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Bad ground in fuel gauge circuit?


Jrw420

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Hey guys i have a '94 ford ranger 2.3L 5speed Manual, the fuel gauge goes back and forth randomly, which tells me everything in the cluster is fine as Far as wiring? If so that leaves me a few possibilities;

-Bad ground in fuel pump or sending unit? I just bought this truck and thats pretty much what the guy told me

-The float is bad? Ive done lots of.reading on this forum and many others and a common consensus is that the brass fuel floats get flooded with gas if you will, and sink. But my gauge isn't always empty

-Bad sending unit? Internal error maybe?

Again ive scoured the.forums,.pulled my bed off twice, went through hell.with a few other issues and now.id like to know at least how much gas is in my truck. Ive been trying to find hot wires and grounds, test for resistance(ohms) because that's how the float works, but have no luck getting readings, i don't know where to connect into , i have haynes repair manual for.diagrams but still, someone please chime in!!
Thanks!
,john
 


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Spott

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It could be a bad ground, which would cause it to jump back and forth between the correct reading and 0. I believe there's a specific ground wire for the sender and pump; it doesn't rely on grounding through the tank body.

It could be a sensor with bad spots in it, which would only work fail at a few specific levels (or only work at a few specific levels), but it would develop those bad spots gradually over time. Your complaint send to be something that developed suddenly.

Erratic readings doesn't sound like a flooded float.

One other possibility is loose or cruddy connectors somewhere between the sender and the gauge. I'd start by chasing wires, connectors, and grounds, before replacing any parts.

There's likely not a hot wire to the sensor, just a ground and a resistance signal. (At least, that's how it is in the 1st gen trucks.) You should be able to get the wire colors off the wiring diagram in your service manual. If not, autozone offers free service info with registration, and it includes more than Chilton's or Haynes (but less than the factory manual).
 

Jrw420

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Thanks for your insight, thats what i kinda figured although i have no idea where to begin. Theres a yellow/white wire coming off the fuel pump to the fuel gauge, along with a black/yellow , black/red and just a plain black coming off the fuel pump.

There is an electrical connector beneath the master cylinder that has these wires. Other than that i can't find them, and i dont know how to test for grounds or how to make new ones. Im sorry, i know i sound very dumb and i dont have much of a clue.

I haven't introduced myself to the forums, but im John, im 17 and this is my first vehicle. I just fixed the parking brake to rear passenger side, and a.few other miscellaneous repairs, when it comes to electrical i might as well be trying to write a letter to the Japanese emperor. Not good

Also you said my complaint seems sudden. The guy said it started a few days before i bought it, which was roughly a week ago. On 8/01/15
 

TxGasRacer

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.It sounds almost like the anti slosh Module is causeing your problems and it is in the back of your instrament cluster and it just fits into a slot, it is a small printed circuit board that is supposed to keep the fuel gage from swinging wildly from empty to full while you are driving. https://www.youtube.com/watch?v=Rvk_A7yJg88
 

Jrw420

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Update:

Just got back home and pulled electrical plug on fuel pump. Tested the yellow/white and black/yellowires, i get 83.1ohms with my dvom, the wires are not hot, i want to ground to the black and yellow wire to the frame, what do you think?
 

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I think you need to look at a wiring diagram to figure out what each wire should be doing, before arbitrarily making a decision as to what you want them to do.
 

Jrw420

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I think you need to look at a wiring diagram to figure out what each wire should be doing, before arbitrarily making a decision as to what you want them to do.
Well i Did lol, i know it would be pretty dumb to do so. I grounded the yellow/black and i had no luck. Fuel.gauge was even more erratic, i got quite a shock from the key when i tried to start it. But.that wire wasn't hot so im a bit confused. I have no idea where to find more.diagrams, ive searched and searched with no luck
 

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Where to find wiring diagrams:

Register for a free account at autozone.com, then click on "My Vehicles" in the top center and add your truck. Hover over "Repair Help" on the right side of the orange bar at the top, and click "Vehicle Repair Guides". Click on "Chassis Electrical" in the center of the page, and then "Wiring Diagrams" in the left side-bar. You want diagrams 23 or 24 (24's easier to read, so check both even if you don't have the Cali engine.)

