Hey fellas, I bought a 1947 Hudson Commodore from a buddy of mine. He is a very competent mechanic, and he had converted the Hudson from 6V Pos Ground to 12V Neg Ground, when swapping the engine to a late 60's 390 Ford. In the process he installed those little voltage drops from Speedway, and everything on the car works fine, except for the gas gauge, which reads backwards (E when Full, F when Empty). The gauge and sending unit are original Hudson, so far as I can tell. I should note that I am only guessing that the car was originally Positive Ground (I know next to nothing about Hudsons), but it is definitely wired Negative Ground now, and that was my first inclination as to the source of my gauge problem. The biggest variable here, is that I don't know for sure if the gauge worked properly before the swap. Could this be as simple as reversed polarity? Thanks, Casey
Could be... the sending unit is nothing more than a variable resistor with a wiper arm. If it were mine I would probably just swap the two wires (at the gauge) and see if it corrected the problem. You have already reversed the polarity and it didn't hurt the gauge so reversing it again should do no additional harm.
I put a Jeep CJ tank under the bed of a mid 60's chevy pickup and the jeep sending unit caused the chevrolet gauge to work backwards... It was still like that when I sold it... Told the new owner that "e" indicates "enough" and that "f" meant that he was F.....
The only gauge that should care about polarity is the amp/volt gauge, I don't think it matters on the gas gauge. You may have a mismatch between the Ohm ranges of the gauge and sending unit, or maybe a bad ground or short?
You know...I am a doofus. The Temp Gauge is swinging backwards too. It's so hard to see that gauge that I didn't even notice. Tried reversing the polarity on the gauges themselves today, and no change. Temp and Fuel Level are the only gauges that use an electronic sending unit on the car, but the temp gauge is hooked up to the Ford sender, so I'd expect that one to be wrong. Hudson gas gauge to Hudson sender, with a Speedway voltage drop, and reverse sweep...I don't get it. I'm no electrician, but I was thinking that since the fuel level sender is basically a rheostat, and the car was originally Positive ground, that the current would have traveled through the rheostat to ground, but now it's constant, and traveling through a varying ground, which seems like it wouldn't work...or am I thinking about it all wrong?
I want to say that those gauges are manufactured by King Seely. Please post a close up picture as a reminder. If so, they are bi-thermal gauges and are not polarity sensitive. The most likely scenario is that you have incorrect sending units, but there are a number of variables to the situation you have described.
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