gas gauge works when it wants, help?

[wwj75c30]

my 75 c30 driver side tank gauge works but when i flip it to passenger side it goes all the way past full and theres only about a couple gal of gas in it not even quarter tank but my driver side usually works but then again not all the time. how can i fix this? new gauge? and also my temp gauge doesnt work. should i just get all new gauges?

[cjlinkster]

If it is going past full; that usually indicates a sorted or open wire. Trace your fuel sender wire (usually pink) from the tank back to the firewall.

I had the same problem on my truck. I changed out the fuel sending unit, but it didn't fix the issue. After a few hours and after pull the gauge, I found the wiring issue. The PO had pulled the wire out of the fuse/ junction box. I stuck the wire tightly back into its place and that fixed it.
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[wilkin250r]

Like cjlink said, the gauge reading way past full is an indication of a broken wire, bad connection, or broken wire in the sending unit itself (you'd understand if you were looking at it). There are ways to test each of these. However, because the driver side often works, it DOES indicate that the problem is most likely not in the gauge itself, so a new gauge probably wouldn't fix it.

When the driver side tank doesn't work, what does it do? Peg past full? Peg as empty? Different symptoms indicate different problems.

[Goldfinger]

It's either a bad ground or a sending unit on it's way out. mine would read at 3 o clock no matter how much was in it

[Firebirdjones]

Originally Posted by wilkin250r

Like cjlink said, the gauge reading way past full is an indication of a broken wire, bad connection, or broken wire in the sending unit itself (you'd understand if you were looking at it). There are ways to test each of these. However, because the driver side often works, it DOES indicate that the problem is most likely not in the gauge itself, so a new gauge probably wouldn't fix it.

When the driver side tank doesn't work, what does it do? Peg past full? Peg as empty? Different symptoms indicate different problems.

This is what I was thinking. If it's working on one tank,,that tells me you have an issue with the wiring on the other tank (ground/short) or the sender in that tank has bitten the dust.

Been a long time since I had to trouble shoot one,,,but I believe if you ground the sending unit wire,,turn the key on,,,,,it should move your gauge to full. Un-grounding the wire and the gauge should drop to empty. If that's the case, your wiring is fine,,,and the sending unit is bad.

When the sending unit goes bad, (potentiometer) they usually peg the gauge and read full just as you describe.

[legendman]

I had this problem not to long ago, it was a bad ground

[slam59]

The gauge on my 74 stays about 3/4 sometimes, even when I add fuel. But most of the time it pegs way past the full mark. Most likely a bad ground or???

[wwj75c30]

Originally Posted by wilkin250r

Like cjlink said, the gauge reading way past full is an indication of a broken wire, bad connection, or broken wire in the sending unit itself (you'd understand if you were looking at it). There are ways to test each of these. However, because the driver side often works, it DOES indicate that the problem is most likely not in the gauge itself, so a new gauge probably wouldn't fix it.

When the driver side tank doesn't work, what does it do? Peg past full? Peg as empty? Different symptoms indicate different problems.

when it doesnt want to its usually past empty or quarter sometimes even half tank

[wilkin250r]

Originally Posted by Firebirdjones

Been a long time since I had to trouble shoot one,,,but I believe if you ground the sending unit wire,,turn the key on,,,,,it should move your gauge to full. Un-grounding the wire and the gauge should drop to empty.

Close, just the other way around. Grounding is empty, un-grounding is full

Your gas gauge is just reading a resistance at the sending unit, with zero resistance being empty (zero resistance is basically a direct path to ground), and 90 resistance reading full.

So if you take the sending unit out of the equation, and ground the sending unit wire, the gauge should read empty (with the key "on"), because you're showing it zero ohms.

Un-grounding the wire should show way past full, because it's expecting up to 90 ohms and you're showing it a million ohms.

If you want to get fancy with it, your local radio shack will have an assortment of resistors for about 10 cents each. Pick a couple under 90 ohms and connect them between the sending unit wire and ground. 45 ohms will read as 1/2 a tank, 30 ohms will read as 1/3 tank, ect ect.

Likewise, you can attempt to read the resistance from the sending unit directly with a standard multi-meter.

[wilkin250r]

Originally Posted by wwj75c30

when it doesnt want to its usually past empty or quarter sometimes even half tank

Hmm, I'm not sure. It's possible you do indeed have a bad gauge. If you got a consistent reading I might know where to troubleshoot, but with readings all over the place I'm not sure where to begin.

How often does it go haywire on the working tank? Is there any pattern, like it only does it when it's cold, or the first 10 miles of driving, or anything like that?

[wwj75c30]

it pretty much will do it whe it feels like every couple starts. my dads friend which who was the original and first owner leaving me the second owner said the sending unit might be bad cause i renember him saying something about he shorted something he thinks

[wwj75c30]

and my driver tank started working once i put more then 15gal in it

[motornut]

yup shorted is empty
wire off is over full,
can you switch the wires to the opposite tank just to test?
or run a temp wire to the back?
check the ground behind the cluster too

[hatzie]

Check tank/sender to frame grounds.
On the 73 to 81 1/2 of the DPDT tank switch switches the gauge. GM used cheap switches. Check the switch.

[wwj75c30]

my dad keeps saying its the sending unit on the pass tank

[wwj75c30]

how hard is that to change?

[hatzie]

IMHO pulling the bed to access the senders is easier. Especially when you're looking over both senders. Less contortionist positions to put the hands in when re-attaching the fuel lines. I only pull a tank if I'm buying it or replacing it.

1 wiring connector in the driver side frame rail for the taillights and maybe plate light connectors. The bed bolts... They should be pretty easy to remove if it's been in the central valley all it's life. Get three buddies and walk it to a set of sawhorses behind the bumper.

Check the frame to tank/sender grounds on both tanks while you're in there.

Make sure you secure the sender ground and signal wires to the frame, with masking tape, where they won't get pinched by the bed cross rails.

[Firebirdjones]

I'm a one man operation so I prefer to stick it on the lift and use the transmission jack to lower the tank. Doesn't take that long.

[wwj75c30]

Originally Posted by Firebirdjones

I'm a one man operation so I prefer to stick it on the lift and use the transmission jack to lower the tank. Doesn't take that long.

luckuky my cousins shop has a lift i can use lol. i just took a bed off today wasnt fun