Fuel Gauge Woes

[cebra]

I built a new wiring harness from headlight to taillight after a small fire and wired in a 1992 Blazer fuse panel and some add-ons (tach/gauges/wiper module/radio/LS harness feed/VSS/etc). Anyways, I cannot figure out what is up with my fuel gauge. I first wired it as seen in the pic...(looking from the back) Switched 12V to the right post and sender feed to the left post. Then I realized the bottom/middle post is a ground and needed to run a ground to that. So I have these 3 wires hooked up and it went from below empty (before adding the ground) to way past full and is stuck there. It should be ~1/2 tank of gas at the moment. Any suggestions?

Side note, I am running a mid-90's S-10 sender and it worked fine before the rewire less 1/8 tank was E and an inch of the sweep went past full when it was full. I was fine with this as I just want to know when I am running low as I use 1 tank of gas every 1.5 days. Thanks.

[cebra]

I have power to the 12V, ground on the sender wire, and ground on the bottom wire. I spent a long time to run all those wires back to make sure they were hooked up correctly. Also, the same power wire runs to my brake light and that works fine and the same ground wire runs to my cluster lights and those work fine.

[VetteVet]

Since you hard wired your cluster you don't have the circuit board but the stock truck cluster has the power feed for the fuel gauge and the temp gauge wired together like this:



This is power from the no, 3 terminal which is key on power for the fuel and temperature gauges and the sender wire for the fuel gauge is the no. 4 terminal on the fuel gauge and the no. 6 wire for the temperature gauge. these sender wires must be stand alone wires with nothing added. Looking at the panel from the back the power wire is on the right and the sender wire is on the left.

I don't know how the !992 blazer fuse panels are wired but if you look at the gauge cluster, it looks like the 67 to 72 trucks gauge cluster
I hope you can sort it out with my diagram and explanation.

The sender wire should be the one on the left (brown/yellow stripe. I assume the two tan wires on the right terminal are power wires for the fuel and temperature gauges. If so they would be correct.
I read that you grounded the two gauges to the third terminal post on the gauges but I also read that you grounded the sender wires which you can't do or the fuel gauge will read empty and the temp gauge will read full hot. If you do not connect the sender wire on the fuel gauge to anything but the sending unit in the tank, the gauge will read way past full like your picture shows. The sender wire for the fuel gauge goes to the stock fuse panel on a terminal there and then on to the fuel sending unit in the tank.

If that wire is grounded some where the gauge will read empty, if it is not connected to anything but the sending unit or if the sending unit and tank are not grounded the gauge will read way over full. This may be your problem. Take the wire off the sending unit (key on) and take the reading, should show way past full, now ground the sending wire, it should read empty, Put it back on the sending unit terminal, If it reads way past full, run a wire to the sending unit flange and see if it reads correctly.

What ever you do, don't put 12 volts on the sender wire to the tank sender.

[VetteVet]

Here is a diagram of the 88 to 98 GMCs with the fuse panel and the instrument panels shown. You may have to enlarge it to read it. Compare it to the 1992 panel you have.

[cebra]

Great info to digest, thanks. On mine, the right terminal is switched 12V, the left terminal (brown/yellow wire) is to the sender, and the bottom/middle is ground on mine. The brown/yellow runs clear to the sending unit bypassing the fuse panel link (could this cause an issue), pink/tan looking is 12V switched from fuse panel, ground is just hooked to a ground block on the firewall. If I pull the brown/yellow wire and put a light tester to it from a power wire, it shows that it has ground through it.

"if it is not connected to anything but the sending unit or if the sending unit and tank are not grounded the gauge will read way over full." I will check the sender ground to see if I have good ground there. Can you clarify by what you mean by "if it is not connected to anything but the sending unit", are you saying if it is connected to the sending unit but the sending unit is not grounded? I am baffled as it all worked before my new harness but my wiring seems correct, I will test by pulling the sender wire and grounding it to see if gauge goes to E. Thanks again.

[VetteVet]

Originally Posted by cebra

Great info to digest, thanks. On mine, the right terminal is switched 12V, the left terminal (brown/yellow wire) is to the sender, and the bottom/middle is ground on mine. The brown/yellow runs clear to the sending unit bypassing the fuse panel link (could this cause an issue), pink/tan looking is 12V switched from fuse panel, ground is just hooked to a ground block on the firewall. If I pull the brown/yellow wire and put a light tester to it from a power wire, it shows that it has ground through it.

