So when I purchased my truck about a month or so ago, I was told that the fuel gauge was not working. When I turn on the ignition, the needle immediately goes al the way to full... and then some. I purchased my truck from a gentleman in my neighborhood, so I drove it home without a problem. Put about 2 gallons of gas in it the other day to insure I had enough gas to get to the gas station and fill it up. Last night went to fill it up and for some reason, I was only able to get about 1.5 gallons in. The pump acted like my tank was full, automatically cutting off and even back splashed a little after I tried to reposition the nozzle. So now I am wondering if the tank is measuring accurately.
Back story, the gentleman I purchased the truck from was no mechanic at all, and would always just take it to repair shops, well about a year ago, he had the fuel tank replaced, looking at it in my truck, it's a shiny silver tank. So when he told me the gauge was not working, I thought surely it was not the sending unit as I would assume that was replaced when they put in a new fuel tank, but you never know. I assumed it was a bad actual gauge or ground.

So my question is....... How can you test if a tank is full without being confident in the reading you are getting? I thought possibly of "knocking" on the fuel tank to see if it sounds full.... but not sure.
Seems like the gauge is functioning at the dash, because it does move, but seems strange that it moves past the full mark, then again, I remember my first car in 1987 (57 Chevy) did the same thing when I filled the tank.

Any thoughts are suggestions are appreciated.

James

 

What you are seeing is one of three things:
1) The tank actually is very full. The gauge needle will normally go past full when it is completely full. (Your only being able to fill in a gallon or so supports this.)

2) The tank is less than full but the sending unit or gauge are reading off.
3) There is a short in the wiring to the gauge and the sending unit wire is grounding. When the sending wire is grounded, the gauge will peg over 'full'.

Easiest way to test is to unplug the green sender wire (on mine there is a plug on the driver's side front of the tank.) If when you do that your gauge drops to empty, most likely you just have a full tank.

One thing to check is how thick your tank is... if the PO replaced the standard tank with one of the big 38 gallon tanks (you can tell the difference by the thickness of the tanks, the 38 gallon tank is about a foot deep) you may have all kinds of fun trying to get a decent reading from your gauge.

 

Houston Dave,

Thanks, so I am going to look this evening, I need to get under the truck and on the drivers side front of the tank, I will see a wire to unplug, any chance of there being some kind of photo of what this should look like? Or is it obvious, like only 1 wire under there to unplug?

James

 

On mine there is a rubber-covered plug pretty much all by its lonesome as I recall. Green or possibly green with black stripe wire, heads off to the middle top of the fuel tank. Essentially your fuel sending unit is just a resistor in the line from hot to ground; as the float goes up and down your resistance varies from (I believe) 70 to 20 ohms. When you ground the hot wire there is no resistance and the gauge needle goes all the way up past full.

 

Its pretty simple, on the sending unit there is 1 pig tail connector, take that off and then look at the gauge.



Heres what the connector looks like<br/>

 

Gotcha, so the black wire will be grounded somewhere, the yellow wire will lead up to the cab and eventually the Fuel Guage and the black cap is the part to take off that is on top of the fuel tank. Is this assessable without removing the tank? from what I remember, the top of tank appears to be right up under the bed of the truck, would I have to remove the spare, then unstrap the fuel tank to get to this connection?

James

 

Is this the rear tank or aux tank thats mounted on the drivers side?

 

If its the rear then yeah its mounted on the top under the bed so you would have hope and pray that the strap bolts will come off easily.

 

Yea, it's the rear tank. The tank is less than 1 year old, so I am pretty confident that the bolts would not be rusted in place and possibly easy to remove. Tough job though for one person that is working out of their driveway with just a floor jack.

James

 

And if it is truly full, it will be heavy and awkward as well. But you'll know pretty quickly too.

If I was a gamblin' man, I'd just throw a can of gas in the back and drive it. If it goes 200 miles you know it was full!
If it sputters and dies in 20 miles, you can then fiddle with it's bits then.

The reason I use the gambling metaphor is that in the old days this was not a big deal, even for someone with just one tank. You just pull to the side of the road, pour gas in and away you go. Nowadays with roads so crowded (depending on where you live) and fuel so crappy, and fuel pumps so prone to failure if you look at them sideways, and trouble re-priming a weak or worn out pump, this little roadside fill-up segué could turn into a full blown overnight adventure!
Hah! So I'm maybe taking a little poetic license here. But since we really don't know the condition of the truck because you just bought it, you never know what's going to be next on the list.

Congrats on the new-to-you truck too though, by the way!
And if you have both tanks on yours, then all of the above is just talk. Once it sputters you flip the switch and keep driving on the other tank.
Assuming you put gas in it of course!

Good luck.

