I know this issue has probably been discussed a hundred times and I have read many threads and watched tons of video's on this, but I really am not sure what to do next. I have a 72 Oldsmobile Cutlass Supreme. The Fuel gauge started reading way past F a while ago. I unplugged the Tan Wire in the back of the trunk and grounded it, the gauge then went to E. So I thought I had a bad Sending unit. Last night I dropped the tank and replaced the Sending unit and reconnected all the wires. I put fuel in the tank and now the fuel gauge is stuck on E. I disconnected the tan wire and the gauge goes straight to Full. I really don't want to have to drop the tank again. Anyone have any suggestions, thank you for help.

 

Drop the tank again.

By disconnecting the tan wire and the gauge going to F, you have verified that the wiring from the gauge to the rear of the car does not have a short. But as soon as you reconnect the tan wire, you have a short in the line because the gauge goes straight to E. The only places that short could be are in the wiring from the connector to the sending unit or the sending unit itself. A pinched wire? I don't see how you can investigate this without dropping the tank and examining the sending unit and the wiring. Whatever you do, don't put the tank back into place without performing the test for a short first. Make sure there is no short and that the gauge reads as it should depending on the position of the float, then put the tank back making sure not to pinch any wires.

 

Thanks for the reply, one thing I wanted to note, when I disconnected the ground wire, nothing happened and the gauge stayed on E. I would have expected it to go to F. Does that mean anything? I will try and see if I can drop the tank down enough to slide my hand in the there and make sure the tan wire is connected all the way. I don't think it's pinched anywhere, but I'll need to look further.

 

Your short to ground sounds like its between the guage and the connector. Before you drop the tank again try troubleshooting that sectionof wire.

 

Do what Jaunty explained in post #2.

 

No. The exact opposite.

He said this:

This test verifies that the wiring from the gauge to the rear connector is fine. The problem is between the rear connector and the sending unit or the sending unit itself.

 

true grasshopper but then he added this

To understand whole problem one must read all replies

 

No. YOU go back and read the replies. He said he disconnected the GROUND WIRE, which I presume means the ground wire from the sending unit, which is the only ground wire there is. This says nothing about the wiring from the gauge to the rear connector. By disconnecting the ground wire, he SHOULD have created an open circuit, but he apparently didn't. This only points more strongly to the short being in the wiring from the connector to the sending unit or the sending unit itself.

 

Well, I"m beyond frustrated now. I dropped the tank again, pulled out the sending unit, hooked up the tan wire and grounded it, I moved it up and down and the gauge went from E to F like expected. I figured that I just didn't have the tan connector pushed in all the way into the Sending unit. So, I put the tank back in position, pushed down on the little black connector into the sending unit until I was sure it was snug and connected to the sending unit. I then, bolted it all back up, connected all black hoses and the ground wire, put about 4 gallons of gas in and guess what........

IT'S STILL STUCK ON E!!!

I moved the tan wire around to make sure it's not pinched anywhere but I have no idea why this is happening, originally I figured it's because it wasn't connected all the way to the sending unit, but this is ridiculous. I know the sending unit is functional because I tested it.....Do you think the float doesn't work?

 

That seems to be about the only thing left. The only thing worse than taking the tank down two times is taking it down three times, but it seems like that's what you'll have to do. You could take out the sending unit and dip it into a bucket of gasoline and see if the float moves up. If it doesn't, I would try another sending unit even if this one is brand new.

 

Before you take it back out put some more gas in it. It may be like my repop fuel sender is, doesn't go all the way to full when it's full and goes to empty way before it's empty.

 

Yeah, I may try that first, I only put in 5 gallons as that's what I had in the can.

 

I missed this comment earlier, and Greg Rogers may be correct. I've worked on gas tanks on two cars in my time ('67 Delta 88 and my current '78 Toronado). The tanks are similar in shape. The fuel pick-up pipe extends down from the top of the tank, and it does not touch the bottom. It's about a half-inch or so off the bottom. If you multiply that half-inch by the cross-sectional area of the bottom of the tank, you get a volume of about two gallons. This is what I calculated for the '67 Delta, and I doubt it's much different for the '78.