"if it is not connected to anything but the sending unit or if the sending unit and tank are not grounded the gauge will read way over full." I will check the sender ground to see if I have good ground there. Can you clarify by what you mean by "if it is not connected to anything but the sending unit", are you saying if it is connected to the sending unit but the sending unit is not grounded? I am baffled as it all worked before my new harness but my wiring seems correct, I will test by pulling the sender wire and grounding it to see if gauge goes to E. Thanks again.

You had a lot to sort out so I tried to cover everything to save on postings,but that's alright. I wasn't sure how you had the gauges wired but I think I see it now. The tan wires are key on power to the fuel gauge, temp gauge, and brake light.

The sender wire for the fuel gauge (brown/yellow) runs from the gauge to the fuse panel on the 67-72 trucks and then on to the sender. That connection on the fuse panel is just a junction and nothing else goes there.

The wire for the sending unit goes on from there to the sending unit and it is the only wire from the gauge to the sending unit. If you have it straight to the sending unit you're fine.

Now if you pull that wire off the sending unit it is connected to nothing and the gauge should read past full. If you ground that wire then the gauge should read empty. It sounds like you're reading past full, which means that the wire is not going through the sending unit resistor to ground. That's why I suggested that you check the tank and sending unit for a good ground.

The test light check means to me that the sending wire is grounded through the sending unit and the tank which is correct. The sending unit is supposed to be zero to ninety ohms so if it is higher than that it would show an open circuit on the sending unit wire. This would be the same as having it disconnected. I'm not sure what the S-10 unit is but didn't you say it worked fine before?

I'll wait for your next post to see where you're at. VV

[cebra]

I tested the sender post and realized I had power on it with it unhooked (the case doesn't have power running through it). I then pulled the little white plastic piece off and replaced with 2 rubber washers and this cured my power to the sender post. I tested all 3 wires and had power on the power wire and ground on the ground wire and sender wire. Lastly, I put a pure ground wire to the sender post (leaving the power/ground wires hooked) and the gauge did nothing. I have 65 ohms across the power/sender posts and nothing on the sender to ground and power to ground. I am thinking that (highly coincidentally) during the rewire my gauge died out on me? I do not see anything incorrect about my wiring and since the needle does not move with (looking at the back) 12V to right post, ground to bottom post, and ground to left (sender) post that the gauge must not be working.

[VetteVet]

Originally Posted by cebra

I tested the sender post and realized I had power on it with it unhooked (the case doesn't have power running through it). I then pulled the little white plastic piece off and replaced with 2 rubber washers and this cured my power to the sender post. I tested all 3 wires and had power on the power wire and ground on the ground wire and sender wire. Lastly, I put a pure ground wire to the sender post (leaving the power/ground wires hooked) and the gauge did nothing. I have 65 ohms across the power/sender posts and nothing on the sender to ground and power to ground. I am thinking that (highly coincidentally) during the rewire my gauge died out on me? I do not see anything incorrect about my wiring and since the needle does not move with (looking at the back) 12V to right post, ground to bottom post, and ground to left (sender) post that the gauge must not be working.

How were you checking the sender post for power? If you used your multimeter and then put the red lead to the sender and the black lead to ground you would see 12 volts because the sender is a path to ground for the fuel gauge. The white plastic piece is called the resistor for the gauge and it protects the gauge internal coil from too much current. It must be on the gauge between the two terminals.

Try putting the gauge like it was before and test the gauge by grounding the sender terminal for empty reading and removing the sender wire to see the past full reading. If you still get no movement on the needle then you may have burnt the coil wiring inside the gauge.

[cebra]

I was testing the wires with a light tester for ground and power. I removed the sender wire and put ground to it and the needle does not move to empty (or at all) from well past full (3 o'clock). I think I must have burnt it up as before I removed the resistor the power was going through the gauge or resistor from the power post to the sender post. Thanks very much for your help, I think the gauge is not functioning and will order a new one and hopefully it is resolved.

[cebra]

Thanks for your help VetteVet, I had my gauge hooked up correctly but I guess I must have burnt the gauge up somewhere doing my makeshift wiring harness conversion. I finally received my replacement gauge from backorder and once replaced everything is working fine now. Some good information here on fuel gauges that may help someone.