1974 fuel guagr sending unit

**toolman0114** · 06-12-12, 05:53 AM

I replaced my fuel tank with a kit that came with everything. Now the fuel guage isn't working, not sure if i now have a wiring issue or a problem with the new sending unit.
My question is:
There are 2 wires connected to the unit, one on the outside edge connected with a flat blade terminal, it's black, is this ground?
The Tan wire in the crnter of the unit has a rubber boot over it, should this wire have voltage to it?

**TimAT** · 06-12-12, 11:43 AM

As you suspect, the black wire is the ground and the tan wire is for the sending unit. With the key on, and the tan wire disconnected, the gauge should go full scale to the right. If you have a 12v testlight, it will show some voltage- the bulb won't be full bright, but it wll glow. And with the testlight in there, the gauge will be somewhere around mid scale. Full ground on the tan wire will put the gauge full left scale.

**toolman0114** · 06-12-12, 04:56 PM

Originally Posted by TimAT

As you suspect, the black wire is the ground and the tan wire is for the sending unit. With the key on, and the tan wire disconnected, the gauge should go full scale to the right. If you have a 12v testlight, it will show some voltage- the bulb won't be full bright, but it wll glow. And with the testlight in there, the gauge will be somewhere around mid scale. Full ground on the tan wire will put the gauge full left scale.

Tan wire disconnected, IGN on - dash needle goes past full
Ground wire removed - Needle goes below empty
tester between tan & black - it lights up, not dim, but not bright, needle goes to about 1/4 tank
I'm suspecting bad sending unit.
What fuse does this hook up to? my short fuse at the bottom of the panel is dead on both sides of the fuse & fuse is good.
What is that little fuse for, it is marked "inst" but my dash lights work when the headlights are on. The courtesy light under the dash and rear compartment light don't work when the door is opened. Do these work off that fuse? if not, which one.
All this stuff worked before I had the car repainted, and fuel tank changed

**TimAT** · 06-12-12, 08:11 PM

Good start. Since the gauge does respond, now it's about accuracy. Check the resistance between the post on the sender (tan wire connection) and a good ground on the tank. GM sending units are usually between 0 and 90 ohms.
I'm going from memory so if I'm backwards, someone will step in and correct me.

Full tank is 0 ohms resistance and full is 90 ohms.

The bulb in the testlight is giving a little bit of resistance, so the gauge is reading 1/4 tank. IIRC, an 1893 bulb in a testlight is about enough to get 1/2 reading on the gauge.
Since your gauge is OK, per that test, it's time to look at the sending unit.

That little short fuse I don't recall what it's for. I keep thinking it's instrument lights, but I don't know for sure.
Usually the courtesy lights are on the same fuse as the clock. Owners manual or GM shop manual will tell you for sure.

**toolman0114** · 06-13-12, 09:44 AM

Since the gauge is responding I assume it has power and not the fuse. I am thinking the sending unit is faulty. Will test resistance again tonight.

**toolman0114** · 06-13-12, 09:24 PM

There was some issue with the sending unit. The tank was about 1/4 full but showed empty. I went through some testing procedures and it seemed to be working. I filled the tank to almost full, and the guage reads between 1/2 - 3/4. So it is working, but not accurately.
I guess the good thing is that when it reads empty, I'll still have 1/4 tank. Is there a way to adjust the accuracy?

**madvette74** · 07-02-12, 06:14 PM

Originally Posted by toolman0114

There was some issue with the sending unit. The tank was about 1/4 full but showed empty. I went through some testing procedures and it seemed to be working. I filled the tank to almost full, and the guage reads between 1/2 - 3/4. So it is working, but not accurately.
I guess the good thing is that when it reads empty, I'll still have 1/4 tank. Is there a way to adjust the accuracy?

I replaced my gauge when it craped the bed, the gauge was way off, so I thgough I would change the sending unit, that didn't change a thing, changed the gauge again still off by A half of tank. I am making this a winter project