Ever seen a gas gauge spin around like a clock?

[TylerW]

I haven't either, until now.

I'm in process of "rehabbing" a '79 Scottsdale SWB that I got cheap. Pristine isn't the word that comes to mind when you look at it.

Anyhow, I spent the last several weeks pulling the top end of the newer crate engine it has in it down and repainting and putting everything back stock. Takes a lot of time.

That's done so I'm moving on to the electrics and instruments.

I noticed that while driving along, the gas "hand" as we call it down here in the south, is jittery, nervous and jerky. Sounds like some people I know. I don't mean the long travel back and forth our '73 always did when stopping and starting away. I mean the needle is never still at any time.

What tops it off is....most times when cutting the ignition, the gas needle makes at least two complete 360 degree revolutions in the space of a second or two, coming back to rest at approximately the right place again.

I wish I was making that up, but I'm not. I even got it on video.

Anybody have an idea what's behind that little scene?

Thanks...

[MountainGoat1966]

WOW, just WOW ........
I thought I had exclusive rights on really weird things going on in my world ......
I don't even know where to begin to troubleshoot that one.
As soon as my brain gets back from the repair shop (provided parts are available) I will assign it the task and try to come up with a suggestion for where you should look first.

[Malo83]

[mrolds88]

Tyler, you need to share the video.....

[MountainGoat1966]

A good friend of mine and excellent mechanic says the newer gauges use a stepper motor.
There's no way the old style gauges can move 360°.
He suspects a bad connection or a cold solder joint.
Stepper motors are cheap to manufacture, but are very specific to application.
I doubt the original '79 gauge used one, but maybe the gauge was swapped out by another person since then ?

[ron350]

Yep the fuel gauge pointer (dial) can rotate 360* with no resistance, no springs, no motor and no center position.
Not sure what would cause it to rotate 360* but I have no reason not to believe it. While working on my cluster once the fuel gauge pointer (dial) moved from full to pointing straight down.

[Nin92]

Sounds like it's imitating what late model cars do!

[chengny]

There is a pulsation dampener (a resistor actually) that bridges the two opposing coils. It's function is to control the rate of movement of the ray (needle) when the sender is generating large fluctuations in a short period of time - like going around a corner fast or slamming the brakes on. If the coil is open, or the entire ceramic plate - on which the coil is mounted - is missing, the gauge will bounce around violently.

The pivot pin on these gauges (1965 -1990's) is free to make full revolutions in either direction. Although people use the term "pegged out", there are no pegs on either side.

I imagine that your spinning gauge issue is because the needle goes flying past the full mark, continues down to the bottom and then after passing the 6 o'clock point, it rises back to the normal rage on the opposite side. It may take a couple of spins before it slows down and settles out.

Some images of a gas gauge out of a 1986 K30 to help explain:












And this from someone who knows more about gauges than I:

Don’t Resist Using The Resistor

For some unknown reason, folks do not like to put the resistor on the back of the gauge; maybe because they lost it or they do not think they need it. This type of ceramic shunt-type resistor was phased in to replace the wire-wound resistor and wafer board found on earlier models. The wafer board isolated the resistor from the gauge housing. They both do the same thing by controlling how fast the needle sweeps across the gauge. Think of taking a turn in your Chevy and the needle moving every time! This makes it tough to get an accurate fuel level reading. If the resistor is not in place on an original or AC Delco replacement gauge, the gauge will not operate properly.

[MountainGoat1966]

Thanks, Chengy, I'm taking notes from this one.
My 83 gauge reads full when the tank is full, burn a quarter tank of gas it goes way past full. Burn more gas and it drops to 1/2 mark.
I think the vast majority of us have some issue with our gas gauges.

[chengny]

I think the vast majority of us have some issue with our gas gauges.


When you say that, it reminds me:

I was burnt out and neglected to mention that the dampening resistor is not specific to the fuel gauge. It is used on all the gauges included in a standard gauge package (i.e. oil pressure, coolant temp & system voltage).

[hatzie]

The small gauges have stop pins. I thought the fuel gauge had one at E... Obviously yours does not.

All four gauges are "Three-Coil Movement" meters See the attached image.

