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Über Genius
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Fuel Gauge Sender unit, explained.

The Opel GT (as well as other models) fuel level sender unit is a VERY simple design.

Inside a protective tube is a float that rides up and down a resistor wire, depending on the level of fuel in the tank.

Built into the float are 4 contacts (two on each side), that touch the resistor wire, and connect one side of the wire to the other. It's assumed that the double contacts, on each side, serve a dual purpose.
1) They contact the wire in TWO places as opposed to one which improves the likelihood of actual contact with the wire.
2) They provide tension between the contacts and the wire.

Below, I am illustrating the various components as well as the best possibilities for restoring an intact unit to full functionality.

Perform this project at your own risk.

First step is to disassemble the unit. You really only need to remove the protective tube to see if your unit is maintainable (repairable).

On the bottom of the fuel level sender unit is a small bolt. It's probably rusted, as shown in this photo.
Carefully bend the holding tabs (already bent flat in this picture) to allow the nut to turn. The nut is NOT put on very tight so if you can't turn it easily with a pair of needle nosed pliers then get some evaporust and soak the bottom of the unit overnight.



Once the nut is removed, the protective tube will come off. If it's stuck a bit, it's likely stuck at the top. Just, CAREFULLY, wiggle things around a little and it will come loose.
Carefully remove the protective tube as there could be contaminants in it that could damage things. Not anything to be afraid of but there are fragile parts inside.

You will see the following, if the unit is intact.

There will be a float, and two very thin wires. The wires are actually pretty stout but still fragile. They are "resistor wires" and they provide the resistance that makes the gauge function. As the float goes up to the top, the bridge in the float bypasses the rest of the wire and produces a LOW resistance. As the float rides to the bottom, the resistance goes up which provides a HIGH (relatively speaking) resistance.



This is important to know because if the float isn't making good contact with the resistor wire, the resistance will always be high and the gauge will read empty.

To service the unit, first make sure the resistor wire is intact. If it's broken, at all, it would need to be replaced to use this unit.

If your resistor wire is broken, you can replace it with 40 gauge Nichrome "A" wire. As near as I can tell, it's an identical match, though my calculations suggest it could be off by as much as 5 ohms, overall. This would just make the gauge go a little lower than "E" on your gauge when you are on empty. Again, I don't know that it's not an exact match.

For now, it will be assumed the wire on your unit is intact.

Make sure you have continuity between the electrical connection tab and the housing where it bolts to the tank. If you do not have continuity, you will need a soldering iron and some good quality solder. It's not a difficult job to drop some solder onto the connections to bring them back to life. I will address that later in the thread when I solder mine. BTW, the solder connection really should be done regardless, but it not necessary to this post.

Assuming that you have continuity, service the wire and the contacts on the float.

It's easy for years and years of fuel to build up a varnish or oxidation on the wires and/or, the alignment shaft (that the float slides upon). The alignment shaft appears to be made of a rust resistant metal but based on magnetic testing, it's not stainless.
You can service the shaft by using some dish soap and a 3m (green scratchy) pad. It should shine when cleaned.



Next, inspect and service the contacts on the float. All FOUR contacts should be clean of debris, varnish, and oxidation. There may be a small groove worn into the contact surface but not worn through the contact. If the wire has worn through the contact, you might want to discard this unit.



To clean the contacts, put a little dish soap on the contact area and slide some waterproof 1200 grit sand paper across the contact surface, NOT THE WIRE.

Next, clean the wires VERY CAREFULLY. Use a small piece of cotton cloth (old Tshirt for instance) about one square inch, coated with dish soap. Run the cloth the length of the wire a few times. Move the float to the other end and clean the wire again. This will make sure the wire is clean from end to end.



Rinse all the parts with water and pat dry. Then put them in a nice, warm, dry, place to remove any remaining moisture. (I, personally, put the parts near a heater duct or in the over set at 150 degrees).

Check the continuity of the unit with the float at the top and at the bottom, before reassembly. You should read near zero resistance when the float is at the top and around 90 ohms when the float is at the bottom. If it is reading fine, reassemble the unit.

When reassembling, make sure the access hole is clear. The access hole is what makes the unit read gradual changes as oppose to having a dancing gauge when fuel sloshes around in the tank. If it's clogged, the fuel gauge will be very unresponsive.

