Skydog9

I could not find any thread that mentions my concern. I have a 72 Cutlass S. I need to replace my fuel tank sending unit. The gauge reads past full. Checked the wiring and confirmed that the sending unit is bad. I have not dropped my tank yet. According to the service manual, it mentions a special tool to remove the retaining cam. Is this something that is necessary?
And can I get one from the local parts store?

matt69olds

There is a special tool to remove the locking ring from the sending unit, but it’s not absolutely necessary. A brass punch and a hammer will knock it loose 9 times out of 10. It’s pretty simple.

Fun71

Are you certain the sending unit is bad? From your description, I would say that you did not really isolate the issue to a "bad" sending unit. A loose connection will cause the gauge to read past full - that happened on my car and all it took was tightening the contact on top of the sending unit so it made good electrical connection.

joe_padavano

This is how I've always done it. Just walk around the ring, tapping on each of the tabs in succession. That's more important when you're reinstalling it.

Skydog9

What I did to confirm that the sending unit is bad is. I disconnected the connector in the trunk. I grounded the wire going to the gauge and the gauge went to empty.
Hooked the connector back up and the needle goes way past full. Disconnected the connector again and checked for any resistance on the wire going to the sending unit. I am reading an open on the wire going to the sending unit. That told me it has to be the sending unit. Am I incorrect?

66SportCoupe

Depending on things they may read above full. However replace the unit is easy with tank down. Take drift and unseal unit replace and install same way. Not difficult at all.

jaunty75

You also have to check the other wire coming from the sending unit, which is the ground wire. This connects to the body somewhere up under the car usually forward of the gas tank. Where it's connected can easily become corroded, and if the connection is poor or non-existent, that will give an open circuit as well. Fixing that connection, if it's bad, could solve your problem. Otherwise, then, yes, you've done all the external tests you can do, and it's time to drop the tank.

olds 307 and 403

These seem to go bad fairly often, not really any different than EFI pumps to change. Pretty sure mine just read Empty but the gauge now works great with a new sender.

Jungle Cat

That ground on bottom of floor as mentioned is a real trouble maker. It’s common not to be able to get that screw off it’s so corroded. I’ve resorted to drilling a new hole.

Fun71

What you did was confirm an open in the circuit somewhere between your test point and ground. As already stated, an open anywhere from your test point to the sending unit, within the sending unit itself, then to ground, would result in the same symptom.

My experience on two different cars has been the connector for the wire that you tested where it attaches to the sending unit. On both cars, the connector was loose and not making a good electrical connection. Simply tightening the connector by squeezing it with pliers corrected the issue.

Skydog9

I sure hope it's as easy as being the ground wire. My tank is nearly full presently. I'll give an update as to what I find. If not the ground, I'll also check the connection to the sending unit before I go about replacing it. Thanks for that tip Kenneth.

Run to Rund

Classic Industries in southern Cal has the best price I have found on the special tool. I got one and it worked well on my 66.

Allan R

I also have a 72 Cutlass and went through the process of dropping the tank and replacing it. Yes, I replaced the sender too (with a brand new faulty one) which means I ended up dropping the tank 2x. Obviously drain the tank first so you don't have to worry about having to balance a fully laden tank. Gas is heavy.

The tank strap bolts could give you grief especially if they are rusted. Soak them in a penetrating oil (WD 40, PB Blaster etc) for a day before you attempt to undo them. Be prepared when the bolts come out because the tank will immediately drop at an angle due to the filler neck. (I'm going on the premise that you've already undone the electrical connections and grounds) If your tank straps and bolts are crusty, consider replacing them. They might clean up with a wire wheel.




With the tank out check the top if you've got a Fremont or Arlington car. You might just find the build sheet!

I didn't have a brass drift to undo the locking ring on the sender. I used a steel punch and gradually worked my way around the tabs to get them moving. Spray some PB or WD on the ring if it looks horrible. Might help reduce the friction. I know that a brass drift is preferred but honestly people use screwdrivers too. The big thing they worry about is steel causing a spark and kaboom. I've done steel tools on 4 different tanks and never had a problem with them exploding, so your call.






Do test the new sender for resistance using an ohm meter setting on your multimeter. Make sure you get the values up to 90 on the swing arm. Also test the float in a small jar of gas. Make sure it floats. I've seen some new ones that sink a little. Also be prepared that the new sender might have a ground wire that's either too short or is attached 180° to the direction you need. I ended up removing and re-soldering mine. This sender tested good, but after installed it crapped out and had to be replaced. Even new parts can fail unexpectedly. BTW make sure the grounding surface is good when you attach the wire, and don't be afraid to coat it with either brake or dielectric grease when done.



Work methodically and don't be afraid to get help removing and installing the new tank. Good luck, it's not a really hard job, just kinda smelly.

Skydog9

Allan,
I appreciate you relaying your experience in dealing with the sending unit and all the pics you provided. One thing I don't understand is the picture of the sending unit with 2 lines coming out.
From all my research relating to the sending unit for my 72 Cutlass, from what I have found my vehicle calls for one 3/8 outlet with no return line. Is the picture of the sending unit with 2 lines coming out for another vehicle?

Allan R

No, it was a mistake ordering and was replaced with the single 3/8" pipe which is the right one. My car didn't have AC or HD cooling when it was built so it came with a 350 2bbl - probably what's in yours?. I believe the Cutli that had C60 (AC) also came with different cooling, and had 2 lines; the main one and a return one. That's easy to figure out just looking at the fuel pump. I apparently wasn't smart enough to look there back in 2012 when I first pulled the tank out. If your Cutlass S is stock as with all original factory 2bbl build (no significant upgrades) the single pipe is your weapon of choice.

It's really not a difficult project. Who are you getting your sender from? The one in my car is a replacement from RockAuto made by Spectra Premium. Not expensive but the float looks a lot different. Bottom line though is it works and is accurate on the fuel gage.

EDIT: BTW I see you did the same thing I did with the grills. I like the look of the 442 grill much better than the factory S grills.

Skydog9

Thanks for the clarification on the picture Allan. Yes my Cutlass S originally came with a 2bbl. It came with the 4bbl when I bought it. And I do have AC. I will go with the Spectra sending unit if and when I do have to replace my sending unit. I haven't dropped my tank yet. I need to do some driving to burn up most of the fuel in the tank. I am running non-ethanol gas and the cost of it is going out of site. I just noticed it yesterday for $3.64 gal To much to be putting it in my lawn mower. One thing I found in the service manual is that no 350 cu. in A-body has a fuel return line. I am hoping I have the same problem Fun-71 had with his Cutlass, that it's either the connection to the sending unit or the ground.