I am trying to replace the portion of the wiring harness that has been cut and removed from under the back seat that went to all the electrical components in the rear of the car (off topic car listed in my profile). I have everything figured out minus the fuel sending unit which feeds the fuel gauge. I have two wires left to choose from. With the key off they are both dead, and with the key on and using a test light, one has a constant voltage and the other causes the test light to flash. My question is which is the correct one to hook to the sending unit?
The sending unit needs to be a GROUND. From frame to sending unit- thru sending in tank- back to the gauge . I wouldn't put 12 volts into that sender without a bomb suit .
All wires were cut and removed from under the back seat to the rear of the car, no wires on the sending unit when I got the car.....
Since you haven't disclosed the type of car, I can't tell you exactly how its grounded to the frame but I guarantee you that it is. Its a rheostat that varies ohms of ground that tell the gauge where to point
If there is only one pole the the unit is grounded via the tank itself. What other devices are still with out power in the rear of the car? I constant feed would seem likely since the engine always needs gas, however it would make sense to me if it was the pulsing wire also as the feed comes thru the fuel pressure regulator. I would figure out what else needs power and try to eliminate a wire that way.
Its a 69 Mercury, Looking at a wiring diagram for a 69 ford and it appears the wire to the sending unit from the fuel gauge would come from a voltage regulator.....that is fed by the ignition, so it would have voltage on it, just not sure if it would pulse or be constant....
The wire to the tank unit comes from the gauge in the dash. It has constant voltage and does not pulse. A wiring diagram should tell you what color that the sending unit wire is. I'm trying to figure out what wire to the rear of the car would pulse other than a turn signal wire with the switch engaged.
Turn on the key and with a test light, find the power at the gauge. The color of the other wire is the color of the wire all the way back to the sender. If there is not a seperate ground from one of the sender screws, make one. The voltage at that hot wire on the gauge should be less than 6 volts.
Edited for clarity. I don't Have the diagram to look at The gauge has too poles, one is voltage , the other is ground. the ground resistance ohms is varied by the sending unit. They have to match in order to work properly btw. Lets just say you were sendind voltage to the sender on its single pole. How would the gauge read that? It can't. If I had a bundle of wires in my hand that I didn't know where the went I'd pull fuses to at least I'd circuits. Pulsing wire to rear of vehical would be turn signals or flashers as my first guess. Also I would think some relay would be clicking unless you have installed some sort of computer then who knows at this point. The wire from the gauge to the sender should be dead on the volt meter. Meaning no continuity to ground and no voltage. If you ground that one wire the gauge should tickle. Might be very subtle but something should happen.
Don't over think this thing. The stock Ford sending unit has only one wire that connects to the center post. That's the way that they work. There is a variable resistor in the sending unit that causes the needle in the gas gauge to move as the level in the tank changes.
Exactly, by varying the resistance to ground. The specifics vary by make and year as to exactly how many ohms correspond to a gauge reading full or empty but the concept remains the same.
The Jalopy Journal
