fuel gauge reads 1/4

[setandreel]

I need a little input please,
87 silverado 4x4 350 tbi. gas gauge reads a quater when full,both tanks, but falls as gas is used to empty. I checked the wires and ground, all good, if I unplug the sendind wires the gauge goes to full this happens left and right tank. I have ordered new sending units from NAPA also fuel pumps( if I have to drop both tanks I am going to replace everything) Is this a likely problem? the 2 units and 2 fuel pumps are $500.00
Thanks
Mike

[VileZambonie]

I doubt you have two bad sending units. I would check the common feed up where it crosses over the LH frame rail. I've seen lots of them melt or get corroded there. Small amount of resistance there causes the gauge to read innacurate. If you unplug it all together it's infinite resistance and will go to full but that resistance in the wire there is probably your problem.

[setandreel]

the common feed is that the plug that crosses over to the pass. tank? the plug looked good, I did notice from the LH tank side of the plug 2 wires so I unpluged it and the LH tank side its female 2 wires ( can not remember the colours) the male end of the plug ( going to the pass side tank) only has one spade in it and one wire comming out, a dummy? I am stumped VileZambonie, the research I've done is mostly for single tanks ( can't find much on duel tanks) and everthing points to either sending unit or a hole in the float, but, like you said both sending units or both floats? unlikly, but I just can't find my problem. Is it possible one sending unit could cause the other to false read also?
Thanks for your time
Mike

[VileZambonie]

Both sending units carry the signal back to the gauge through the same wire. They just install a jumper harness for dual tanks. If you were to unplug where they connect and go over to the pass side of the fram you'll see the gauge move. I just removed the dual tank set up in mine and went to single tank so I just had the same problem in my 85 but as I followed the wire from the tank sure enough it had high resistance due to corrosion where some of the insulation had melted off. Just follow the wire from the LH side up to the gauge and inspect it for anything obvious. If it looks ok check for resistance in the wire.

[setandreel]

Do the  sending units go to the dash switch then to the gauge? also does anyone have a wiring diagram showing from the sending units to the dash?

[DnStClr]

According to the schematic from GM for my 87, the signal goes from the sending unit in the tank directly to the instrument cluster on a pink wire. I wish I was literate enough to add the wiring diagram to the post, but when I copied it and uploaded it to my Car Domain thing, the pic was automatically shrunk to an unusable size!  
On edit, I do have an extra wiring manual for the 87s that's doing nuttin but sitting on the shelf. You're welcome to it . My email  
dnstclr@yahoo.com

Don St.Clair
Culleoka Tn

Edited by: DnStClr at: 11/18/06 11:08 am

[setandreel]

Thanks all, turned out to be the shunt on the back of the gauge, new gauge and all is well, Don e-mail is on the way
Mike