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Discussion Starter #1 (Edited)
Here's how you can return your original sending unit to as-new performance, get a REAL empty, and not have to add on any gimmicks like gas gauge correctors and whatnot.

I did this in mid 2007 on two B-bodies and it's still working perfectly on the original fuel pumps (with 149k and 163k miles) as of April 2012.

First, drop your tank and pull out the sending unit. NAPA carries an internal harness you will want to buy and install while you're doing this. It's less than $20. Airtex also makes one, AIRTEX Part # WH3000.

BEFORE you do anything else, remove all the rust off of the outside portion of your sending unit and paint it. I prefer to bake mine in the oven for 4 hours at low heat (170F) which really does a nice job curing the paint. Do not go any hotter than that or you will burn the paint. Where they sit in the vehicle, these sending units get no breeze and so they are very susceptible to corrosion because dirt, salt, and moisture just sit up there.

Okay, now that it's rustproofed, let's refurbish it:

Using an ohmmeter, ohm out your sending unit (purple and black wires) to see what it reads full (float up) and empty (float down). Factory spec is 90 full and 0 empty. Low fuel light comes on at 10 Ohms.
Mine was not doing well:



This is caused by a combination of things:
1. Lousy connections filled with gasoline varnish
2. corrosion and burn damage on the ends.
3. The stock GM resistor setup isn't always calibrated right. You'll see what I mean as we go.

Note that my Dremel was dead at the time so I did a lot of cleaning via "scraping" with a jeweler's screwdriver. You use whatever method you want to clean these metal areas I show in the pictures.

Look how heat-damaged my fuel pump wires were. See the insulation melted back?


Take the sending unit apart with a Torx bit:


Current is conducted through the spring to the resistor. Don't lose the spring.


Start cleaning every contact:
Dirty wiper: (be delicate on the wiper)

Clean wiper:


Clean the rotator connection any way you can. Here I'm pointing to it with a jeweler's screwdriver:


Fine sandpaper works well:


Clean where the spring mounts to the assembly:


Clean the corresponding resistor area with a pencil eraser:


Be VERY gentle with that resistor. The wire isn't real strong. If you break it, it's dead. Also clean the edge where the wiper rides.

Clean the harness connection:


Now, once everything is clean, put it back together and check EMPTY with your ohmmeter. If yours was like mine, it was still reading a lot of ohms, because the wiper never got close enough to the spring to have no resistance in the circuit path. In that case, you need to RECALIBRATE the resistor. If you're good with a soldering iron, this is no problem. Here's how to do it. Find the highest point on the resistor where the wiper travels. I showed mine with a pencil. Then, run a bead of solder all the way from that point up to the spring mount, so there is a direct, low-resistance path from the wiper to the spring when the gas tank is empty. NOTE that the jaw is NOT grabbing the resistor itself, but the plastic housing. Be gentle with that resistor wire!


Now, you need to run a bead of solder from the spring mount to that point. Use some good paste flux to make sure the wire is super clean so you get a good bond. Start at the mark and then work your way towards the spring mount. Be careful not to let the solder flow farther away, although all that will do is give you a slightly false low, which isn't really a bad thing. Cleaning carefully will ensure that the solder doesn't bond to anywhere you don't want it to (solder HATES dirty metal and refuses to bond to it). I'm just starting here (and the solder quality doesn't look too good - see how it's beading up?)


And now I'm done. See how the solder has flowed nicely onto the resistor wires? This is what you want to see.
 

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Discussion Starter #2 (Edited)
Here's the finished product.


Now, bolt everything back together. Next step is to install the new harness. Here's some more tricks:
1. DO NOT install the plastic connector bodies. Just solder them directly to the top of the bulkhead like this:

No more vibration or contamination on these connections! Don't worry, they'll never need to come off. HOWEVER, for the fuel pump, cut the connectors off the ends of the leads, strip 1/4" of insulation off the wires, and solder them directly to the fuel pump leads side-by-side so if you ever need to replace the fuel pump, you just touch the hot soldering gun to the joint and the wires come right apart.

