Fuel gauge issues on m818

[cessnatwin]

Heres the problem! My M818 reads pegged beyond full on both fuel tanks when switching. The gauge does not move at all, just pegged over full! Both sending units can't be bad, so probably a bad gauge. Does any one have any thoughts on this or a gauge?

 

[taylordc15]

Had this issue on my deuce, turned out to be a ground issue, tightened the gauge so it made better contact with the dash, problem solved.

 

[Armada]

Ditto ground. Gauge reads full when ground is lost.

 

[bigelk50]

Ditto on my M818. Ground issue on one and a bad sending unit on the other. Now the needles jumps around quite a bit while on the road. fine at a stop though.

 

[808pants]

similar gauge issue here: during 'dry' testing with two different senders, my gauge needle 'vibrates' right around the correct levels as I raise/lower the float arm. (both senders show about the same resistance range - 0 to 35 ohms). I thought I would replace the gauge to see if that would dampen the vibrations for some reason, since I didn't want to find out after reassembling dash and tanks that something was still wrong. But the new gauge, well-grounded, sits at a bit less than 1/4 tank without any movement whatsoever. Huh?

Repeated test with old gauge - got the same vibrating-but-correct variations from E-F, then back to new gauge - same non-varying reading. No grounding problem in this case. I hate to conclude it's a defective brand-new gauge, 'cause that's gotta be rare, but...what else?

 

[cranetruck]

Ditto ground. Gauge reads full when ground is lost.

Right, and any difference in the ground potential between the dash (instrument) ground and sending unit ground will produce an error.

What brand is the gauge? The older SW or the new plastic Faria?

 

[cessnatwin]

The fix was adding ground straps to each tank! thank you! Taylord. armada, big elk and others that fixed it perfectly!

 

[808pants]

Right, and any difference in the ground potential between the dash (instrument) ground and sending unit ground will produce an error.

What brand is the gauge? The older SW or the new plastic Faria?

wait...who, me? I think I have eliminated any grounding issues (alligator jumpers to known grounds), but anyway, old gauge is steel S-W and new is mostly-plastic (Faria, I presume)

 

[jimmcld]

I have the jumping issue on one of my tanks. I found a broken wire about 6" up from the sending unit. The insulation is still good on it but when I wiggle the wire at that point, it causes the guage to jump. It has been a problem for some time because it had all sorts of ground wires added before I got it from GL.

 

[808pants]

...broken wire about 6" up from the sending unit. The insulation is still good on it but when I wiggle the wire at that point, it causes the guage to jump. It has been a problem for some time because it had all sorts of ground wires added before I got it from GL.

So it was the ground wire that you were wiggling, or the gauge wire? I'm thinking the more grounds, the merrier (if uglier). If one ground wasn't quite doing it, another one would have. So maybe it is/was really the wire leading up to the gauge that had parted internally? (On my truck, I've found the insulation will go to **** and fall away before there's any problem with the very-flexible stranded conductors inside - I've spliced in some sections to deal with insulation that's been destroyed by chronic fuel exposure, going well past the damaged area and into the main harness in some cases just to be sure - but I've not yet found compromised conductors.) According to my two senders, resistance is 0 ohms (empty) and 30-33-ish ohms (full). Based on this measurement, a short to ground in that gauge line (say at some pinch-point in the harness, or where the wire is scraping over the frame where a grommet fell away in chunks years ago, etc) should make your gauge swing to E momentarily or indefinitely depending on how bad that short is. (This diagnosis doesn't help in my set of circumstances!)

Anyway, the seemingly-simple gauge fix has been elusive to me, and I really want/need it to be dependable in order to eventually run a twin-tank alternative fuel setup of some kind. I haven't found anything that goes into any useful detail on fuel gauge faults in the TMs - anyone? Outside of that, the attached text/illustration I copied & put together from...some tpub link, I think it was... is what I believe a detailed schematic would consist of, generically, for this particular gauge type ("magnetic fuel gauge"). Sorry about the crappy quality - best it can be from the thumbnail size illustrations. Maybe there's something more to the real setup that I am missing that would explain my ongoing problems?

 

[jimmcld]

On mine it was the wire going to the guage. There were no signs of wear on the insulation and it was not near anything that could pinch or rub it. However, the conductor is broken internally. I say "is" because I still have not fixed it. I just pushed the conductor back together withing the insulation and it now works 90 % of the time. Depending on the sensativity of your ohm meter, it could read zero ohms resistance and still fail under any kind of load. I was just lucky enough to be able to watch the fuel guage while I was wiggling wires and noticed that the guage would fluctuate wildly when I wiggled this one wire. The guage would peg the full mark and then go back the empty violently while being wiggled. Then I applied pressure on each side of the break, to push the internal conductor back together, and the guage held steady. This is the wire going to the sending unit and about 6" up from the sending unit.

Someone had already added additional ground straps in an effort to fix the problem. My truck had gone through a depot rebuild in '91 and I believe it has had this problem since. I have also found other problems with the rebuild. If someone else has already worked on it, you cannot assume anything!

Jim