Fuel Guage help

[65-72 chevytoys]

Last weekend I found the break in the wire that caused the fuel guage to go past E i.e. no resistance. Spliced the wire back together and connected the tan wire into the fuse block. Now the guage stays on empty after putting about 5 gal in the tank. I started tracing that same tan wire from where I found the break to the back along the frame rail. I could not find anywhere it was grounding out, but I could only go to where the wire went over the top of the tank. I could not reach in between the blazer tank and bottom of the bed to where it connects to the sending unit. Anyone know how to check the wiring or the fuel sending unit without dropping the tank? I want to check of the wire is grounding out or if the sending unit is bad, but I can't get to it. Any ideas?

[Critter]

IIRC, 0% resistance (completed circuit) would cause the guage to go to full, actually past full. If you take the the tan wire and ground it out anywhere along the frame it should move the needle to full if everything is working right. When it was broke in two, (100% resistance) It caused the needle to go to empty.

So I would ground out the tan wire as close to the tank as possible and if the guage goes to full it is your sending unit or a connection right at it, etc. If nothing changes, then you have wire problems farther up the line closer to the guage or the guage itself. I had a similar problem and after trouble shooting I found it was the guage. Changed it and everything worked fine. Hope this helps.

[Jtrux]

Originally Posted by Critter

IIRC, 0% resistance (completed circuit) would cause the guage to go to full, actually past full. If you take the the tan wire and ground it out anywhere along the frame it should move the needle to full if everything is working right. When it was broke in two, (100% resistance) It caused the needle to go to empty.

So I would ground out the tan wire as close to the tank as possible and if the guage goes to full it is your sending unit or a connection right at it, etc. If nothing changes, then you have wire problems farther up the line closer to the guage or the guage itself. I had a similar problem and after trouble shooting I found it was the guage. Changed it and everything worked fine. Hope this helps.

I hate to admit it, but he is pretty good at electrical stuff, not great or anything by any means but good. I'd take his advice if I needed it but I never need help so that's not a problem.

[Critter]

Originally Posted by Jtrux

I hate to admit it, but he is pretty good at electrical stuff, not great or anything by any means but good. I'd take his advice if I needed it but I never need help so that's not a problem.

I hate YOU AND YOUR AVATAR!

[Jtrux]

Originally Posted by Critter

I hate YOU AND YOUR AVATAR!

I actually laughed out loud when I read that.

[65-72 chevytoys]

Thanks, I found the break in the tan wire that was causing the disconnect in the circut and spliced it back together. Now the guage reads empty, so I think it is grounded out somewhere. I traced the tan wire along the frame to the tank, but did not find any grounds. However I cannot reach the fuel sending unit to see if it is gournded there too close to the bed. I think I may have disconnected the fuel sending unit from pulling on the tan wire because the guage went back to over full. Any ideas on how to get to the sending unit without dropping the tank? If not how hard is it to drop the tank?

[Critter]

Originally Posted by 65-72 chevytoys

Now the gauge reads empty, so I think it is grounded out somewhere.

The opposite would happen. The tan wire should be thought of as a negative or "ground wire". The way the system works is that the gauge has a constant 12V power to one side of it. The other side (the tan wire) is the negative. Basically the tan wire runs from the gauge back to the sending unit to complete the circuit. The sending unit varies resistance and regulates the amount of "ground" that gets to the gauge. When the tank is full the sender is at top and giving 0 resistance which is considered a good closed circuit showing a full tank. When the sender is down it creates 100% resistance and the tan wire doesn't get a ground. This causes and "open" circuit and the gauge goes to empty.

So keeping this in mind, if the tan wire is grounded out to the frame, this would complete the circuit (before it had a chance to get to the sender) and send the fuel gauge to full. If the wire is broken fully in two that would cause an open circuit and send the gauge to empty.

If the gauge is stuck at empty you should be looking for a broken wire, not a grounded out wire. That is why I suggested for you to PURPOSELY ground the tan wire and see if the gauge goes straight to full. Hope this makes sense.

[65-72 chevytoys]

Originally Posted by Critter

If the wire is broken fully in two that would cause an open circuit and send the gauge to empty.

If the gauge is stuck at empty you should be looking for a broken wire, not a grounded out wire... Hope this makes sense.

This is confusing me because the exact opposite is happening. The guage was at full at the 3 O'clock position with disconnects in the tan wire. I started tracing the tan wire and found a break and I fix that. I also found the tan wire at the fuel sending unit was disconnected and I fixed that. I traced the wire back to the cab to make sure nothing was grounding out and that was ok. After those two fixes no breaks in the wire and reconnected to the sending unit the fuel gauge stays on empty even after putting 16 gal in the tank. So what sould the gauge read when the circut is connected empty or past full??

I want to make sure I have the wiring figured out before I start replacing the sending unit or gauge.

[jorgensensc]

Maybe it is working correctly. Maybe your float is bad. Just ground the wire first before dropping the tank. And no you can't remove the sender without dropping the tank. It's a pain, but it may need to be done.
Shawn

[cparman]

IF you could get to the connector at the tank, and disconnect it, you should have a TINY BIt of current. The way to check this is to use a test light. ground the light, and touch the ice pick end to the connector, or where ever the break in the wire is. If you look carefully you will see a VERY DIM blinking light. That will tell you the everything IN FRONT of you is ok. a sunk float can wear you out.

[65-72 chevytoys]

I dropped the tank and it was completely full so it must be a float problem, but I put my meter on it anyway and had about 8 ohms. I don't know if this is normal or not. It should have a high resistance when the float is full but I don't know if 8 ohms is high enough or not. It should have low resistance when empty or if the float is sunk.

[cparman]

I am going to go check. 0-90 empty to full I think?? be back soon

[cparman]

Cannot find what I want quickly. I am ALSMOST sure the a GM sender is 0 OHMS at empty and 90 OHMS at full. I will keep looking, but ------. I would guess the float is sunk, you will know when you remove the sender, and shake the float.

[65-72 chevytoys]

Even before I dropped the tank, I suspected a bad float. After checking the readings that confirms it for me. Thanks for the info.

[RLONG]

GLAD TO FIND THIS THREAD SO OK WHEN I HAD A BROKE WIRE THE GUAGE WAS WAY TO THE RIGHT OF FULL WHEN REPLACED SHOWED 1/8 OF TANK FILLED UP THE TANK NO CHANGE THEN AFTEr A WHILE OF DRIVING AROUND STARTED JUMPING AROUND AND BACK TO 1/8 [ I THINK WHEN I CHECKED CLOSE TO THE TANK SHOWED 6 OHMS BEFORE FILLING ANY IDEAS