Gas Gauge Needle Wobble

[MIPS]

Now I know that fuel senders and gas gauges from this era didn't have a whole heck of a lot to compensate for the contents of the tank sloshing around but I'm not sure what I did this time to make it even worse.

The last two times I had to dig into the cluster for unrelated reasons, the gas gauge decided to misbehave once it was all back together. The first time it seemed "too tight" and for the first tank of gas it was very jerky in movement but it did eventually settle down and become pretty linear with it's wobbling as you turned corners. This time around even though I was careful not to touch the needle while the cluster was disassembled and during cleaning it now acts "too loose". Even getting into the car is enough to send it on a wobbly trip up and down the dial and makes it a lot more annoying as you try and guess how much is left in the tank.
Suspecting I damaged a wire (or one of the riveted terminals, since those don't seem to like being handled excessively), I pulled the cluster again (I sure am getting good at this!), tested the grounds, tested the battery and ignition power and tested that all the wiring back to the sender was good using a spare cluster. The wiring was fine and the gauge itself is actually good. If you test it on the bench an open or shorted signal path pegs it beyond the E of F but with the sender attached it tracks cleanly between the sender end stops, but with no lag or hesitation, so indeed the gauge has become very, very sensitive.

I know some gauges compensated for extremely sensitive readings by using some sort of a grease that maintained viscosity across a wide temperature range. I can only guess it's been wiped out. Problem is that unlike the Eagle's speedometer and tachometer, the fuel and temperature gauges have the dial riveted on the front and the assembly is riveted on the back to the cluster circuit board, leaving only a small opening to reach the front needle shaft. Having just got off repairing the mistake of oiling the speedometer needle shaft I'm being extra cautious how I approach this problem since if I make a mistake there is no easy way to fix it.

Again, this isn't a sender calibration or an electrical issue. The gauge has simply lost its ability to compensate for variations as the fuel moves around in the tank. Google is absolutely useless for answering a weird problem like this.

[AMC of Houston]

That affliction may be normal!   The first paragraph in the Fuel Gauge Diagnosis section of the MR251 shop manual (page C-98 in my copy) seems to be a disclaimer that implies that the fluctuation is normal.   Kind of worded a bit weaselly, but that's the way I'm interpreting it!

[maddog]

I I've never had mine fluctuate like that. I myself have a different issue with my gauge as it will not go below the quarter of a tank mark, other than that it's perfectly accurate.

[djm3452004]

Non-tach clusters are still available pretty cheap used, from about anywhere -- here, eBay, Marketplace, etc.  I'd grab a couple of those for salvage and swap their fuel gauges into your cluster to see if there's any difference.  Final-gen AMC cluster parts are so plentiful that it would be pointless to spend a bunch of hours trying to diagnose a bad fuel or temp gauge unit.

For me pulling the cluster is such a pain, particularly with the speedo cable disconnect/connect step being one of the worst parts, that I'd probably just leave the needle wiggling, provided it's somewhat close to accurate.


David

[MIPS]

The gas gauge is one of my few pet peeves on the Eagle. Because I've been keeping a logbook on the fuel consumption for almost six years now I have now spent many hours calibrating, fine-tuning and keeping the reading perfect as to preserve my experiments across multiple senders and two tanks. I mean as long as the needle isn't whacking its own end stops (which I confirmed it's not) the extreme wobble isn't damaging it but you're making a long-distance trip and there it is out of the corner of your eye wobbling about and it's just.... ugh. >_>

[MIPS]

TIL:

The temp and fuel gauges look like they are riveted to the PCB on the cluster but they are, in fact, pluggable modules.



I thought you meant look for a whole replacement cluster and was throwing a fit. This has no goddamn reason to be this modular and serviceable. It's amazing.

[djm3452004]

TIL:

The temp and fuel gauges look like they are riveted to the PCB on the cluster but they are, in fact, pluggable modules.



I thought you meant look for a whole replacement cluster and was throwing a fit. This has no goddamn reason to be this modular and serviceable. It's amazing.

Sorry, I knew this and should have clarified.  I assumed you knew based on how many times you've had the cluster in/out!   

[MIPS]

That's fine. I pulled it out again tonight anyways to install the tachometer. Man that thing is useless on an automatic.

Anyways, the spare gas gauge now that it's installed is not much better. I don't get it. I didn't touch it and yet somehow being jostled around it's no longer "sluggish".

[MIPS]

I had to reach out to the professional audio crowd about what I was talking about and a suggestion they came up with is that when the gas gauges are assembled, they use a damping grease and after 40 years it has dried out and that is why the needle waves around more than usual. This also then explains why this problem seems to apply to all pre-electronically dampened gas gauges. You are not wrong if you remember at one time long ago the needle didn't wave around as much.

Now the audio industry does not use it in meters and gauges but in knobs. As a result there's quite a few vendors selling damping grease for sliders, knobs and digital rotary encoders, for example Tribosyn:

https://youtu.be/2H5HNDy5hcE

Of note it's stable between -40 and 130c so it's ideal for vehicle applications.

So I suspect that a very light viscosity damping grease if a meter is dismantled, cleaned and regreased will even out the tank sloshing without hacking on external electronic bits. The big problem is finding it in very small quantities and the finding a light enough viscosity that it drags the needle but you don't end up with a gauge that takes ten minutes to go to F when you fill up. I wonder if this is something you could go to an audio repair shop, give them a dollar and get some on a toothpick.

[MIPS]

Further research and investigation is travelling into a dead end. The spare gas gauge has also now decided to act up in the extreme heat which means I again need to find another cluster to steal the gas gauge out of or I need to look into something else.

I find it hard to believe that with how crazy some people are about vehicle restorations there is nobody that could restore the original state of gauges beyond the tachometer or speedometer. Turns out there is, in fact. Several options.
I'll fire an email off to https://www.clocksandgauges.com/ and see what they think and then I guess I'll consider having the original gauge sent in for service. It will cost more but my sanity is preferred.

[vangremlin]

That's fine. I pulled it out again tonight anyways to install the tachometer. Man that thing is useless on an automatic.

Isn't that the truth!  I was so excited to get a tach in my Eagle, it's like watch paint dry with it's slow motion movements!

I've used Clocks and Gauges on a couple old school clocks.  They do good work.