Also, note that there is a notch at the top of the protective tube that aligns with the housing at the top. You MUST align this tab and notch or the nut on the bottom won't go onto the rod.

Finally, before installing the unit, check the continuity again to make sure it's still working and nothing was damaged during reinstall.

Helpful tips:
*While the unit is out, before disassembly, use a dremel and wire wheel to remove oxidation from the top of the unit. You need a GOOD connection between the tank and the sending unit for the gauge to work.
*Solder the connections between the wire and their respective connection points.
*Replace the gasket between the sender and the tank
*Clean and/or remove the fuel sock and other debris from inside the tank while the sender unit is out.
*Inspect the inside of the tank.
 

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Resident Whippersnapper
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I remember in the early days of my GT ownership that my fuel gauge stopped working. I tested the gauge and wiring and that all worked, so I pulled the sender. It was very clean, and so was the tank, but there was enough corrosion where one end of the resistor wire grounded to the case that it rendered the sender inoperable.

Memory's a bit fuzzy, but I think I strategically inserted (shoved) copper wire into the area where the corrosion was. Checked with an ohmmeter, stuck it back in the tank, and it seems to be working well to this day.
 

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Just Some Dude in Jersey
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Great write up.

I installed a brand new sender unit in my Red Baron car's oem tank about 9 years ago. The original sender was all funked up and the alignment shaft had crumbled to a pile of rust. The resistor wires were still servicable. I paid a radiator place to clean and line the tank. It lasted about 6 months, the tank was too far gone, the lining didn't work, and I started getting clogged fuel filters. I removed the sender, had an aluminum copy of a GT tank made, and reinstalled the sender. 6 months later the fuel gauge wouldn't indicate changing fuel level. I removed the sender again and discovered the central alignment shaft was all pitted with barnacles of rust, snagging the float. I had previously, when the sender was brand new, removed the protective tube to see what a brand new one looked like inside and all was chromy/brassy shiny and perfect when new.

So, basically, a brand new sender ended up with an alignment shaft that rusted bad enough to stop the sender from functioning after just a year or so being in a car that was mostly idle and was only driven a couple of hundred miles.

I cleaned up the shaft by lightly rubbing it with some emory cloth and put it back in the new aluminum tank. I haven't had any problems since. That was 8 years and about 5000 miles ago. I have a new sender on the shelf just in case.

:veryhappy
 

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Opeler
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I had a bad sending unit one time that had poor connections at the terminal lug. After doing all the above cleaning I still had bad continuity at the terminal. There is a brass rivet that ties the outer contact lug through an insulator and to the wire on the inside. That whole assembly developed corrosion and needed cleaning up. Had to carefully take out the rivet and clean the parts then reassemble. Yet another area to add to the above maintenance steps.
 
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Über Genius
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Discussion Starter · #5 ·
I had a bad sending unit one time that had poor connections at the terminal lug. After doing all the above cleaning I still had bad continuity at the terminal. There is a brass rivet that ties the outer contact lug through an insulator and to the wire on the inside. That whole assembly developed corrosion and needed cleaning up. Had to carefully take out the rivet and clean the parts then reassemble. Yet another area to add to the above maintenance steps.
I was going to address that this morning. The unit I am taking pictures of is having the same issue but it's the ground that's the issue, not the sending lug. They are all connected so fixing the brass rivet IS the proper way to fix it all but I'm attempting the solder route.

When soldering the connection, the trick is to have CLEAN metal. It's difficult to get a wire wheel in there without damaging the Nichrome wire. It's held on with a solder connection so if things get too hot, that connection will melt and the Nichrome wire connection. Then you are back to square one.

Also, solder doesn't want to stick to the pot metal that makes the top of the sender unit. It will adhere to the metal flex plate that creates the ground connection but not, easily, to the pot metal. I will be working on a fix for that.

As for the guide rod crumbling. I do know it's magnetic which implies it's just a (assume nickel) plated steel rod. It should be easy, enough, to source a brass or stainless rod to take it's place, though it would need to be threaded on each end to work properly. If one were to build a new sender assembly from scratch then it would be possible to have a rod that's only threaded at one end, to be the top, and then clipped in all the right spots at the bottom end.

If I can't find a way to fix the ground connection in the unit I am working on then I will do a write up on replacing the rivet connection at the top of the sender. I'm reluctant to do it because that will, surely, damage the Nichrome wire. Although replacing the Nichrome wire wouldn't be difficult, it's not on my list of things I WANT to do.
 