DO NOT heat-shrink any of these connections unless you have heat shrink tubing you KNOW is gasoline resistant. Most of them aren't, and if you're in doubt, just take a piece of what you have and put it in a little jar of gasoline and give it half an hour. If the surface feels gooey after the half-hour, it's not resistant. Honestly, you don't need to protect the connections. They don't move around any, so they won't touch. Mine have been in my two cars since 2007 with no problems.

Okay.. Moment of truth:
Check at the sender itself: Empty should read 0 Ohms:
Excellent!

Check at the harness:
More Excellent!

Check the Full level:
Wonderful!

Okay, you're all done. Take the sender and go plug it into the vehicle harness, then turn the ignition on. You can verify the gas gauge circuit is working by watching the gauge on the dash respond to you moving the float. Then bolt everything back together and put the fuel tank back in.

Notice how much higher pitched your fuel pump sounds! It's quite noticeable.

And that is how you "refurbish" your sending unit.
 

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D'ho! I wish you would have posted this yesterday so I could have done it to mine. If I have to drop my tank agian anytime soon, i'll be doing the F-body mod.
whats the fbody mod?

on another note, do you think I would benefit from this? My guage is extremely inaccurate. It will move a quarter tank just from going to a complete stop to taking off, or turning a corner.
 

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F-body mod

sorry to threadjack sherlock9c1

To remove the fuel pump on a F-body is a real pain. You have to remove the whole rear end, torque arm and exhaust from the cat back to drop the tank. Had to do this to my buddy's car and it took forever, looking around forums I noticed some people were cutting an acess hole in their trunk directly over the fuel pump. I performed this mod on his car too. Cut the hole and put a sheetmetal cover over it. This would allow you to get the fuelpump out in a few mins versus hours. Sorry I don't have a picture.
 

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Discussion Starter #5
If your gas gauge is jumping around a lot, try a bottle of fuel system cleaner with your next tank of gas. Removing the varnish deposits may help the problem.

That said, B-body gas tanks are such that fuel slosh becomes a problem when you get down to around 1/3 to 1/4 tank, and so the gauge fluctuates accordingly. Most B-body drag racers will race with at least 3/4 tank for this reason. My 9C1 starts to run funny at WOT at 1/4 tank. Fill it up, and it drives nice again.

On the access hole, I've wondered the same thing. The thing is though, once you drop the tank, if you refurb the sender, put a new harness and a new fuel pump in and solder the connections like I recommend, you probably won't ever have to go in it again. It's a Catch-22. I will say though, the worst part about the whole job is disconnecting the fuel lines. They're in a terrible spot.
 

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Joel, do you have a part# or link to this harness at NAPA? Or is it just a replacement FP Sender harness for a 94-96 B-Body?
The one I used is an Airtex brand part number is WH3000. I purchased it at Advance auto.
 

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Very nice write up, thanks. Will come in handy since I have to replace the pump on my 92. I'm going for the F body mod though cause the car is immobile and sitting beside the garage. Plus the tank is full to the top.

I saw that NAPA carries a complete sender/pump assembly (I think it was Delphi) for about $300. Other stores have the senders/pickup for about $140. Now that this is posted though people have cheaper options :)

The airtex harness is available from most any parts store for less than $20.
 

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Discussion Starter #8
Good point. The in-tank harness is not unique to B-bodies. Many, many GM vehicles use the same 3-wire setup. That's why the wires are a bit long.

BTW, BE CAREFUL how short you cut the fuel pump wires. Although there is extra length, most of it is there for a reason.
 

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Good point. The in-tank harness is not unique to B-bodies. Many, many GM vehicles use the same 3-wire setup. That's why the wires are a bit long.

