fuel gauge and fuel tank problems

[Prerunner1982]

some of you may know that i am taking a 12hr road trip on sat. i had planned on running a crossover fuel filler neck to the driver side. well i attempted to gather everything i may need to day, i looked at dropping the tank to clean out the grudge and crap since i havent used the tank since my wreck a year or two ago. I decided to check and make sure how much fuel was in there and realized that it wouldnt switch over to that side. i guess after not being used for 2 years it will go bad. I am not going to ask how to fix this here, i will do a search or Brandon can IM/email me and help me out. what i would like to as and i am sure i have asked before and i know a few others have, but i am gonna ask again. my guage reads 3/8ths of a tank as full, when i unplugged the connection to the valve for the tanks the guage read true full. Is this just a default type thing or what? if i remember correctly (though my user text will probably prevail ) a bad ground would make the guage read past full. Is the sending unit bad as it is only reading the driver side tank due to the fact i cant switch it over to the PS and check it that way. Could the bad valve have something to do with it, the guage has been like this for a while and i dont know how long the valve has been bad though i know it worked when could actually fill the PS tank. thanks for any help.

[SanitysBane]

my truck does the same thing, not a clue as to why

[blink32]

If I read that right it sounds like you have a bad gauge or secondly a bad sending unit. If its reading only 3/8's of a tank when you know its actually full, and when it only goes to the Full mark when the sending unit is disconnected it doesn't seem like the gauge is working right. When you drop the connections to the sending unit the open ground should make the gauge do a full sweep to its furthest passenger side. Mine have always gone 1/4-1/3 PAST the full mark which leads me to think its more the gauge than the sender. But it could be a combination of the two. I would imagine its cheaper/easier to get a new gauge to swap in than a sending unit to test.

[nn2004]

Same thing here. Seems to be a common problem. My gauge is stuck at full, but solely because all the valves for the aux. tank have been disconnected. However when I ground the fuel gauge (pink) wire with a test light, I get a 1/4 tank reading and the tank is in fact full. People keep saying to check the ground, but I dont think that is the case. It's gotta be the sender or the gauge, now just which one is it? Somebody has to have the answer seeming as everyone and their mama had this problem at sometime.

[SanitysBane]

on mine its the gague, i check both sendning units with an ohm meter, and then i put in another one that reads 0 at emtpy and 120 at full, it still only goes to its 3/8th spot. im thinking to get another fuel gague and slove the problem. sence my truck does it with the switch disconect all together.

[NeCrOmAnCeR]

To blindly say it is one or the other is impossible on just the descriptions on this board. Because of the rationale used when designing the fuel guage/sender on GM vehicles where 0 Ohms is empty, and 90 Ohms is full, any additional resistance caused by a dirty ground could cause a guage to read say 1/4 tank when it is actually empty. And any time the guage wire is disconnected will gause the guage to peg past full as mentioned. So if the tank selector valve is faulty or the wires are disconnected this will happen.

nn204 mentioned using a test light to ground the guage wire, while I understand doing this to try and prevent damage due to excess current, for our systems the additional resistance from the lamp in the test light may be responsable for the high reading. Try grounding the wire to see if the guage reads empty.

To check the ground(s), first check to see if the sending unit body is grounded. There is a small wire with a spade connector which connects to the top of the unit, and the other is bolted to the frame. Because both ends are exposed to the elements and road salts (etc) corrosion is very common. Because the sending unit is insulated from the tank by the rubber o-ring, only the lock ring acts as a direct connection, and again corrosion is common on 30 year old trucks. A properly mounted tank has "squeek" pads between the tank and straps so again the only true ground is the small wire. SO break out the multimeter and measure resistance between the tank and the frame, it should be less than 1 Ohm, and measure the resistance of the sender both full and empty to check it. Ground the guage and see if it shows empty then leave it disconnected to see it peged full. If the sender and guage check out ok, be sure to check the ground wire.
Sorry, the always present "check your grounds" statement will always be involved.

[nn2004]

It appears in my case it is either that ground, or the sending unit. Grounded the gauge wire - empty. Disconnect - past full. When plugged back in, reads 3/8 regardless of the full tank I do have. Mostly I was using a test light solely because all of my fuel system wiring below has been disconnected. They removed the left tank, and wired an aftermarket relay directly from the fuse panel to run the right tank's fuel pump. So after finding the right wires to reconnect by seeing which ones had any power, I hooked them up and now I get a 3/8 reading. Gotta be the sending unit or bad ground to the unit. So for future reference, does anyone know anywhere that carries these sending units .. cheap??

[SanitysBane]

you can have my 2 old sending units, very cheap. i got the same problem with mine, not a clue what the deal is, i think its just the gague. let me know us know if u find the cure.

[nn2004]

Are the units any good? How much are we talkin bout?

[SanitysBane]

one has a whole in the sock and i belive is the origal one, the other has a scrap at the bottom and was replaced some time. http://67-72chevytrucks.com/vboard/s...773#post889773 is the link to the furom that all my stuff is for sale at.

[86-C10-V6]

All the guages in these trucks have this problem. The guage has two coils that drive the needle movement. One coil has current flowing through the sensor, in this case its the 0-90 ohms from the fuel tank. All the other guages(oil pressure and temperature) use the same linear scale. The other coil has current that flows through a calibration resistor on the back of the guage. The resistor is a white creamic block with etched resisitive track and two connection pads. The resistor makes electrical connection through the nuts on the back of the guage. Because of the metal in the stamped nuts on the guage, the surface will tarnish and the resistor will stop drawing current. This is why the guage shows 3/8 when full. Remove the guage, then remove the two nuts and the ceramic block. Use a pencil eraser to carefully burnish the pads on the resistor, they should be bright shiny copper. The resistor should measure 90 ohms after cleaning, if it is open you can make one from parts from Radio Shack, just make sure you use a 5 watt 90 ohm resistor. I used 2 180 ohm 2 watt resistors wired side by side. I like to lightly sand the contact surfaces of the nuts. Re-assemble and re-install the guage.
Hope this helps.

[nn2004]

Ok, doesnt sound too hard. Thanks for the useful info. 1 quick thing - how the heck do you remove the instrument cluster?? I took off the whole dash pad and still could not get it out. I was affraid the whole thing would brittley shatter on me, so I gave in and looked at other things first. Is there some kind of angle these come out at??

[86-C10-V6]

With my setup, once you remove all the screws, I tilt the steering wheel all the way down, then push down and pull out at the top of the instrument panel cover.
Should be fairly easy if you have the dash pad removed.
Can't remember if you have to remove any knobs, my trucks at the tire shop so can't look at mine to compare.
Hope this helps.

[ocbaud]

wow, that trip really came up fast. it seems like only last week you said you were taking it.

[nn2004]

Mine doesnt have tilt wheel, but I'll have to give that a try. Thanks again.

[86-C10-V6]

I got my truck back from the tire shop and I can see where I normally remove the radio knobs and the light switch knob. You also need to slide up the gasket around the steering shaft and remove the top plastic trim thats on top of the steering shaft before removing the instrument cover.
Good Luck

[nn2004]

I can get the plastic bezel off w/no problem, but i dont see how the gauges come out.

[Russell]

Take a look around the outside edge of the bezel -- You'll see some silver screws you need to take out

I'd recommend only removing the gauges, and not the whole bezel, you're very likely to break a tab if you take the whole thing out... BTW -- this would be a great time to paint the back of your dash bezel with some chrome paint Just be sure to mask off the lights so you don't paint them up!