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Super Moderator
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Further to FO1981's excellent thread, here is a write-up I just did for an Opel Facebook page. The photo comments are below the main text, in the order of the photos:

While I continue to re-assemble my Opel GT restoration project, I need an occasional diversion, typically related to helping other Calgary area Opel'ers with their cars. Vince had a fuel level sending unit that wasn't working, and I have had success in repairing several of them by re-grounding the resistor wire plate to the body of the sending unit, either by soldering or using a screw to ground the plate to the body. But this time the resistor wire broke. So I loaned him a spare unit I had, and ordered the proper resistor wire from eBay. The wire is "NiChrome", and to get the correct resistance, it needs to be 40 AWG (VERY fine wire!), which is 220 ohms/meter (67 ohms/foot). Turns out that it is used commonly in the "vaping" world and it readily available on eBay and Amazon. It took me about a half hour, and after cleaning the float slider contacts, it works perfectly. If any other Canadian members need their fuel level sender repaired, let me know and I can either send you a length of wire for a couple of bucks, or send me your sender, and for a small fee (I haven't thought this part through, maybe $20CAD?) I can repair it for you

First step is to bend the tang back and remove the nut and tang on the bottom, and then CAREFULLY slide the canister away from the base

While is tricky to get solder to stick to the pot metal base, if it is scraped cleaned and fluxed, I can usually get a decent solder connection between the base and the copper spring plate. The alternative is to drill a small hole and screw the plate to the base

Here is the new NiChrome wire soldered to the copper spring plate at both ends. The trick is to pre-load the plate (I inserted a 1/8" drill bit between the plate and the base) , solder the wire ends so it is not loose, and then remove the bit so that the spring plate tensions the wire

It is critical to ensure that the float slider contacts are clean and the the wire is between the tabs

Also critical to ensure that the wire does not touch the centre post

The finished product. I taped the float to stay at the top of the post while I installed the new wire and soldered the ends

A tiny bit higher than the OEM 90 to 93 ohms when the tank is empty

After I cleaned the float slider tabs, the "full" reading went down to 7 ohms, which is a tiny bit higher than the 3 ohms of the OEM wire, but will still provide a pretty accurate gas gauge reading
 

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Detritus Maximus
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Recently cleaned out my fuel sender for the second time. The last was 19 years ago. Here is a few pics of what it looked like inside. The pile of rust dust next to it is what came out of it. You can see the condition of the center rod.
 

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Recently cleaned out my fuel sender for the second time. The last was 19 years ago. Here is a few pics of what it looked like inside. The pile of rust dust next to it is what came out of it. You can see the condition of the center rod.
All that in just 19 years, Wow. I'm scared to think about mine. Well I guess we'll see if she cranks today or what. With my luck, it will be what the H***. Ah well, that's life and years of sitting. Jarrell
 

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Opeler
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Recently cleaned out my fuel sender for the second time. The last was 19 years ago. Here is a few pics of what it looked like inside. The pile of rust dust next to it is what came out of it. You can see the condition of the center rod.
A year or two ago, I pulled a Bosch sending unit out of my Porsche (nearly identical to the GT unit) after a seven year storage and it looked a lot like yours. The nut was rusted on and all of the internals were either rusted or disintegrated and the wires broken. As the surgeons say in the movies, "damn it, we can't save them all".
 

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Detritus Maximus
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Well, it has sat for much of the past 10 years. I had the tank cleaned out and the guy said the best thing to do is to put Stabil in the tank first and fill it up. That will keep the gas good for a year or two and the full tank prevents rust due to condensation from the water in the gas or the air cycling thru the vent lines. If you could seal the tank airtight, you could use nitrogen to displace the air (oxygen). Anyway, a full tank will rust less, but you'll want to drain and replace every year or so.
 
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Great Article on how to fix the sending unit. My GT shows 3/4 full when the tank is actually filled to the top. Rather than go through all the restoration (its been in my GT since it was new). I am going to order a replacement from Gil, just because I suspect the worst, and it's a heck of a lot less work to change it out. The new price is $69 plus shipping, don't see how I could go wrong. If you are like me, I would rather drive the GT than work on it.