BTW, BE CAREFUL how short you cut the fuel pump wires. Although there is extra length, most of it is there for a reason.
To be more specific, here's what I found using the AIRTEX Part # WH3000 ($7.33) at Rock auto

BUICK (1984 - 1996)
CADILLAC (1985 - 1996)
CHEVROLET (1985 - 1997)
GMC (1985 - 1997)
OLDSMOBILE (1984 - 1997)
PONTIAC (1984 - 1996)

Pic is here:

 

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I had to get a new sender unit this week, I was planning on doing this sender modification while I had everything out but when I went to replace my fuel pump I found the steel lines leaving the tank were rusted quite badly and I didn't like the way it looked..

So I called up my local AC Delco distributor and ordered the ACD#FLS1072 GM#19179521 which is the part number to supercedes the original 25028955

Looks like GM redesigned the sending unit, correcting everything that gave us issues with the old one.. Here are some pictures of the new unit.

The only contact that isn't well secured from varnish and corrosion is the one between the arm and resister and it appears to be self-cleaning.









 

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I REALLY like what I'm seeing so far on that new unit.

What do the connections on the fuel pump and bulkhead look like?
They appear to be unchanged. I followed your lead and soldered the top bulkhead connections today to avoid any future charring issues.
 

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I completed the install today. I am running the Walbaro high pressure/255 pump and needed to change the pump connection anyway. So the larger spade style connections are on that end now.

The greatest news.. Put it all together and the gauge is correct for the first time in forever. I had to remove my fancy fuel correction circuit board since it was interfering with the correct reading. Once that was removed the tank actually read empty and after going to fill it up.. I read correctly at the full line.. I did Ohm out the sender before putting it in. 0.02 Ohms empty, 90.02 ohms full. I just hope it stays that way after a year or two of use.
 

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Discussion Starter #14 (Edited)
I just wanted to write some clarifications on some questions raised recently:

1. The main problem with the factory in-tank harness is NOT the wires, it's the CONNECTORS. If it was the wires, the entirety of the insulation would be melted. Every single factory harness that's got heat damage has it only at the connectors, which means the high resistance and thus heat generation is at the connectors only, and not on the wire. This is why soldering in a new harness solves the problem.

2. The factory fuel pump resistance in my car measured around 3.5 ohms. This means at 14V (max voltage), the stock pump will draw 4 amps of current. The factory wiring is plenty sufficient for that.

3. If you are running an aftermarket fuel pump such as the Walbro 255, at 45psi it draws about 9 amps per this site. That's still not too many amps for the existing harness, although the margin is smaller. Keep in mind that the harness is constantly getting fuel slosh so it is not likely to sit and cook like a wire in an insulated house wall.

4. If you decide to replace the in-tank harness wiring with thicker gauge wire, remember that it MUST be gasoline and ethanol resistant. Otherwise, the insulation will dissolve and clog your fuel strainer over time.
 

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Here's a tip that some of you might find helpful when trying to remove the corrosion from the parts of the sender that affect resistance: clean them with ketchup. Yes, ketchup. It contains a mild acid that does a really nice job of removing mild corrosion with no real scrubbing required. Just coat the parts liberally with ketchup, let them sit for a few hours, rinse with water, and air dry. You'll be amazed at how nice and clean they'll be.
 

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Or, avoid the mess, and use the active ingredient in ketchup: vinegar ;)
The nice thing about ketchup is that it's thick and stays in place. With vinegar it might be harder to keep the part covered.
 

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Discussion Starter #18
Just to be clear, unless you've had a lot of water in the tank for some reason, mainly what you're fighting in the sending unit assembly is varnish and not corrosion. I'm not sure if it would be removed well with acid. On the sending units I've done, it just wipes off with a very mild abrasive, like my finger or a pencil eraser.
 

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Just to be clear, unless you've had a lot of water in the tank for some reason, mainly what you're fighting in the sending unit assembly is varnish and not corrosion. I'm not sure if it would be removed well with acid. On the sending units I've done, it just wipes off with a very mild abrasive, like my finger or a pencil eraser.
I've already done one sending unit with ketchup after a mild abrasive didn't quite do the trick, especially under the rotating wheel. Whatever was covering the metal contact parts came off completely, and with no rubbing there's no risk of breaking any wires.
 
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