'51 Super's "dancing" fuel gauge

[Kestrel]

It seems whenever my fuel gauge needle drops below 1/2 tank, the needle begins an erratic dance, fluttering around nervously and intermittently pins over onto the full side reading for a second, before returning to the more accurate reading of between 1/4 and 1/2 (but still continues to flutter.)

 

I can simulate the needle jumping around by pushing up and down hard on the rear fender with the car in park and idling in neutral.

 

Gummed up sending unit ? When tank is full or about 1/2 full, she reads nice and steady. I don't want to drop the tank... just yet anyway.

 

Any thoughts appreciated ! 

[hchris]

Sounds like the sender, little wire coil gets dirty and you get bad resistance. However, shouldn't have to drop the tank as sender unit is usually mounted externally, at worst you may have to drain the tank if the sender is mounted low down. 

[Bloo]

I agree, but it could be a loose wire or a bad ground, so in the interest of not dropping the tank try grounding your tank and see if it helps. If not, drop the tank and if you don't find a loose nut or connection at the sender wire, it is probably the sending unit's fault. If/when you drop the tank, I highly recommend adding a ground wire from one of the sending unit mounting screws to the frame.

[Kestrel]

Thanks guys. I'll try grounding the tank first with a dedicated wire and see what happens. I never looked, but I don't suppose Buick installed an inspection port in the trunk floor to easily access the sending unit mount. ??

 

On my '32 Chevy I added an extra ground wire to a nearby frame location, as you recommended. No issues thereafter.

[Ben Bruce aka First Born]

Yep, you suppose correctly.😁

 

  Ben

[gdmn852]

Hello I have a 49 and it did the same thing, apparently the arm in the sender wasn’t contacting the small wire windings due to wear . I replaced the sender, fortunately   it has a access plate so tank didn’t have to be dropped.

[Kestrel]

"gdmn852",

Did your gauge give a steady accurate reading when the car was at rest, as mine does ?  With the engine off but with the ignition on, if I simple push on the rear fender, up and down, it flutters. That creates the identical erratic movement when driving down the road.

 

I clamped on a temporary ground wire from the sender pickup tube to the frame but it didn't change anything.

 

You gotta love that access plate.

[Ben Bruce aka First Born]

Charles, I knew a fellow one time, along time ago,  who MADE an access hole.  I bet the tank can be dropped just as quickly, though.  

 

  This is one place where replacing as opposed to repairing is probably a good thing.

 

  Ben

[gdmn852]

Hello on my 49 if full to 3/4 full it would read ok after that it would bounce up to half full then down to empty,I did add a ground wire but didn’t help,also on sender the first repo didn’t work correctly,didn’t have high enough resistance, ohm rate was low. this was as car was driving, stopped it would still bounce.

[Kestrel]

Update on my problem.

I pulled the tank, removed the sender and bench tested it’s resistance  with my old Simpson 260 analog meter. 

 

I was able to duplicate the erratic dash gauge movement exactly by watching the needle (set to 1x ohm) begin to swing steady up, then jump to the far left then back again as I continued lifting the sender arm. Apparently there’s a short or open circuit inside the sender rheostat thing. I don’t know how a modern digital meter would show such behavior so clearly.

 

For me anyway, electrical theory makes sense only after actually seeing something tangible happen.

 

I'll test the new sender first, then install, with a back-up ground wire as suggested.

[rodneybeauchamp]

Had similar issue on my ‘38 where gauge was good for half a tank then keep flicking. Found the backing for the rheostat winding was broken creating an issue. Was able to repair from another sender which worked out well and much cheaper as my labour rate is AUS $0.01 per hour. 😀😀😀😀😀

 

Nah, I just don’t like reproduction stuff and prefer to diagnose and fix if I can!

cheers

Rodney 😀😀😀😀😀

[Kestrel]

Rodney,

I took mine apart after reading your post. I found the little spring with the contact point was weakened to the point it no longer made contact at the half way position on that resistance winding. I simply bent it back to increase the tension against the winding so it now makes full contact throughout its range.

 

It worked. Could have saved $60.00. With my luck, it would weaken again and fail before too long. 🥵

[rodneybeauchamp]

Charlie.

 

That’s great news. Put it back together, wire wheel the bits that are rusty.

 

Now put it up on Evil Bay as “OEM fuel gauge sender suit ‘51 Buick. Reconditioned and tested. $100.00 plus postage.”

 

Bad luck you already bought the reproduction one, the original would last another 40-50 years. 🙁🙁🙁🙁🙁🙁

 

Rodney 😀😀😀😀😀😀