You know, you might actually be on to something with polarity. Those old sensors have something strange going on around the contact pads that I've not seen on mine or any of the other posted pictures. Maybe there is something galvanic going on there. I would pay particularly close attention to the pinning of the connectors on both sides of the bulkhead plate. The blue (positive) side was soldered to the inner contact pad (the non-segmented one).

Edit: Also it sounds like you got senders from two different sources.

It might be worthwhile verifying that the sender circuit boards are indeed identical and measure maximum resistance with the float arms all the way up.

 

Yes, Rock had just one, so the other came from GM Parts Direct. They are identical part numbers. I used both of my old floats. I got the old ones out last night for pics. Today I got the meter out, set it to ohms, and connected alligator leads to the meter leads and cgecked both the old senders. They seem to work fine, with ohms/resistance changing in a fairly linear pattern on each sender. For the Hell of it, I reversed polarity on each sender. Made no difference in readings. Now, how the PCM sees things, IDK. If one side is reversed, and the other isn't, does that make a difference? IDK that, either. The shop manual doesnt address my stupidity/confusion/hypothetical situations!! I got about 43 ohms at "empty," and 198 at full. Interestingly, that 198 ohm reading was actually at about 3/4-7/8 travel. Going higher with the travel resulted in the ohms going to an entirely different reading, like maybe .26? It didn't change from there until the float ran out of travel (brim full tank). I attribute this as the reason why the gauge reads full in most any car for 80-100 miles, then the gauge will start dropping quicker in the next 100 miles. Meaning what we all know, that when your cars gauge reads 3/4 or 1/2 tank, it's really a bit lower than that, because the gauge won't move for the first 100 miles or so. Hope this all makes sense. If you read my earlier post, I mentioned I "cheated" a bit, because the left/fuel pump side was giving me trouble. So I just changed the sender on that side, and reinstalled the unit. The sender seemed to move freely, though, in both sides. Hope this all makes sense....

 

I also took pics of the old senders from a distance. Note the difference in the float arms. The float arms are from the new senders I installed, because I reused the original float arms on the new senders. Confusing, sure can be!! Whew!!!!!

 

Ok first question, did you transplant the little black plastic clip onto the new senders when you installed them? - the one that's over the central pivot point of the old sender in the above picture? This is a critical mechanical component of the sender unit and is responsible for keeping the wipers in full mechanical contact with the sender's electrical contacts, if that makes sense.

Edit: Wait a minute, that's my picture, complete with my home-made LS valve spring compressor tool on the right side. Dammit, how are we supposed to help you?

I think you're over-complicating things by interjecting all the crap you've read about this problem over the years. Wipe that out of your mind and start thinking methodically. You can't solve problems by thinking that it can't ever be this, because the bible (FSM) says that's not possible. Belief systems aren't compatible with systematic troubleshooting.

 

Yes, I did use the black plastic clips. Sorry for the confusion. I am trying.....HARD!! But those sender float arms look exactly like mine, the new senders were installed with the old float arms. I think you missed my point about the FSM. That was that they arent going to list hypothetical things, like me reversing polarity. I'll take a photo of my own new sender float arms, mounted on the old senders, and post them.......

 

OK, here are my new, unused sender float arms, mounted on the old senders....

 

Do you have the tanks open yet?

Did you invest any energy in the suggestion I made of logging the individual sensor inputs via ALDL (a scan tool)? That would give a definite avenue to explore, though you'll want to pull your fuel pump fuse before trying it with the tanks open.

I truly want to help, but it's difficult to do so when the recipient finds any and every way to resist my efforts.

 

I'm certainly not trying find ways to resist you!! I have no scan tool. I'm going to open them, then drain them this afternoon. First thing I'll check is whether the wiring is installed correctly. The only other thing I can do is make certain the float arms aren't impeded. I've had a lot of time at the Dr and dentist lately, going again tomorrow. I'm going at a slower pace than I did at 60 years old. Nothing I can do about it, so please bear with me.....

 

OK, turns out the right (Pass.) side is wired as it should be. This is a pic of my new senders w/ old float arm, just as it was installed in the car. It makes very smooth readings on the meter when moved. Next pic will be of the same sender, but a more distant shot in order to view the float arm.....

 

Here's the shot of the float arm...

 

I have to meet my daughter out of town today. First time I've seen her in 8 months! So I'm putting the sender back in, as I dont want the garage to fill with gas fumes while Im gone. I'll tackle the other one tomorrow. Thank you for all your help.....

 

I thought this paragraph #16 was VERY interesting. It is in the 2000 FSM, Volume #2, page 6-926, under engine controls. IDK how important this step is, but I haven't done it. The right side sender moves freely, and is reinstalled. No fuel yet. I have the float itself pointing inboard, towards the center of the car, and near the inner edge of the fuel tank, as that is the only way it can go......