This means that the gauge will read empty and the car will run out of gas when there is still two gallons of fuel in the tank. That two gallons can never be gotten out. So when you put four gallons of gas in your tank, only about two of those gallons would be sensed by the sending unit, and that's not much gas for it to sense. Given the relative slop in typical fuel gauge readings, you probably need to fill the tank at least half way before you can be sure that the sending unit is or is not working. I wouldn't be surprised if you discover that the sending unit does, in fact, work, and that it has always worked, but you just never put enough gas in the tank to truly test it. Just my thought.

 

Thanks, yeah the more I think about it that may be issue. I know for sure that my original fuel sending unit was messed up as it was showing way past F, and now that is no longer the case. I'm going to filler up and see where it's at then just go from there.

 

Kirk, the fuel sending unit in your Olds is probably grounding against the body of the car when you install the tank. This happened to me in my 71 98. Check out this thread:

https://classicoldsmobile.com/forums...71-98-a-73750/

Since the fuel sending unit doesn't reach the bottom of the tank you can use fuel rated hose with a hose clamp to extend the fuel pick-up to the bottom of the tank. Just make sure you cut a notch in the end of the fuel hose so it won't "suck against" the inside of the tank. This is a common issue in Ford Bullnose trucks from the 80s. I read about the work around on Ford websites but haven't done it to my 86 F250 yet. No reason it shouldn't work for an Olds too.

 

This would make me nervous. If this were a worthwhile thing to do, why didn't Olds include something like this from the factory?

There's a significant advantage in NOT having the fuel pickup line reach to the bottom of the tank and that is that you don't suck in the crud that invariably builds up in fuel tanks over the years. My guess is that this is exactly why tanks are designed NOT to suck the last drop of fuel out of the tank. Yes, there is a filter on the end of the fuel pickup line, but that can become clogged if too much crud gets sucked into it, and the filter can wear out over time. I think the fuel system is designed so that the in-tank filter stops any crud from getting into the fuel line and has it fall off the filter to the bottom of the tank. In other words, the filter is designed to stop the crud but not trap it. This would be defeated if the fuel pickup were continually sucking on the bottom of the tank as that crud would be sucked into the filter over and over.

I would leave the fuel pickup line just the way it is. What's more important? Sucking that last drop of fuel out of the tank and running the risk of a clogged fuel line, which will require dropping the tank to fix, or living with the fact that your as-advertised 21 gallon fuel tank has a usable volume of 19 gallons? I'd take the latter.

 

Good point, and also remember that gasoline is lighter than water, so any water in the tank will be right on the bottom.

 

Good point! Another reason you don't want to be sucking on the bottom of the tank and probably another reason why the system is designed so that it doesn't suck on the bottom of the tank.

 

Thank you everyone for the help. I added another 5 Gallons to my tank and the needle finally moved to a little over a 1/4 of a tank. So just not having enough fuel was my issue with the new sending unit. I'm glad I didn't have to drop the tank for a third time.

 

Excellent!

 

Yeah it was a little irritating to find the way these repop China senders work. Now I have been using my Cutlass I notice the fuel gauge never goes completely to full, even when gas is dribbling out the fill tube. It goes up to about 7/8 and starts to go down rapidly as driving. When it was down to about 1/4 according to gauge I filled it and it would only take 7-8 gallons. My guess is when it reads empty there probably is still as much as 1/2 tank or about 10 gal. Glad you got it !

 

Excellent!

 

The most common brand of aftermarket fuel tank and sending unit I see when I search, including for a '72 Cutlass Supreme, is Spectra Premium. This company is based in Canada with manufacturing plants there. I don't see any mention that their parts are made in China. Even if they were, it doesn't really matter where in the world parts are made as a reputable company will want to be sure of the quality before putting their brand on it. Many people trust the AC/Delco name even though many parts sold under that name are made in China.

 

Yeah I guess I kinda put all this repop stuff together and labeled it "cheap stuff from China", That was not correct, and not fair. I stand corrected...