How does the fuel sending sending unit sent the signal to fuel guage on a 71 dart.

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hobart63

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My fuel guage on my 71 dart works when it wants. Is there a way to check sending unit for proper operation. I cleaned the single point connection by the sending unit but, don't know how exactly it's suppose to work. Please help:-k.
 
It's all about the ground make sure the sending unit is grounded there should be a ground strap between the steel line at the sending unit and the line to the pump
 
The ignition switch sends 12V to the cluster, and powers the voltage limiter (regulator) for the temp/ fuel gauge, and oil gauge if you have one Power goes through the gauge, and the sender terminal of the gauge goes down the dash harness to the connector in the left kick panel, where the rear harness goes under the sill and to the rear of the car. The sender wire hooks to the sender, which changes resistance according to fuel level, and ground.

First determine for sure that the temp gauge is working. This will show that the voltage limiter is working, which supplies both gauges

Then ground the sender wire, and turn on the key, watching the fuel gauge. Don't leave the key on longer than it takes to observe the gauge. It should quickly head for "full."

If this works, go back and check the sender with an ohmeter. Approximate resistances are:

L = 73.7 Ohms (empty)
M = 23.0 Ohms (1/2)
H = 10.2 Ohms (full)

Factory cars came with a little clip on the sender tube which jumpered from the sender tube, over the length of the rubber connector hose, and clipped to the fuel line. If this is missing, devise a ground. One way is to take a fuel line clamp, scrape the sender tube clean, and wrap some clean stripped wire around the tube, then clamp it with the hose clamp. Run the other end of the wire to a bolt on the body.

If the sender resistance seems OK at the sender, hook up the wire and re--check at the connector in the kick panel. If that seems OK, time to pull the cluster

Some problems at the cluster include---


Bad instrument voltage limiter or bad "socket" connections where it mounts

Bad connections at the PC board harness connector, loose corroded connector pins

Bad connections at the gauge, loosen/ tighten the nuts a few times to "scrub" the connection clean

Of course the gauge unit, or the sender, depending on test results, can also be bad.
 
Thanks, I appreciate all the help that can get. I will check today as soon as I get off of work.
 
Works when it wants to is typically one of two faults.
First , The ground strap on the fuel libe loosing contact. That can make the gauge come and go with the weather.
Second is the gauge itself loose at the printed circuit board. The studs and nuts that support the gauge are also its contatcts. those studs have a square shoulder poked through another piece of board. That board distorts with age. The studs get quite warm with gauge operation. All this combined equals loose nuts.
You might be able to reach in from underneath with a disc type ratchet and 3/8 socket to tighten those nuts but dont over tighten then and chance breaking the forementioned board they are mounted in. Most importantly disconnect the negitive battery cable before reaching in there.
Good luck
 
I grounded the sense wire and it shot up to full. I did the ohm test on the sending unit and got 62.7 ohms which may be right. I only put $10 of fuel when I take it for cruise cause I don't want the gas to run out. I also tightened all 4 nuts behind the cluster. I even ran an extra ground wire and clamped it to sending unit. The real test is when I put gas and hopefully the guage will move.Any other suggestions on what I can try? I'll let y'all know when I put gas in wich should be in a 1hr 1/2.
 
I had the same issue...I ended up pulling the sending unit and found that the Copper float had cracked, would fill up with fuel and sink to the bottom of the tank. I let the fuel evaporate out of the float and soldered back up the cracks and voila it worked again. I know they sell replacement floats and the sending units themselves are pretty cheap but i had to get the car back on the road. Hope that helps.

-Jake
 
I filled up the gas tank and guage did not move. Instead I had a gas leak coming from pipe that goes into gas tank. Any other suggestions on what I can test?
 
You can measure the gage itself with a multi-meter and check against the factory resistance signals. Can you post a photo where it is leaking? Sounds like it might be the big rubber fill tube grommet ($12 ebay). Worst case is you will have to pull the gas tank, which is fairly easy if almost empty. Kill 2 birds.
 
It is leaking from the fill tube grommet. I will look on eBay for it. Thanks I will recheck it with my meter(ohm).
 
I could go through my achives and find pics to prove what I think is wrong with your fuel sender. You could ask Mullinax95 to confirm what I'm saying..
The sender has a moving contact that scrubs a risistor winding to create the different ohm values. Eventually the scrubbers wears through so the end becomes much like a fish hook. once it catches the resistor wire, thats the only value it can send.
One other common fault is a saturated float.
Either failt equals new sender.
I'll go a little further and predict the sender you remove will be shaped different from the replacement you purchase. Both designs work and both have their flaws. Nothing lasts forever. Hope this helps
 
I bought a new sending unit and a fill pipe grommet. I will install them when they arrive and let y'all know if it fixed the problem. When I grounded the sense wire, fill guage would shoot up to full right away.
 
Sounds like your fuel gage is working. Down the road, consider replacing the factory 5 V Vreg on your dash with an electronic one. Search "voltage limiter" and "Plymouth" on ebay, ~$25. Your factory one is a little silver box w/ 3 male spades. They are notorious for failing, sometimes taking out the fuel and temp gages ($$$). If you have a tachometer (Rally dash), your Vreg is built into your fuel gage.

When you get your new sender, you will likely find that the new lock ring that comes with it is too small to get a tight fit. Some bend the tabs out, but most re-use their factory lock ring. While you have your tank out, a great time to clean rust and repaint the body, plus the tank. Also shake out the junk that fell in (pens, wedding rings, ...).
 
I installed the new sending unit and fill tube grommet. The fuel guage works good. The old sending unit had a hole on the float causing not to float. Also filter screen was torn.Thanks every for yalls advise.
 
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