Fuel Gauge for 75 blazer question

[mraby11]

Does anyone know how to fix the fuel gauge on my blazer?? Its reading past the full mark. Ive removed the wire attached to the sending unit and the gauge drops further past the full mark, so I know Im getting power to the guage.

 

[dyeager535]

"Short" that wire to ground and see if the gauge goes all the way the other direction. If it does, everything forward (towards front of truck) of that wire is good.

 

[mraby11]

Thank you dyeager for the quick response. When I removed the wire, ithe needle dropped all the way and pointed directly at the floor board. When I grounded out that wire, it went back to its previous position, just past full. Any other suggestions??

 

[dyeager535]

Something with the gauge (and that could be dash connectors, etc) or sender wire.

IF the "forward" portion of the fuel gauge circuit is working, the gauge should go well past full with the wire disconnected (open), and with the same wire grounded, the gauge should go well past empty. Or vise versa, can never remember which. GM says you should wait a couple of minutes for the grounding/opening of the wire to take effect, but in my experience, once the needle stops moving, it's as far as it's going to go.

Pretty unlikely, but make SURE you are getting that wire grounded good. Might even want to make sure the frame grounds are good.

The gauge sender is a 0-90 Ohm setup, theoretically if you knew the level of the fuel in the tank, you could check resistance on the wire terminal on the sending unit and know if it was working correctly as well. But since the gauge isn't working properly, need to fix the known issue first.

 

[mraby11]

Ill give that a try. I think Im just going to drop the tank and check the sending unit. thanks again for your help. Its greatly appreciated.

 

[sweetk30]

also there is a direct ground wire from sender to frame rail. check to see if thats there and clean ground.

0-90 ohms is the working range . so if you had a few resitors you could test it . or good DVOM meter set in the 100 or 1k scale of resitance and 1 lead to tank or frame. then other to the sender pin .

and lots of the trucks of this vintage if the tank was full of fuel the gauge would be past the full mark. just a quirk thay had.

 

[dyeager535]

Ill give that a try. I think Im just going to drop the tank and check the sending unit. thanks again for your help. Its greatly appreciated.

You can drop the tank, but from what you described, it's not the tank that's the problem.

The tank sending unit gauge portion is essentially a large rheostat. As the float moves up and down, it makes contact on different spots of the wrapped wire, which changes the resistance to ground, which is what the gauge "reads".

When you disconnect the sending unit wire, you are tricking the gauge into thinking that the wiper (what the float is attached to, that wipes the rheostat to make contact) is so far off the rheostat it's not even touching it anymore. Conversely, when grounded, the gauge is tricked into thinking the rheostat has 0 resistance to ground. It is no different than hooking the sending unit up outside of the tank, and manually moving the float up as far as it will go, and as far down as it will go.

The rheostat CAN be cleaned (they seem to develop a black coating that doesn't conduct properly), but if the sending unit is in need of repair, replacement ones can be had for not much money.

If the sending unit was the problem, your gauge would max out either direction depending on whether the sending unit wire is grounded, or open, which would indicate the gauge and all it's wiring is good. All the gauge does is determine the amount of resistance to ground, and reflect that as a spot on the gauge face. Nothing more, nothing less. All the sender does is vary that resistance based on the float level.

 

[mraby11]

Thank you everyone for the help. I really appreciate it. Ive tried all the advice and nothing seems to work. Ive removed the ground wire and the needle points directly to the floorboard. I grounded out the wire from the sending unit and again, it points directly at the floorboard. Ive traced the wires down the frame rail and didnt see another ground. Ive tested the wire with a test light and it lights up, pretty dim, but I know its getting power. Once I put everything back where its suppose to be, it goes right back to where it always is, about a 1/2 inch past the full mark. I tried attaching pics to show what Im talking about, but apparently thats too advanced for me.

 

[JDNobodi]

Sounds like something wrong with the gauge. Grounded or open, it should read full or empty. It should not read the floor board.
However, it sounds like the gauge is moving when ground and unground the sender wire. Maybe the needle is on the wrong position on the gauge and needs to be reset.

 

[dyeager535]

You aren't understanding the circuit properly.

Putting a test light on the sending unit wire doesn't tell you anything useful. It is NOT a power OR ground circuit, it's the signal circuit.

Power and ground for the gauge to read correctly is handled AT THE GAUGE. You have to disassemble the dash to properly test those.

As poor as the instrument panel connector is (copper on a flimsy plastic backing) contact issues are a very common issue.