Bob
 

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Detritus Maximus
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I could live with an erratic sender, but I had to pull the tank to clean it way.....so, might as well make it work again.
 

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Great Article on how to fix the sending unit. My GT shows 3/4 full when the tank is actually filled to the top. Rather than go through all the restoration (its been in my GT since it was new). I am going to order a replacement from Gil, just because I suspect the worst, and it's a heck of a lot less work to change it out. The new price is $69 plus shipping, don't see how I could go wrong. If you are like me, I would rather drive the GT than work on it.Bob
Exactly what I was thinking too. Plus if you do rebuild the old one, you have a spare. I guess that's the way I'll go too. Hard enough to get to the tank and get it in and out, so a replacement would be the lesser of 2 evils. Jarrell
 

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Further to FO1981's excellent thread, here is a write-up I just did for an Opel Facebook page. The photo comments are below the main text, in the order of the photos:

While I continue to re-assemble my Opel GT restoration project, I need an occasional diversion, typically related to helping other Calgary area Opel'ers with their cars. Vince had a fuel level sending unit that wasn't working, and I have had success in repairing several of them by re-grounding the resistor wire plate to the body of the sending unit, either by soldering or using a screw to ground the plate to the body. But this time the resistor wire broke. So I loaned him a spare unit I had, and ordered the proper resistor wire from eBay. The wire is "NiChrome", and to get the correct resistance, it needs to be 40 AWG (VERY fine wire!), which is 220 ohms/meter (67 ohms/foot). Turns out that it is used commonly in the "vaping" world and it readily available on eBay and Amazon. It took me about a half hour, and after cleaning the float slider contacts, it works perfectly. If any other Canadian members need their fuel level sender repaired, let me know and I can either send you a length of wire for a couple of bucks, or send me your sender, and for a small fee (I haven't thought this part through, maybe $20CAD?) I can repair it for you

First step is to bend the tang back and remove the nut and tang on the bottom, and then CAREFULLY slide the canister away from the base

While is tricky to get solder to stick to the pot metal base, if it is scraped cleaned and fluxed, I can usually get a decent solder connection between the base and the copper spring plate. The alternative is to drill a small hole and screw the plate to the base

Here is the new NiChrome wire soldered to the copper spring plate at both ends. The trick is to pre-load the plate (I inserted a 1/8" drill bit between the plate and the base) , solder the wire ends so it is not loose, and then remove the bit so that the spring plate tensions the wire

It is critical to ensure that the float slider contacts are clean and the the wire is between the tabs

Also critical to ensure that the wire does not touch the centre post

The finished product. I taped the float to stay at the top of the post while I installed the new wire and soldered the ends

A tiny bit higher than the OEM 90 to 93 ohms when the tank is empty

After I cleaned the float slider tabs, the "full" reading went down to 7 ohms, which is a tiny bit higher than the 3 ohms of the OEM wire, but will still provide a pretty accurate gas gauge reading
Bumping this thread so I can find it as I rebuild one of these. Great info, thanks to the folks that put it together.
 

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Here is my sender. Not the one out of my tank but one I bought to rebuild and install. I get good resistance for full and empty at the solder tabs. I do not get continuity from the terminal to the housing.
 

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Here is mine sender. Not the one out of my tank but one I bought to rebuild and install. I get good resistance for full and empty at the solder tabs. I do not get continuity from the terminal to the housing.
Got lucky with mine. Replaced mine during the summer heat, never again, learned the hard way, take out the seats. Spent more time getting to the sending unit, the actually replacing was 10 min, getting to it, 2 1/2 hrs and 2 1/2 hrs putting it (rear tire tray, etc) back. But I lucked out. I had A NOS sending unit. I kept the rebuilt one, and the old one to rebuild and put on the shelf. I got "stuff". Jarrell
 

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Terry that looks like an after market one. I f you look at pictures from kwilford and first Opel 1981, you can see what it should look like.
 

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Well that's strange my unit looks nothing like any of yours.
What the turd is going on here? :bs2:
Are there any ID names/numbers on this unit? I have never liked the design of the OEM unit and have looked at more "conventional" units from time to time (but the fuel system problems are pretty far into the future for me right now)!

Doug
 

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absolutely nothing at all slracer and here I thought I had the last remaining antique will never find this piece again to put in the rare opel parts :no: case
Another item to meet Mr. trashcan I guess
 
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