Topic: Gas gauge 61 GMC truck

[Gurp]

Stays on full
Thought it was a bad ground.
re grounded the sending unit no change. any suggestions?
new tank and sending unit.

[jgger]

I believe the sending unit in the tank should put out different voltage as the float are raised or lowers. Maybe  put a multi meter on the output wire and operate the the float by hand. If that gives you a change in the reading then trace the wire to the cluster plus check the guage.

[scottly]

The sender is a variable resistor, with one end grounded, and the other end connected to the gauge. When the sender is at the empty position, the resistance to ground is high, and the gauge reads empty. When the sender is at the full position, the resistance to ground is low, and the gauge reads full. With the sender wire disconnected, does the gauge read empty?

[Don R]

Also ground the gas tank float mount? (sender)

[Gurp]

The sender is a variable resistor, with one end grounded, and the other end connected to the gauge. When the sender is at the empty position, the resistance to ground is high, and the gauge reads empty. When the sender is at the full position, the resistance to ground is low, and the gauge reads full. With the sender wire disconnected, does the gauge read empty?

even when it was ungrounded it read as full.

[BigJimG]

The sender is a variable resistor, with one end grounded, and the other end connected to the gauge. When the sender is at the empty position, the resistance to ground is high, and the gauge reads empty. When the sender is at the full position, the resistance to ground is low, and the gauge reads full. With the sender wire disconnected, does the gauge read empty?

even when it was ungrounded it read as full.

Ungrounded (open), on an old GM truck should read full, shorted sender (zero ohms) will read empty.  To check fuel senders here is the ranges for popular vehicles

Empty - Full

Ford up to 1986 - 73-10 Ohms
Ford 1987 & up - 16-158 Ohms
GM up to 1964 - 0-30 Ohms
GM 1965-1997 - 0-90 Ohms
GM 1998 & up - 40-250 Ohms
Mopar up to 1986 - 73-10 Ohms
AMC 1950-1977 - 73-10 Ohms
Autometer -240-33 Ohms is the most common however other ohm ranges are made
Classic Instruments - 240-33 Ohms (excluding vehicle specific gauge kits which use factory ohm range)
Dolphin - 0-90 Ohms
Dakota Digital - Programmable to work with most Ohm range senders
VDO - 10-180 Ohms

Make sure you're getting proper voltage to the gauges and sending unit.  Its been a while since I've worked on old GM's, but old Mopars (especially trucks) are notorious for the voltage limiter going bad and frying the gauges.

[Gurp]

Thank you Jim this is the info I feel will do me the best!!