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#877591 - Sat Aug 25 2012 04:21 PM Fuel gauge reading backwards?? HELP!  
loadmaster235  Offline
Wrench Fetcher
Joined: Dec 2006
Posts: 51
Athens GA
49 GMC 1 ton
Converting to 12 v
235 6 cyl
Original fuel gauge and sending unit
Running power through a resistor.


Okay you wizards of all things old trucks- I need your help! I recently tested my fuel tank sending unit and my fuel gauge to make sure they both worked before I install them in the truck. As info, I did check the sending unit,which came out of a 49 GMC 1 ton. It "ohmed" as you would expect throughout the full range of motion. They were hooked up as follows:

The sending unit was hooked up directly to the hot side on the fuel gauge. Power came directly from a 12v battery through a voltage reducer, to the power side of the fuel gauge, the ground wire came off the battery to the ground connection on the sending unit and also grounded to the back of the shell of the instrument cluster. Please note, I read every entry I could find in the tech tips section of the forum regarding the correct way to hook up a fuel gauge and sending unit. I certainly did not want to ruin either my sending unit or fuel gauge by hooking them up backwards.

Here's the problem, when everything is connected, and the sending unit is resting on empty, the fuel gauge is sitting in about one quarter full. However when I activate the sending unit to read a full tank,the fuel gauge needle actually moves toward the empty side of the gauge. In other words, the fuel gauge seems to be operating backwards! Please help an ol boy figure out what's wrong.

Thanks in advance

Last edited by loadmaster235; Sat Aug 25 2012 04:24 PM.


#877593 - Sat Aug 25 2012 04:39 PM Re: Fuel gauge reading backwards?? HELP! [Re: loadmaster235]  
Flxible  Offline
Boltergeist
Joined: Feb 2002
Posts: 16,437
north of the 49th
just reverse your connections at the gauge, won't hurt anything and will no doubt read correctly .... the sender hooks to the ground side of the gauge, the battery hooks to the "hot" side .... if the gauge is inaccurate you can just live with it, knowing what the gauge says when the tank is actually empty, or you can try bending the needle

Bill


"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir
"When we tug a single thing on an old truck, we find it falls off" ~ me
Some TF series details & TF heater pics & Rust-a-holics Unanimous parking lot

#877604 - Sat Aug 25 2012 05:55 PM Re: Fuel gauge reading backwards?? HELP! [Re: Flxible]  
loadmaster235  Offline
Wrench Fetcher
Joined: Dec 2006
Posts: 51
Athens GA
Thanks. Question. I thought of reversing the terminals at the gauge, but that red tag on the back says hooking the power to the wrong post will ruin the sender unit?


#877678 - Sat Aug 25 2012 08:30 PM Re: Fuel gauge reading backwards?? HELP! [Re: loadmaster235]  
Flxible  Offline
Boltergeist
Joined: Feb 2002
Posts: 16,437
north of the 49th
"The sending unit was hooked up directly to the hot side on the fuel gauge. Power came directly from a 12v battery through a voltage reducer, to the power side of the fuel gauge"
again, the sender provides the ground that makes the gauge work, it is not/can't be hooked to the "hot" side, which is what would be the battery or positive side - the only way the sender would be damaged is maybe if you run a full 12V thru it to ground it might fry the resistance in it - when testing just don't make 'permanent' connections, have the battery + wire in hand and just touch it to the post to watch the reaction

what are you calling the "ground connection on the sending unit"? the sender is grounded by it's mounting flange screws, the wire terminal on the sender is the wire to the gauge .... the circuit is >
battery + to the + terminal on the gauge;
battery - to the gauge case and sender case;
sender terminal to the 2nd terminal on the gauge

someone else might have a better idea of why a gauge would work backwards, but to me if it reads backward, the wires are on backward

Bill


"When we tug a single thing in nature, we find it attached to the rest of the world" ~ John Muir
"When we tug a single thing on an old truck, we find it falls off" ~ me
Some TF series details & TF heater pics & Rust-a-holics Unanimous parking lot

#878818 - Wed Aug 29 2012 04:17 PM Re: Fuel gauge reading backwards?? HELP! [Re: Flxible]  
k10  Offline
Shop Shark
Joined: Aug 2000
Posts: 2,071
NH
GMC = positive ground. You are now running a neg. ground. Fix it fast or blow it up forever.


#879682 - Sun Sep 02 2012 10:36 AM Re: Fuel gauge reading backwards?? HELP! [Re: k10]  
sps  Offline
Shop Shark
Joined: Sep 2008
Posts: 186
joppa , maryland
Thats what I thought, GMC was a positive ground. Might try soldering the send wire on the other side of the resistor of the send unit, just a thought.


#879728 - Sun Sep 02 2012 03:02 PM Re: Fuel gauge reading backwards?? HELP! [Re: sps]  
bartamos  Offline
Master Gabster
Joined: May 2005
Posts: 5,093
State48 Ranch
Also always run a short wire from one of the sender mounting screws to a bare frame point. Many tanks are isolated from ground due to strap pads and rubber hoses. The factory TF assy manual shows a ground connection as I described. Just scribble out the F and put E and scribble out the E and put F. Just kidding.


1960 GMC K10 Shortbed Wideside BBW CC BBV6 (The Oreo SpeedWagon)
Is it me, or am I weird?


Moderated by  Rusty Rod 

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