Rebuilding and bench testing a fuel gauge?

[CraigLandrum]

In the process of restoring my central gauge panel, I was checking all the gauges and noticed that when I ohmed out the terminal on the fuel gauge, it was open. Took it apart and examined it. There are two electromagnetic coils to the right and left of where the needle goes and a flatter coil directly underneath the needle pivot mechanism. After carefully disassembling the needle area I discovered that the flat coil was toasted so I stripped off the old wiring and rewound it with the same gauge enamel magnet wire (it has 200 turns if anyone is interested), and made sure that the ends were properly touching the two terminals. Put everything back together and now I get continuity between the terminals and about 112 ohms between each terminal and the case (I believe this is measuring the ohms of each electromagnetic round needle deflector coil).

I want to bench test this but for obvious reasons don't want to just hook the two terminals up to a 12 volt battery since all that would do it turn my new gauge coil into a nice temporary light filament.

I have variable bench power supplies and any resistors that may be needed. Anyone out there ever bench test one of these puppies? I don't have the fuel tank handy (its being reworked) but did want to verify correct operation before installing my new panel.

I assume that the case is ground and one terminal is hot and one comes from the fuel level sender.

Suggestions welcome.

 

[TR3driver]

Check out the article at
https://mgaguru.com/mgtech/electric/fg_01.htm

Written from an MGA point of view, but the TR3 gauge is nearly identical. Same testing procedures should work.

 

[CraigLandrum]

Exactly what I was after and the gauge in the article is exactly like mine except for minor differences in the faceplate.

Thanks!

 

[MGTF1250Dave]

Aloha Craig,

I used that article to build a bench tester, I think the parts cost about $20 and I got everything from Radio Shack except the DC power supply. I had one from an old answering machine or something. I still need to do a little fine tuning with the gauge in the car, since it's operating voltage is more than 12V. If you can get a DC power source that is equal to what the gauge will get when installed it shouldn't need any further adjustments.

 

[CraigLandrum]

Followup - looks like the main thing that goes wrong with these gauges is that the "flat" coils directly underneath the needle pivot burns out. That's the coil that provides 154 ohms using 200 turns of resistance wire (I didn't know about the resistance wire and initially replaced it with regular enamel magnet wire which is incorrect and won't work). Rather than replace it with more resistance wire, I used three 1/2 watt 470 ohm resistors (from Radio Shack or Jameco, etc) in parallel which provide 156 ohms which is close enough.

 

[Geo Hahn]

Well done... and you obviously understand all this.

I used that website to rebuild mine and now the gauge operates correctly... but I'm still not real sure how it works.

 

[dklawson]

Excellent idea on adding the series resistors to the magnet wire. Good job.

 

[CraigLandrum]

To be absolutely clear, the three 470 ohm resistors take the PLACE of the older flat resistance wire coil which can be removed and chucked. Also, there's nothing magic about using 470-ohm resistors in parallel - it just let me get close to the spec'ed 154 ohms. I'm sure that if I wasn't so impatient that I could order a 150-ohm 2 watt resistor from Jameco or somebody for a bit cleaner solution, but since I'm the last person that is likely to look inside that gauge, I think I'm pretty safe

 

[dklawson]

Oh... I think the choice of 3 parallel 470 resistors was a good one. You're selling yourself short. The 2 Watt resistor would probably have been too large to fit inside the case. The only problem with the use of the parallel resistors is that IF one of them burns out or fails (for whatever reason) the remaining two will carry the current load and effectively appear as a single 235 Ohm load. That would mean the gauge goes out of calibration and the magnet wire might overheat from the increased current. However, resistors don't fail that often do they?

Again, I think your fix is great. Let us know how the calibration worked out.

 

[TR3driver]

The odds of a commercial resistor failing are VERY low without some serious electrical or mechanical abuse being applied.

And even if one of the new resistors DID fail, it wouldn't "burn out" the magnet coils. The calibration would change, but the maximum dissipation of the "empty" coil (the one in parallel with the resistor) remains the same while the maximum dissipation of the "full" coil (the one in series) goes down. Much the same result as having the entire resistor burn out (which is where Craig started).

Even with 14.4 volts applied and the fuel sender shorted (tank empty), the dissipation from each 470 ohm resistor only hits about .44 watt. So a 1/2 watt resistor should last effectively forever in that circuit.

Personally, I wouldn't tear a gauge apart just to change the resistor, as I've had several of them and never seen one fail that way. But it is an excellent solution to the problem at hand : cheaper and easier than rewinding the original resistor.