fuel gauge

[cwilson jr]

immediately after having my fuel sending unit replaced by a garage (i moved away from), my gauge dances and mostly reads way past full at bout the 3 o'clock position.

now, everything i have been reading suggests the ground.  my question is there something outside as a strap i can check first, or am i going to have to remove the bed to get to the sending unit?

this is on an 85 K10 with a 350 and 4bb.

[bd]

Assuming your referring to your 85 K10, a ground wire is affixed directly to the sender and bolts to the frame rail.  Perhaps the garage didn't tighten it down.  Why did you replace the sender in the first place?

[cwilson jr]

yes.  the 85 is correct.  fuel delivery was cutting out and they actually replaced the sending unit before the pump. 

[cwilson jr]

i'm not even seeing the ground at all.  that would be coming out of the top near the lines wouldn't it?  all im seeing is a line with a rubber grommet and i assume that is for the gauge read?

[bd]

i'm not even seeing the ground at all.  that would be coming out of the top near the lines wouldn't it?  all im seeing is a line with a rubber grommet and i assume that is for the gauge read?

There are a couple of different style sending units.  One style sender has a roughly 18" ground wire welded to the sender frame and a single insulated post to attach the gauge wire.  The other style sender has two insulated posts, one to attach the gauge wire, the other to attach the ground wire.  But, all styles have two wire connections.

[cwilson jr]

ok, took a look at this and i located the ground.  small black wire is connected to the post on the snap ring and also bolted to the frame.  appears it has solid connections.  really thought this would turn out to be the ground from everything i've read.

1.  what could i look into next?
2.  if i have to open up the sending unit, am i able to drop the tank or do i have to remove the bed?  (been hearing both)

[bd]

Temporarily ground the gauge wire at the sender and check the fuel gauge with the ignition switched ON.  The fuel gauge should read Empty.  If it does, replace the sender.  If the sender is under the cab, drop the tank.  If the sender is under the bed, raise and safety block the bed or remove the bed.  But, be careful of rusty and seized bed bolts.  There's a previous thread addressing this.

[cwilson jr]

ok, just checked the ground and actually temporarily even grounded it to the battery with no difference.  this was with the key turned on as well.

i did however notice moving the other single electrical line on the post on top of the sending unit, the gauge will sometimes read from the 3 o'clock position way past full to about a single line below the full line.  i can't seem to even get this reading constant.  i even made that connection tighter and this is still not the correct reading as i have no more than a 1/2 tank and probably a little less.
 

[bd]

...moving the other single electrical line on the post on top of the sending unit, the gauge will sometimes read from the 3 o'clock position way past full to about a single line below the full line.  i can't seem to even get this reading constant....

What 'other single electrical line?'  Are you referring to the ground wire?  Can you post a pic of the top of the sender showing the wire connections?  Sounds like a lot/all of your gauge symptoms are due to connection troubles, or maybe a broken wire inside the insulation.

[cwilson jr]

i followed the line under the plastic cover, but then lost it before it got dark.  is that accessible through the firewall and goes to the gauge i presume?

[bd]

You're correct.  The pink wire off the top of the sending unit is the signal lead to the fuel gauge. 

Since the symptoms began immediately when the sender was replaced, you probably have connection issues either between the pink signal wire and the sending unit post (your picture), or between the sending unit post and the rheostat inside of the fuel tank (internal to the sending unit).

Purchase a 47-ohm, 1/2-watt resistor from a local electronics supply, like Radio Shack (under $2 - see image below).  Remove the pink signal wire from the top of the sender and insert the resistor between the signal wire and a good clean ground.  The fuel gauge should register steady at a half-tank with the ignition switched ON.  If it does, replace the sender again.  You might be able to finagle the sender out of the tank, but you'll need to be very careful going back together, so as not to damage the new sender.

If the gauge isn't steady at one-half-tank, with the resistor still connected wiggle the pink wire where it enters the rubber boot while an assistant watches the fuel gauge with the ignition ON.  If the gauge is erratic between half-tank and 3 o'clock, you have a break in the wire where it enters the boot and you'll need to replace the wire end.

If the gauge indication is steady, but registers between half-tank and full, you may have a problem with the gauge, as well.

Be systematic and post what you find.

[cwilson jr]

ok, used the resistor and with it the needle slowly backs down to one line above 1/2.  im not sure if its the sender or it has to be exactly on the 1/2 to determine that.  if there is an issue with the gauge, i did follow the line all the way to the harness already in the cluster and there are several pink wires.  the haynes appeared to not have that information.

UPDATE:

I believe i have located the correct pink wire for the gauge lead.  when the cluster harness is plugged in and say you're facing behind the cluster, i am using the top right pin with two pink wires.

I ran a wire directly from the sending post to the cluster pin and it reads one line below full.  this is not correct.

SO, i then removed the sending unit, everything looked good and extremely clean as it's fairly new and verified that there is well below half a tank of fuel remaining in the tank.  i then moved the float and the gauge read accordingly so.  i carefully re-installed the sending unit and NOW, it's doing the same dang thing.  the needle reads one line below full.  i am really scratching my head at the moment.

it almost seems like when the sending unit is touching the tank it is being affected by something?  has anyone ever come across this before?

[cwilson jr]

any ideas?

[bd]

ok, used the resistor and with it the needle slowly backs down to one line above 1/2....

I believe i have located the correct pink wire for the gauge lead.  when the cluster harness is plugged in and say you're facing behind the cluster, i am using the top right pin with two pink wires.

I ran a wire directly from the sending post to the cluster pin and it reads one line below full.  this is not correct.

SO, i then removed the sending unit ... and verified that there is well below half a tank of fuel remaining in the tank.  i then moved the float and the gauge read accordingly.  so i carefully re-installed the sending unit and NOW, it's doing the same dang thing.  the needle reads one line below full.  i am really scratching my head at the moment.

it almost seems like when the sending unit is touching the tank it is being affected by something?  has anyone ever come across this before?

The "resistor test" verified that the gauge, and the wire to the gauge, are functioning correctly.

The "pink wire" you've identified for the fuel gauge behind the dash is the correct wire. 

Did you peer into the tank in all directions when you had the sender out?  Check if the float arm is hanging up on something inside the tank.  Verify whether the sending unit is "indexed" so it fits into the tank in the proper direction.  It's always possible they installed the wrong sending unit.  Or, perhaps, the original installer re-bent the float arm to more easily fit the sender into the tank.

Originally, I think, you had two problems:  (1) poor wire connection(s) to the sender forcing the gauge to 3 o'clock, and (2) a sender having restricted movement of its float arm because of interference in the tank and/or a misshapen float arm.  It seems you've resolved the 'connection' issue.  Now you need to figure out the calibration problem that makes the gauge read full with less than a half-tank.  I think you're on the right track to finding the solution. 

[cwilson jr]

well, i got her working and it appears there were the following two issues:

1.  while having the sending unit out i was moving some wires around and getting the needle to jump.  turns out the small plate where the ground wire is soldered was sticking out a little.  i bent it in and it corrected the grounding issue.

2.  however, when i would place the sending unit back in, the gauge would be working properly, but still not not reading in sync with the amount of fuel i had in the tank.  so, i took some measurements and bent the float arm accordingly and all is well.

i couldn't work on the truck for a while and also wanted to confirm this corrected the issue as well before posting.

thanks everybody and especially bd as u were extremely knowledgeable and helpful.

i hope this helps someone else out in the future and cuts out time spent on this.