Looking at the wiring diagrams, here's what I see:

Pink/black is 12V to the fuel pump.
Black is ground for the fuel pump.
Yellow/white is the signal wire between the sender and the gauge.
Orange is the ground for the sender.

I don't see any anti-slosh module on the diagrams, the yellow/white wire runs straight from the tank sender to the gauge (I assume there's a couple of connectors in between, as well.)

It appears you may have different colored wires, for whatever reason, but if you can't tell for sure, you can pull the pump/sender assembly out of the tank and it should be easy to see which wires run to which pins on the tank connector.

You need a few things for that circuit to work properly:

1. You need constant "12V" from fuse 16 to one side of the fuel gauge. If that's intermittent (bad wire or connector somewhere between them) then your gauge will also be erratic. To test: Set your meter to 20V DC, and hook it between that post on the back of the gauge and a good ground, turn the key to "run", then see if it varies at all while the gauge bounces around. Read it again with the engine running, as well. It should be close to either 12v (engine off) or 13.8V (engine running) every moment that the key is in "Run". If not, trace that wire (Red/Yel) from one end to the other looking for damage and loose or cruddy connectors. Make sure the nuts on the gauge post are tight, if it uses threaded studs.

2. You need a working gauge. The gauge actually measures electrical current to get its reading, which changes as the resistance in the tank sender changes. To test: Don't do this test yet, it's complicated, so see if the problem can be found elsewhere.

3. You need a good signal input to the gauge. This comes up the Yel/Wht wire. To test: Turn the truck off and pull fuse #16 (it should be labeled gauges or panel or something like that). Get a long piece of wire. Set the meter to the smallest resistance (Ω) range. Hook one end of the loose wire to the "other" post of the fuel gauge and hook the other end of the loose wire to one probe of the meter (you basically extended one of the probes.) Pull the tank connector off and hook the other probe to the pin for the Yel/Wht wire. The meter should read close to 0Ω (or whatever small value you get when you touch the probes directly together.) If not, there's a bad wire or connector. Then, keeping the probe in contact with the tank connector pin, flex the connector and wires around, and rattle the wiring harness and connectors all the way up the chassis, including the dash connector, and see if the reading changes. If it changes, (while shaking or not) you've got a bad wire or connector, so trace that Yel/Wht wire until you find a bad spot or a loose/cruddy connector. Remember to replace fuse #16 when you're done.

4. You need a consistent resistance value from the sender, varying with fuel level. To test: pull the pump/sender assembly out of the fuel tank. Set your meter on resistance, on the smallest range that will give you a value with the float arm at the bottom of its travel. Hook the probes to the two connector pins for the fuel sender (for the yel/wht and org wires, according to my diagram) and very very slowly move the float arm from the bottom to the top of its range. You should get a smoothly decreasing reading on the meter, and it should never jump to an erratic value (This works better with an analog meter with a needle, but we work with what we've got). If it's erratic, then your sender is bad. Also, wiggle the wires between the sender and the connector pins and see if the value gets bad, if so, those connections are loose or bad (which may require replacement of the sender anyways, if they can't be tightened). If there's no measured problems with the sender, make note of the minimum and maximum resistance values you get at the ends of the float travel.

5. You need a good ground for the sender. To test: Set your meter on the smallest resistance range, and hook one probe to the ground pin on the wiring-harness side of the tank connector (orange on my diagram) and the other to a good frame ground (it must be clean and shiny, bare of paint, rust, and crud). It should read 0Ω (or the same as when you touch the probes directly together). If it's very high, or "open circuit", or gives you erratic readings when the wires are moved around, then there's a bad wire or bad/loose connector in the ground wire path somewhere, so trace it and check. Also, inspect the major ground points for the truck, there should be big heavy ground "straps" (either wide flat metal braid or fat wires) between the engine and the frame, the engine and the cab (probably at the firewall) the cab and the frame, and the battery and the frame. Each must connect to a clean bare spot and be fastened tightly at both ends. I don't think it's the main ground straps, because you'ld have more problems than just the fuel gauge, but it never hurts to check and maintain them.

If I were doing the work, I'd test #5, #3, #1, and then #4 in that order, based on likelihood of failure and difficulty to access.

After finding any problem, correct/repair it and then see if your gauge is still erratic, before moving on the the next test.

Let me know what you find for each test you do, and if everything tests out well, I'll come up with a method to test #2, the gauge itself.
 
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