fuel gauge testing

[babydumpling]

Hello new at this so wish me luck. I have a 96 Ranger regular cab,3.0 with auto trans. Fuel gauge stays on empty at all times, thinking it is the sending unit float. The manual shows the Yellow/White wire going from send unit to cluster. Was going to put a ground into it at the 16 pin connector in front of tank on inside of left frame rail. My thought was that the ground would send the gauge to full like we use to test in the old days. When I split the connector apart the gauge went to full with the key on . I did not have to ground the yel/wht wire. Am I thinking right that it is in the sending unit and the gauge is ok? Or is there another way to do this testing? Thank You, Andy D.

 

[RonD]

I would ground the Yellow/White wire and see if gauge goes to the opposite end, just to be sure gauge is working as it should, but it does read like it is.

Yes, probably the sending unit, float(hole in it) or anti-slosh circuit in the sending unit.

Most Fords after '89 used sending units with 16 OHMs empty and 160 OHMs full
So grounding(0 ohms) should show Empty on gauge
Disconnected(infinite ohms) would show Full

100 ohm resistor from ground to Yellow/White wire(gauge) should show about 1/2 tank


You could hook up an OHM meter to the sender wire, if it shows 14-18 ohms then float is probably at fault, below 14 ohms is probably the sender at fault.
Doesn't really matter since all should be replaced once you get to the tank.
Drop the tank or pull off the bed.

 

[babydumpling]

Thanks Ron I will do some OHM reading and let you know. If it is in tank I will replace complete unit even though truck runs ok that pump is 16 years old. It sound like it is in the tank just did not want to do a lot of labor for nault. Thanks,AndyD.

 

[babydumpling]

Hello Ron , going to do that testing today. Now let me see if I understand this. In the old days if you took off the sending unit wire at the tank and put it to a ground the gauge would read full. Now you are telling me that by just disconnecting the wire it will read full? Remember when I disconnected that 16 pin harness plug that disconnected all wires to the sending unit,though the other wires I would think would only be for the pump. If I am doing something wrong it shows that I have gray hair and cars have changed! Thanks Again, Andy D.

 

[RonD]

Each car maker has their own best way to operate gauges but that changes over the years, lol.

Resistance gauges are pretty simple and tend to last a long time so are still used.
Water temp and fuel level are still resistance gauges on most cars/trucks.

What changes is the way they are wired, high resistance(or open circuit) can show either Full or Empty depending on how it is wired, on your year the fuel gauge will go to full on high resistance(or unplugged wire).
So unplug the wire and gauge goes to full, ground the wire(0 resistance) and gauge goes to Empty.

On another model/year car or truck gauge could do the opposite, Full when grounded and Empty when disconnected.
Testing is the same.
If a resistance gauge only stays at one end, disconnected or grounded, then the gauge or wiring would be where the problem lies, not in the sender.

Don't read to much into these types of circuits they are pretty simple.

Water temp gauge can be tested the same way, doesn't matter if it goes Cold or Hot when wire is unplugged or grounded, as long as it does go both Cold and Hot when both ways are tested, open and grounded.


Often manufacturers will look at past failures of senders, i.e. do they fail more often "open" or "closed"(grounded)
Then they make the gauge circuit for best driver reaction.
If fuel sender fails would it be best for gauge to show Full or Empty?
Empty would be best so driver doesn't run out of gas on the road thinking tank is full.

 

[babydumpling]

Thanks Ron, It was the hole in the float deal. Replaced all and all is fine. Thanks Again for all the education and help. Andy D.

 

[RonD]

Thank you, and Good Work on getting it fixed