 

I think I see your problem. That looks exactly like the float arm that was in the LEFT (DRIVER) SIDE tank of mine. The picture of mine that you re-posted a few days ago shows the original arm for the RIGHT (PASSENGER) SIDE that I originally posted here is still on the old, dirty sender. Go back to that and re-read carefully. The second picture I posted there shows the arms swapped. Notice how the foam float on the arm I was about to put back in the tank was wrapping around the jet pump, not pointing away from it like in your picture.

I suspect you just need to swap the correct arm on, and that what you have in it now is contacting the tank somewhere and can't swing all the way to the top, or is simply shaped in a way that doesn't give correct readings.

It's not important. All it's saying is the PCM won't reset the fuel system diagnostics if the fuel levels are the same as the last drive cycle. Filling the tanks to full will do the same thing. It has nothing to do with how the PCM calculates the summed fuel level signal that drives the gauge.

That's not how mine is installed. Note my picture here where I'm about to install the jet pump with the arm from the old sending unit now installed on the new sending unit. The foam float is pointing outboard, away from the tunnel. The arm with the double-bend near the pivot point that would be pointing towards the tunnel is the correct arm for the LEFT (DRIVER) SIDE and is only setting on the workbench for comparison. It's not the one installed in the tank because it's the wrong shape.

Edit: At the end of the job, the floats on both arms should be pointing away from the center of the vehicle (tunnel). I suspect you have yours reversed, and the floats are making contact with the wall of the tank impeding their travel at the top of the range. I know you say you checked the float travel is un-impeded, but how do you actually test that while having the pump bulkhead completely installed against the gasket / tank wall? The answer is you don't (without using a scan tool).

 

I hope you're right! If you are, IDK how in the flyin' f_ _ _ I managed to do that!!!! Thank you so much!! I thought the end of the float arm was mighty close to the edge of the plastic tank. My abilities are clearly waning. I can't imagine making that mistake 10 years ago. I'll take care of that, but it makes me wonder wtf I have in the drivers side???!

 

I could've sworn I only removed one sender at a time.....

 

Looking at your picture here, the float arms are swapped for the side they belong on.

I hope you don't think I'm trying to make you feel bad, but I hope I'm right because it's the simplest explanation for your problem. We're all human. Here's my least proudest moment - what happens when you have a piece of weld spatter stuck inside one of the oil feed holes of an LT1 crank that comes un-stuck. Thankfully it was round enough not to cause immediate catastrophic carnage.

 

I could've sworn I only removed one sender at a time.....

 

Wow. You got pretty lucky. I wonder where the weld bead came from? Well I just swapped arms. I found it interesting that as soon as the last bolt was loosened completely, the entire unit shifted away from the inside of the tank. Not much, but slightly, so I'm sure that was it. I've already swapped it, and reinstalled it. I'm going to forget the other side for now, and see if this works. I will say if I'd thought about this a bit, I would've caught it. Why? Because I had to order 2 new left side senders, as the right is now unavailable. That said, I should've then had a total of 3 left side arms, 2 new ones, and 1 old one, and 1 right side arm, from the old one. If that right side arm was on my bench when I was done, there's no way I'd have another, correct right side arm in the tank. Because all I bought were left sides. SO......I'm sure the correct arm is on the left side. It has to be. So I'm putting everything back together, and leaving the left side as is. I'll go to the gas station, and dump 6 gallons into the car. With the roughly 3 gallons in the can, this should give me slightly over half a tank on the gauge. If that checks, I'll fill it up. I'll report back Thank you very much.....

 

If you still have it up in the air and not too much fuel to drain out... if it were me I'd pull the left side out far enough just to confirm it has the arm with the double-crook bend near the pivot. Then if you still have problems you have my permission to panic (not really). Getting these cars on jack stands is enough of a job in and of itself, well I don't mean to undervalue your time but wouldn't tempt fate.

My bobweights came in higher than the stock piston + rod assy so weight had to be added back to the crank. Because it was nodular iron, the shop filled the original drill holes with high-nickel rods. That little bugger fell into the #7 throw oil feeder and stuck good. I spun it to 6900-ish rpm multiple times and it was smooth as butter so the balance job was perfect, except for this tiny piece of spatter. How it didn't eat itself when that broke loose is beyond me. Anyway, back on-topic... :-)

 

I'm lucky enough to have a lift. I will say this, though; it was easier to work on cars at 35 years old with jackstands than it is at going on 70 with a lift!! Anyway, no, it's all together. I'm gonna put the gas I have in it, then go to the station and slowly fill it, watching for gauge needle movement. I'll report back either way. If weather allows, as it's supposed to rain, and has been off and on. I'm keeping my fingers crossed....🤞