge will cause the gauge to go past full to about a 3 o'clock reading.

Don't know when they switched from 30 to 90 ohm senders and gauges. The 73-87 family are 90. They are the ones I am most familiar with.

[REDROCKER652002]

This is gonna be a stupid question, but is the float intact? Reason I ask is my son's float had a small crack in it, so that was a small part of the overall problem. Also, his was a grounding issue I think. But I would check to make sure the float is not cracked and filling up with fuel. If I am out of line, I apologize. Good luck

[The Rocknrod]

Originally Posted by Dead Parrot

If you have a 30 ohm sender, the reading on your ohm meter of 18 ohms seems about perfect for a half tank of gas. Pretty sure senders are linear in response. 0 = empty, 30 or 90 depending = full. Open circuit on a 73-87 gauge will cause the gauge to go past full to about a 3 o'clock reading.

Don't know when they switched from 30 to 90 ohm senders and gauges. The 73-87 family are 90. They are the ones I am most familiar with.

Thats what I thought also but it still doesnt work. Also see post #11 in Red. I use an ohmmeter and read between the two posts on the back of the gauge (nothing else hooked to them) I got 37.4 ohms.

[The Rocknrod]

Originally Posted by REDROCKER652002

This is gonna be a stupid question, but is the float intact? Reason I ask is my son's float had a small crack in it, so that was a small part of the overall problem. Also, his was a grounding issue I think. But I would check to make sure the float is not cracked and filling up with fuel. If I am out of line, I apologize. Good luck

It's a new unit. But that doesn't mean anything right? I may have to drop the tank and look. I thought about that today and cringed.
But on post #3 I got 18 ohms at the sender, in my mind that should be a tad over half a tank. Which is what I have.
And besides that it was pegged out at full until I fooled with it (posy 11).

[Dead Parrot]

From post #3 where you said you had 17.5 ohms at the input wire. If that was the dash input from the tank sender with the wire connected to the sender and the sender grounded, seems that the tank sender and related wire are working as intended. Might test by adding some gas to the tank and see if the ohms reading goes up. 3/4 of a tank should read about 24 ohms on a 30 ohm system, assuming a linear response. I wouldn't fill the tank all the way so there is a noticeable difference between an open circuit(max full or over full) and a partially filled reading.

Instead of adding gas, if siphoning is easy, pull out half of what is in there and see if the ohms fall roughly in line with the amount of gas drained.

If the ohms change as expected, I would concentrate on the gauge and related dash conections as the prime suspects.

[The Rocknrod]

I can do that easy enough. I will report back.

[The Rocknrod]

My only fuel tank had gas in it. I poured it in. Now i'm reading 24.6 ohm across the two connections at the gauge with them being pluged into the gauge. Means nothing.

[The Rocknrod]

Okay this am I read 24 ohms at the tank 24 ohms at the gauge. That's with the extra 5 gallons in it.
Had 18 ohms less 5 gallons in it.
Gauge still doesn't move off empty.
I have 12V key on to the gauge.
I have verified grounds to the gauge and the sender at the tank.
And remember in post #11, I ran independent test leads to all points without using the onboard wire harness just to verify the same readings.
I dont know if it means anything but my Tachometer works, My clock works, my temperature gauge works, my voltage gauge works, I have gauge panel lights, wiper motor works... and it wouldn't if I didn't have grounds.

[Dead Parrot]

Since the ohms went up with more gas and the ohms reading is the same at the sender post and the dash wire, IMO your sender and frame wire from sender to dash are good.

Seems to point to a bad fuel gauge or bad dash connections for the fuel gauge. Especially since the other dash gauges are working.

[The Rocknrod]

Well the ohms change at the connections to the gauge so...

[The Rocknrod]

Okay so I removed the sender from the tank. I grounded the senders mounting flange and ran a test lead from the sender to the fuel gauge sender input. Turned on the key.
Gauge went direct to max Full (over-full).
The must have a break in the wire someplace doesn't hold true here. Bad gauge or not?
I will swap the leads to the fuel gauge and report.

[The Rocknrod]

Just in case anyone wants to know For Sure. Looking at the back of the fuel gauge (my 64 C10) from the rear, the sender wire is on the left the 12V wire goes on the right.
I forget who I purchased the gauge from but they were swapped.
If you have continuity everywhere as I have, it will then work correctly.
Made it easier with the sender in hand in the cab running test leads and all to find the issue.
Latter, just to make sure I ran test leads from the sender in hand back to the tank area thru the wires on the truck harness to ensure it works properly.

[VetteVet]

After re-reading your post I saw that the sender was 30 ohms. I thought the last year was 63 for the 30 ohm senders. They work about the same however. Also after reading the thread again I thought that the grounding tests were good and it occurred to me after seeing your post 11 test, that the gauge stays where it is when power is removed. By using a separate power source you were able to make the gauge move off full. But now it stays on empty. I think you might have a power to gauge issue or an internal gauge issue.

Have you checked Tom TBones gauge thread for a solution?

http://www.67-72chevytrucks.com/vboa...d.php?t=640615

Originally Posted by REDROCKER652002

This is gonna be a stupid question, but is the float intact? Reason I ask is my son's float had a small crack in it, so that was a small part of the overall problem. Also, his was a grounding issue I think. But I would check to make sure the float is not cracked and filling up with fuel. If I am out of line, I apologize. Good luck

Forum rules prohibit stupid questions therefore there can be no stupid questions, except by moderators or other forum staff.

[The Rocknrod]

All fixed Vettevet. Post 23 fixed it.