Fuel gauge moves around while driving

[Charlie]

Recently replaced tank and sending unit. Gauge has not worked since purchase in 2015. Indicator needle always stayed at 3:00 position. Gauge works now, just moves back and forth near the fuel level while driving. This may be normal operation but cannot remember this was normal in 1974. Been too long for the old memory. Has anyone found an easy fix for needle not moving around?

 

[rpcraft]

It's a little bit normal but it should only take a moment or two to balance out when you come to a stop, unless you are stopped on an incline. The later vehicles that had baffles in the tank suffered from it much less usually.

 

[crazy4offroad]

Unbaffled tank, they started putting baffled tanks in em around, what, 85 or 87? Whenever it was they switched over to TBI. I think the Chinese float assemblies are more likely to react more extremely than say a Delco-Remy unit or something.

 

[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.

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

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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.

 

[Charlie]

Thanks chengny. It does settle to approximate fuel level when stopped for a minute or two. That may be just what I need. Where to buy?

 

[bucket]

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.

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

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

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.

Well I just learnt sumpin. And now my Stepside pickup has yet another item on the "to fix" list.

 

[chengny]

Thanks chengny. It does settle to approximate fuel level when stopped for a minute or two. That may be just what I need. Where to buy?

I seriously doubt that it is possible to buy just the dampening resistor anymore - you probably never could. Do you have an unused gauge laying around that you could cannibalize? It doesn't necessarily have to be from a gas gauge - all the gauges have them and they're the same. If I knew the ohmic value, you could measure the resistance of yours and make an informed decision whether to even pursue that course of action.

I have several gas gauges - some even new in the box. I could send you a resistor from one of them - or even a complete gauge - if you truly feel that you have eliminated all other possible causes.

From which vendor did you source the new sender? If you say elemcee...well, there's good chance that the problem is product quality.

 

[Rusty Nail]

Ow. Burnnnn!

 

[Honky Kong jr]

Ow. Burnnnn!

Don’t you mean

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[Rusty Nail]

Yes.
That's it.

 

[hatzie]

I seriously doubt that it is possible to buy just the dampening resistor anymore - you probably never could. Do you have an unused gauge laying around that you could cannibalize? It doesn't necessarily have to be from a gas gauge - all the gauges have them and they're the same. If I knew the ohmic value, you could measure the resistance of yours and make an informed decision whether to even pursue that course of action.

I have several gas gauges - some even new in the box. I could send you a resistor from one of them - or even a complete gauge - if you truly feel that you have eliminated all other possible causes.

From which vendor did you source the new sender? If you say elemcee...well, there's good chance that the problem is product quality.

Not only could you buy damping or shunt resistors right from GM in the 60's 70's and 80's... you can buy em now from Corvette and F-Body parts purveyors. You need to know the correct ohmage but that ain't too difficult.
http://secure.classichq.com/GAUGE-RESISTOR-84-OHM-PINK--P816.aspx
If you don't feel like finding an OEM version just buy a 10W wire wound resistor in the correct ohmage from Mouser or Newark or ...

Sometimes all it takes is an external tooth star washer under each nut to increase contact.

The Fuel and Oil Pressure gauges are 90Ω senders. The TEMP gauge uses a wildly different sender and likely a different Ohmage Shunt. Adding more to the fun is the FACT that GM used several different temp senders across the 73-91 model year range. I did a writeup on the common TEMP senders you might encounter in the light duty chassis but that doesn't include the Medium duty chassis that had Cummins and CAT diesels.

That being said some fault lies with the sender but not all. I've had this issue with brand new AC Delco senders as well. When the gauge was brandy spankin new it wasn't as loose as it is 40+ years later. Some rude comments come to mind but I'll hold my tongue. Gauge restorers add some silicone goo to the inside of the coils on C2 & C3 Corvette fuel gauges to damp the needle. I've read about the same technique being used on 67-81 F-bodies.

 

[Charlie]

I have seen range of resistor from 78-90 ohms. Just was not sure which value I should get. I was thinking about the 86 ohm, still not for sure. Any help is appreciated.

 

[hatzie]

Pretty sure it should be 90Ω or thereabouts. Not Positive tho. These old ceramic resistors are likely 10% or higher tolerance.

Honestly I'd put some internal/external tooth washers under the nuts and carefully snug them back down. This may fix some of your problem. Another partial issue is probably the gauge itself is pretty free and loose after 40 years of rough service.

Make DOUBLE SURE the ground wire from the sender ring to the frame is SOLIDLY ATTACHED with ZERO resistance from the tank to the frame.

The C3 Corvette guys claim the following color to ohmage relationships; https://www.corvetteforum.com/forums/c3-tech-performance/2122378-help-fuel-gauge-resistor.html
Lt blue-123Ω
green -89Ω
yellow-88Ω
orange-86Ω
blue black-81Ω

 

[Rusty Nail]

Nice work! @hatzie

 

[Charlie]

Honestly I'd put some internal/external tooth washers under the nuts and carefully snug them back down.

Would you be more specific to location of these washers?