The following is a fairly technical description of the Three Coil Gauge Movement plagiarized from from Page 285 of "Automotive Electrical and Electronic Systems" Chapter-13 on Gauges etc
The function of the fixed resistance is described as well.
----------------
"Three-Coil Movement gauges depend upon the field interaction of three electromagnets and the total field’s effect on a permanent magnet attached to the rotating pointer.
Two coils are wound at right angles to each other. These are the minimum-reading coil and the maximum-reading coil. Their magnetic fields will pull the permanent magnet and pointer in opposite directions.
A third coil is wound so that its magnetic field opposes that of the minimum-reading coil. This is called the bucking coil.
The three coils are connected in series from the ignition switch to ground.
A fixed resistor forms a circuit branch parallel to the minimum-reading coil.
The variable-resistance-sending unit forms a circuit branch to ground, parallel to the bucking and minimum-reading coils. When sending resistance is high, current flows through all three coils to ground. Because the magnetic fields of the minimum-reading and the bucking coils cancel each other, the maximum reading coil’s field has the strongest effect on the permanent magnet and pointer. The pointer moves to the maximum-reading end of the gauge scale.
As sending unit resistance decreases, more current flows through the minimum-reading coil and the sending unit to ground than flows through the bucking and maximum-reading coils. The minimum-reading coil gains a stronger effect upon the permanent magnet and pointer, and the pointer moves to the minimum-reading end of the gauge scale.
Specific three-coil gauges may have slightly different wiring, but the basic operation remains the same.

In a three-coil gauge, shown below, the variable resistance-sending unit affects current flow through three interacting electromagnets. (GM Service and Parts Operations)"

[hatzie]

Originally Posted by MountainGoat1966

Thanks, Chengy, I'm taking notes from this one.
My 83 gauge reads full when the tank is full, burn a quarter tank of gas it goes way past full. Burn more gas and it drops to 1/2 mark.
I think the vast majority of us have some issue with our gas gauges.

• The sender is a wire wound potentiometer or rheostat. http://sastramech.blogspot.com/2013/...ntiometer.html
• One end of the rheostat coil is grounded and the other is not hooked to anything.
• The gauge sender wire is attached to the wiper.
• The float moves the wiper across a 90Ω resistance wire coil. The resistance between the wiper and ground is your sender reading.
• Gauge reads "Full" = 90Ω resistance and empty is almost a dead short to ground... Maybe 1Ω-3Ω at the Empty position.

Your sender has an open (non conductive) spot in the windings just below the area the wiper contacts with the float at the "Full" position. Replace the sender and it'll work again.

[MountainGoat1966]

Thanks for the info, Hatzie ...
Now I am looking forward to dropping the tank to replace the sender. (not really, but it is now on my list of things to do to the old truck).

[whiskeydented]

The coolant gauge in my 78 spins 360 degrees lightning fast. Just started doing it this weekend, well first time I started the truck in 4 months. It's pretty wild to watch.

[MountainGoat1966]

After all the weird things that happen where I live, I might suspect space alien activity messing with our trucks.

[Rich84]

I have no idea what the heck you guys are talking about..LOL. But if my gauges start to spin I'll definitely look this thread up.

[hatzie]

Originally Posted by MountainGoat1966

Thanks for the info, Hatzie ...
Now I am looking forward to dropping the tank to replace the sender. (not really, but it is now on my list of things to do to the old truck).

I've always yanked the bed. Depends on what PITA you want to go through.
If you can line up 4 buddies and 2 sawhorses the bed comes off real easy.

[MountainGoat1966]

Originally Posted by hatzie

I've always yanked the bed. Depends on what PITA you want to go through.
If you can line up 4 buddies and 2 sawhorses the bed comes off real easy.

Sounds good, but I can't even line up one person to help here, I'm on my own.

[hatzie]

I've raised the bed with a jack and slid it back on 4x4 timbers.
It may be just as easy to pull the tank if you can't bribe some help with beer or food.

[Keith Seymore]

Originally Posted by TylerW

I noticed that while driving along, the gas "hand" as we call it down here in the south, is jittery, nervous and jerky. Sounds like some people I know...

Maybe it would feel better if it took up smoking.