Electrical color coding

Pete7 · Jun 18, 2010

On a old tractor with a positive ground system was the negative battery cable red or black? I'm wondering if red always stood for positive. Or is red more of an indication that the cable /wire is "live"?
Reason I'm asking is because I bought a new (but old stock) fuel gauge for a Farmall 340. The tractor had a positive ground system when new and long ago it was converted to a negative ground system. This new gauge had a red tag on one terminal. So did the red tag mean it's for a positive or a negative wire, back in the day? Need to knew what polarity the red stood for back then.
Darn confusing.
Pete

CNKS · Jun 18, 2010

I think it means the hot wire gets attached to that terminal, positive or negative, so I doubt if it makes any difference if the tractor is positive or negative ground. If you have an operators manual, the wiring diagram will be in it.

Challenger · Jun 18, 2010

my memory says that + was always indicated with red, but on my old 6 volt Cub the main cable to the starter which is positive is a heavy 1 or 2 gauge blue insulated cable, so go figure.

Al L. in Wisc. · Jun 18, 2010

pete7; from my Operator's Manual Farmall 340: Ref. No. 10. Fuel gauge receiving unit to fuel gauge sending unit cable (dark blue). Ref. No. 15. Push button starting switch to fuel gauge receiving unit cable. No color is specified for this wire, so...maybe red was the color of choice for the gauge manufacturer or happened to be what they used. Ain't those Farmall 340's a neat little tractor! Good luck w/your project.

Janicholson · Jun 18, 2010

Most Fuel level gauges use a resistor in the sending unit to send current to the meter. the meter then converts this to needle movement. (usually using a bimetal heat sensitive bar with a heater wire wrapped around it, or by an electro magnet movement sensitive to current.
In most applications, the gauge gets power through a IVR (instrument Voltage Regulator) or directly from the ignition terminal on the key switch. From the gauge the current flows to the tank sending unit, and from the variable resistor in that to ground. It may not make a difference which post is connected! I hope this helps some. Jim

gene bender · Jun 19, 2010

You need positive ground if you want the guage to work otherwise it will read backwards at least id did on 340 i helped with the new harness.

Electrical color coding

Pete7

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CNKS

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Challenger

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Al L. in Wisc.

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Janicholson

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gene bender

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