Need help locating grounding issue with fuel gauge

rangerdoc · 04-24-2011, 08:31 PM

I put my car back together last month. Last weekend I took it for its first cruise. The gauge was reading 1/2 so I filled it up. It took about 10 gallons and the needle was pegged. But since then the needle has not come back down. It should be no more than 3/4 after the driving. The fuel sender is new because it wasn't working prior to the car assembly. When I installed the sender the needle went from E to 1/2.

I grounded out the gauge with the sender disconnected at the tank and it pegs out, so I figure I have a grounding issue. But for the life of me I can't see the problem. I've put electrical tape on so many surfaces inside the gauges that there is nearly no metal left to look at. The gauge is original and has the little cardboard pieces on the inside and outside of the cluster. I have the washers on top of the circuit board holding the gauge in place.

Any ideas what to do?

Danimal · 04-24-2011, 08:42 PM

If you ground the wire at the tank and the gauge pegs, the wiring and gauge should be ok. Sounds like you have a bogus sending unit. Don't feel too bad, I also have a bad one I installed in my car. Gotta get another one myself.

slim · 04-24-2011, 08:51 PM

Sounds like It's the sending unit.

rangerdoc · 04-24-2011, 08:59 PM

Now that is dang frustrating! Especially with a new sender and a near full tank of gas!

Just curious...is it possible to put a sender in upside down and if so, how would it react? I don't think I did, but thought I'd ask as a possible reason. I guess the only way to know now is to figure out how the connector tube should be oriented. Anyone know?

Ponyman66 · 04-25-2011, 08:53 PM

Add my vote for a bad sending unit. The system actually works through a ground, being located in the ground leg of the circuit as opposed to being in the positive (or power), leg. The sending unit has a variable potentiometer (basically, fancy electrical term for variable resistor), which increases or decreases resistance to ground as the tank level rises and falls. One other check (with the symptoms and checks you've listed). Turn the key on to run and see if your needle still reads full. With the key still on, disconnect the sending unit wire and let it hang. The gauge should now read empty. Counting the check you've already done (grounding it and getting full deflection), you've now taken the system (minus the sending unit), through the entire range, from zero to infinite resistance. That would confirm the overall system is functioning properly and the sending unit is indeed faulty.

Regarding installing the sending unit upside down, I don't think you can. With that said, which direction is the hose fitting pointed? It should point up. Good luck with it.

rangerdoc · 04-25-2011, 09:29 PM

I'll have to go look, but I'm pretty sure it is pointing up.

Besides the fact that I have a near full tank of gas and the current sender is brand new, the worst thing about all this is I had already taken apart the dash looking for a grounding issue. That'll teach me to not post my questions first!!

Danimal · 04-25-2011, 09:33 PM

I took my gauges and dash all apart because none of the cluster backlighting worked, fuel gauge was inop and so was the temp gauge. Turns out I all two bad sending units and a bad headlight switch. What are the chances.

rangerdoc · 04-26-2011, 12:04 AM

Ah man...that is some serious bad luck having all that at the same time. I would have done the same thing. Funny thing is I had the same problems. Bad oil sender, bad headlight switch and bad fuel sender...though that was before I put the car back together.

rangerdoc · 05-30-2011, 01:52 PM

Sooooo...I've driven the car a few more times and now it looks like the fuel sender is working. I don't know if it just showed extra high with a full tank and once the level dropped it was able to give an accurate reading or if it is still not correct. I think I'll keep my gas can in the car in case the level is actually lower than indicated. Maybe when I get the level lower I'll drain it and see if the amount is close to indicated.

Is there any way to measure the accuracy of a fuel sender without draining the tank?

22GT · 05-30-2011, 02:29 PM

There are a lot of crappy sending units out there.

Nutz4mystang67 · 05-30-2011, 07:42 PM

Some times the repo float arms get bent in the box. I had to rebend mine to read right ....
Mine stayed on E for 78 miles untill I ran out. I pushed the car 1/2 mile to the station and it took 5 gallons to read full, I stopped and went home and pulled the sender and rebent now it works.

rangerdoc · 05-30-2011, 08:16 PM

Yeah, I'm going to be keeping a close eye on it to see what happens. I'll certainly be keeping my 2 gallon can full whenever I go someplace.

rangerdoc · 05-06-2012, 06:06 PM

Did you know you can't start the car when it is out of gas? I swear, it really won't start!

Took me a bit to finally realize the car was empty and I got the chance to change the fuel sender...a year later! But now the fuel gauge is accurate. The old new one that I put in appeared to have a hole in the float so it was full of fuel.

65BlkCoup · 05-06-2012, 10:17 PM

LOL, as I was reading though my first thought was a bad float. I just went through the same deal with a plastic float. Swapped it out for a copper float and all is working.