Paul

 

Paul,

I think I will try that first, I have a little 2 gallon gas can I can put in the bed while I drive around, surely it will move eventually.
Right now I am going through plenty of things popping up on this truck trying to get it Road worthy. First the carburetor, Then all 4 shocks.... thought I was ready and realized my right turn signal was not working, fixed that and my backup lights do not turn off (Still need to address that situation) and last night when I drove to the gas station, I noticed some slight looseness in the steering and a shake. I have yet to take it over 40mph so who knows what to expect then. I have a feeling a tire balance and rotation is in my future, at the minimum. The truck sat in a driveway for about 6 months so hopefully the tires did not rot at all.

James

 

No need to try and drop it yet, you can get up under the bed and see the top of the sending unit and just use a manly grip and pop the sending unit plug off, just 2 pins.

"Disconnect the wire that goes to the fuel sending unit at the tank. Ground that wire to the frame, making sure the frame is clean. Turn the ignition to the "run" position and watch the fuel gauge. If the gauge swings all the way to full, either the sending unit itself is bad or it has a bad ground. If the gauge stays on empty then either the gauge is bad or there is a wiring problem.

To pop the plug off for the test, you can barely get up in there with your head and see the plug. While looking at it, VERY CAREFULLY use a screwdriver to pry it off. AND I MEAN VERY CAREFULLY. It is only pushed on like a 1/8". This is only if you can’t get it with your hands.

Once you have done the test (above) and decide it’s the sending unit. And you have cleaned the ground and that did not work. Then disconnect the ground again and fuel line and read on….

If you can't get either one disconnected, no worries you will have a little more room once the tank is dropped down a little. But make sure you have some wire slack in the plug connector.

Run the tank dry or almost or siphon it empty, I would not do it with ½ a tank of fuel, but if you just have to, just use a big floor jack and a piece of 2x2 plywood. It’s not heavy at all, unless you have it full or 1/2 full of gas. JUST SIPHON DRAIN THE TANK!

Loosen the filler and vent hose clamps at the tank end and maybe you can pop the lines off? But most of the time you have to just loosen the clamps and wait till the tank is about out.

With the jack in place, or not, just loosen the hdwr (2 bolts) on the front straps, and remove the tank straps aft hdwr all together, then swing the straps out of your way.
Slide the tank to the dvrs side about 1/2" and then pull out or let the passenger side come out of the frame. Then disconnect the elec connector and eng feed fuel line if you could not get to them earlier.

Then slide it as far as you can to the right and pop off the filler/vent lines. Sometime you need to carefully put a flat tip screwdriver in between the rubber hose and the tank to break it loose. The filler neck and vent lines get stuck to the metal pretty good sometimes. Once the tank is removed there is a lock ring that comes off with a screwdriver or punch, look for a small tab that has been bent over to lock it. Un-bend it, tap off the lock ring, remove the sending unit.

Insp the underside of the tank where the support straps rest, lots of dirt and moisture build up there, and that causes the bottom side corners to rust out. You can also come reinstall time replace the anti-chaff material. Old 2" ratchet strap work great, glued to the support straps.

Insp the inside of the tank, might be a great time to replace it all together? Before you reinstall anything connect the electric connection and ground to the sending unit. Wire it up under the truck so it is not hanging by the connection. And with the key in the 1st (on position) you should be able to move the float arm and see a difference in your gauge?? It is always easier to do this with a helper.....

Also before install, paint the complete tank with some good paint. Even spray on bed liner or undercoating??"

 

Referring to my earlier - when I say to unplug the sending wire, I do NOT mean to remove the two-pin plug from the sender on the tank!! At least on mine, the hot wire to the sender (yellow in the pic above, I seem to recall green) has a connector plug near he front of the tank. Much easier to disconnect THAT and run a jumper to ground. Like I said - if you unplug that wire and your gauge goes to empty, then when you ground the wire the gauge goes to full - most likely you just have a full tank of gas.

 

Ok, I'll try to locate that wire and jumper, hopefully be able to take a pic and get more advice, I'll be honest, I am not really even sure on how to run a jumper to the ground, what do you mean by that? Bear with me, I am a novice when it comes to working on these trucks.

James

 

Meaning just stick a grounded wire into the plug (the part coming from the front of the truck will be a female plug; ie. it will stay insulated even when the connection is unplugged - good safety feature to prevent unintentional grounding if it is accidentally unplugged.) You can use pretty much any old scrap of wire - make sure one end of it is getting good contact inside the plug and ground the other end to the frame. If you want to feel really good about it, scrape the crud off the frame there to assure good contact. An invaluable tool for working on anything is a longish piece of wire with insulated alligator clips on both ends so you can do things like bypassing much of a circuit to test things like washer pumps, or even test light bulbs by connecting one end to the + on your battery, holding the side of the bulb against the - clamp, and touching the hot wire end to the contact on the base of the bulb.