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This past winter I purchased a new gas tank sending unit. Luckily, I bench tested it before dropping the tank and installing it. The unit registered backwards. After some back and forth with the manufacturer (KM Lifestyles) I got the correct unit and when testing it on a spare gauge I have, it worked smoothly and accurately. Then, once installed, it seems to work when the car isn't moving, but underway, there is a wild 'flicker' to the needle. The way it jumps abound is definitely not the float oscillating and I'm left wondering: bad ground? bad wire between tank and gauge? or maybe my actual gauge is bad. In as much as all of these are a fair bit of work to change, was wondering if anyone has any advice on what the likelihood of the root cause of this could be?? Anyone experienced this before? Much apprec. for any input of suggestions. Chris.

By far the simplest thing to try first without messing with pulling a tank would be add a good ground. I don't know about a 39 tank but most later tanks have a sort of flange around the middle where the halves are joined together. Usually there is enough metal sticking out at the corners where a small hole can be drilled in the flange at a distance from the tank big enough for a wire terminal and ground wire to be screwed on and then run over to the frame or body. Harder to check but a good ground also means the gasket has not shrunk and the sender is still fastened securely. If you can get to them one of the screws holding the sender could also be used to attach a good solid ground connection.

After that it would be checking the tightness of wires at the gauge and sender terminals and look for any issues with inline connectors where sections of loom might join together.

If everything is tight that leaves the gauge and sender. Gauges usually work or don't so not the most likely to be acting up. The sender on the other hand could have a spot in the resistance winding where the wiper is barely making contact. I don't know if KM makes the sender from scratch with new parts or rebuilds or uses old NOS items but any abrupt change or loss of wiper contact could be enough to make the gauge go wild until fuel level moves the wiper past the area. That loss of contact is a frequent cause of issue with the 51-6 senders when the phenolic support the resistance wire is wound on warps.

Thank you Howard. Just in for lunch after spending the morning under the car. My tank is a custom stainless job that came with the car, but when I installed the new sending unit, I did make sure to have a new and clean connection from one of the screws on the flange to the frame of the car. I have an electric pump mounted at the tank for priming and used that bolt to attach the wire. It is clean and tight. For the sake of elimination, I am in the process of running a new wire from gauge to sending unit, and will see if that effects any change. I tend to agree with you that the gauge is a work/no-work scenario, so leaving swapping that out as a last resort. I was also thinking that maybe there is some kind of 'interruption' with the power supply TO the gauge. Does that make any sense?? I have checked all the connections and put a hand socket on all of them and nothing seems amiss. I guess I'll know more (or less!) after I finish with the new wire. Really appreciate your info and advice.
Thanks again. Chris.

Gauge feed should be a wire coming straight from the ign switch so assuming it is tight and the ign switch is good then not much to go wrong. You are replacing the wire to the sender so that is good but diagrams show there is an inline connector. It is probably somewhere under the car or maybe in the trunk if the wires run overhead. Those inline connectors getting wet -- even in Packards day -- resulted in some oxidation and bad connection problems with intermittent issues.

Hi Howard,
Well, all for naught - replacing the wire did zero to alleviate the flicker. Stopped, it is pretty static, but once moving, it's all over the place. You say the diagram shows an 'inline connector'; is that in the power feed or the S.U. side of the circuit? I have replaced the wire right from the tank (or within 6" of where it connects to the S.U. as I can't get at that without dropping the tank) all the way to the eyelet on the back of the gauge, and I soldered and shrink wrapped all connections. You mentioned in your previous post that maybe there is a 'blank spot' or bad connecting area in the winding of the S.U. and my thought was to add gas and see if that changes anything. My Dad also suggested jury rigging the spare gauge I have up with ground and power, and see if it works any better. I could have it hanging off the bottom of the dash and it would tell the story. When I dropped the tank to install the new S.U. I found the old cork float sloshing around in the gas tank. It had broken off and that explains why I had no reading at all. Now, if stopped, I can at least tell the approximate level, but it would be nice to be able to monitor it whilst driving. Maybe that's asking too much!! LOL Chris.

Chris, the factory 39 drawing shows the connector for the sender. It is the top one where the arrow points and I am guessing it might be under the car somewhere around the battery.

Adding the extra gauge would be a quick but not necessarily an easy way to prove the culprit. I believe 39 gauges are magnetic so in addition to moving or adding the wires to the terminals be sure the case has a good ground if it is hanging loose.

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Howard,
I get the connector in the wiring harness you are referring to; but by running a completely new wire from sender to gauge, would I not be bypassing that? (or eliminating it from a possible issue?)
FWIW, I drove the car to town this evening and filled it with fuel. The needle is pinned on 'Full' and is rock solid. I guess I will have to wait and see what happens as the tank drains down and the gauge starts to show less-than-full status and see what it behaves like.
And yes, with all it would take to set up the spare gauge with power, ground and S.U. connection, it would probably just be easier to install the damn thing!! I really thought the new sender was going to solve my problems with fuel reporting, so I am pretty disappointed and frustrated with this latest wrinkle. I seem to always go 3 steps forward and 2 steps back with everything I do to/with this car. Most annoying... Chris.

Chris, I am playing around with the same thing. I installed a different gauge that now has some bounce in it. I did notice that the gauge seemed to move a lot faster when I tipped it back and forth than the older one did. This was when the gauges were uninstalled. I am told that the gauges are designed to counter the sender bounce. I plan on looking at the original gauge that seems to glide between stops slower versus bouncing to see if I can figure out why. It may be a little while, but I will report back if I learn anything of interest.

Hi Buster,

Thanks for weighing in on this. I am still in the 'evaluation stage' of this problem with my car. As stated in my previous post, (I think) my new sender unit has a definite damper on the float to eliminate 'slosh-readings' on the gauge, and the flicker I'm getting is too erratic to be that anyway. Then when I filled the car right up, the gauge correctly reads 'full' with no flicker. As I have driven the car little since, I still have no real 'results' for how it will behave as the needle descends towards 'empty'. Howard (HH56) had stated that if the windings on the little coil of the sending unit don't have even contact with the 'wiper' of the sending arm, then that can cause flicker in one spot. If my gauge is more or less reliable until I get down to say 1/8th to a 1/4 of a tank, I will probably just live with it. I do have a spare gauge and if it DOES 'twitch' overly as it goes down, I may try switching that out, but I sure don't relish the task of re-dropping the tank and messing with the sending unit again, as that is quite a task. I too, will report back, once I have more info. Chris.

Another possibility is that your custom stainless tank is missing its baffles, which is causing major sloshing of the petrol......... Just a thought.Quote: