Actually, I've never seen a thread where a fuel gauge was proven to be bad. I'm not saying it hasn't happen, just that I've never seen it. These gauges are actually very reliable. Most of the time, when a fuel gauge starts acting up, it's because the sending unit or it's wiring harness need attention.
(Note: In cases where both the fuel gauge AND the temp gauge are giving erroneous readings, it's almost always the 7 volt regulator or its associated wiring).
If your fuel gauge isn't reading properly (and your temp gauge is OK) or, if you're in the process of cleaning your fuel tank, you should remove your sending unit for cleaning and inspection. Never, ever, leave a sending unit in place while cleaning a tank. By the time a tank needs cleaning, the sending unit almost always needs attention as well. In addition, many tank cleaning methods will damage a sending unit, or worse, destroy it. Always remove the unit from the tank and give it the attention it deserves.
Removing the sending unit
Remove your seat and look at the top of your fuel tank. The sending unit is located towards the rear of the tank and has two wires attached to its top. Remove the two wires, taking care not to put undue 'side to side' stress on the female hot lead when removing it from the sending units male connector. This connection point has a seal that you do not want to damage. I suggest that for the removal of this lead, you grip and immobilize the sending units male tab with a pair of needle nose pliers while you gently remove the female connector. Don't wiggle the female connector from side to side, simply pull it straight back and off. Lubricate it if it's stuck.
Once both the wires are disconnected, you'll need to be sure that your fuel level is lower than the tank opening before pulling the sending unit. If need be, siphon some fuel out through the filler neck until you're sure the tanks fuel level is at least an inch below the sending unit opening.
The sending unit is held in place by a locking ring. (There's a specific Honda tool made for removing this ring however, they're impossible to find). Remove the locking ring by turning it in a counter clockwise direction. In lieu of the Honda tool, place the business end of a large screwdriver against one of the upright tabs as shown below. Then, with a hammer, tap the head of the screwdriver. Rotate around to each tab, tapping on each to rotate the ring in a counter-clockwise direction. Do this gently, as distorting this ring will be a 'show-stopper' for you during re-assembly.
With the ring off, insert the screwdriver blade under the rim of the sending unit and gently attempt to pry it up. It should pop up easily. If not, don't force it as you'll distort the body of the sending unit. Instead, work a knife or box cutter blade under the edge and move it around the circumference of the unit to break the seal. When the unit comes up, remove the old rubber gasket and toss it. Order a new one from Honda. Don't use any other seal than the one from Honda.
With the sending unit out of the tank, you'll be looking at a version of the two units in the photo below. The top unit is about average for a tank that exhibits light to moderate interior rust and is a good candidate for restoration. The bottom one came from a badly rusted tank and may not be salvageable. (Well see as we open it up and get into it.)
Next, we're going to remove the little box that covers the inner workings of the unit. To do this, gently bend up the three small tabs holding the cover in place. Don't bend the tabs back any further than that which is necessary to remove the cover.
Remove the cover and you'll be looking at this. Interesting that they're as bad, or worse, inside as they are on the outside. That's way you always remove these units for cleaning and inspection.
In the photo below, note that a wire coming from the right side of the winding is soldered to the units 'hot' post. These windings consist of one continuous wire, wrapped about 90 times around a fuel-proof mounting board. The wire end coming from the right side is soldered to the 'hot' post while the other end is tucked into a notch on the lower left corner of the board. It's either cut off flush with the board or is left protruding, giving the appearance that it's broken off from the 'hot' post. Not so. Only the wire from the right side should be attached.
Cleaning the sending units
When new physicians take their Hippocratic Oath prior to starting in medicine, they promise (among other things) to, "...never do harm." This is a good principal to keep in mind as you proceed through these next steps of cleaning, repair and adjustment. If you find yourself wondering, "What would happen if I just bent this thing a little" or, "I heard that you can....", please stop. Unless you're trying to correct an obvious condition caused by a PO, anything you do outside these instructions could make matters worse. As you proceed, refrain from making adjustments like, 'bending the float arm' to correct your fuel gauge. Don't think of this unit as a toilet bowl float. It doesn't work that way.
Let's move on to cleaning. If possible, cleaning the rust from the sending unit should be done with one product only, and that is EvapoRust. EvapoRust is incredible stuff. Safe, water-based, non-toxic, 'green', non-corrosive to metal (it's not an acid) and it only attacks and removes rust. As a result, it will not dissolve the fine wiring and other thin parts that make up the sending unit. If you do not have access to EvapoRust in your part of the world, consider having a member here mail you a quart. If that's not possible due to postal restrictions (I won't tell), the only other non-corrosive cleaning method I'm aware of is electrolytic or electrolysis. If neither of these are available to you, the only 'acid' method I can recommend is Phosphoric Acid. Phosphoric Acid is unique in that it has an ability to dissolve iron oxide (rust) very quickly while etching iron very slowly. This means that you can leave metal in Phosphoric Acid much longer than you could with, say, diluted Muriatic Acid, Citric Acid or vinegar, without the structural damage that those acids can cause.
To start the cleaning process, place your sending unit and cover into any container that will hold liquid and fill with ER until the parts are completely covered. Let them sit until all of the rust is gone. Note: You cannot leave metal in EvapoRust 'too long'. ER has no damaging effect on metal so, it can be left in indefinitely. Also note that ER will not remove rust 'stains' from metal. So, once the surface rust is gone, don't be concerned if some rust staining remains. Do not remove the unit prematurely and 'scrub' or 'scrap' at the rust while the ER is working - you're not accomplishing anything. Simply let the ER do its job. The same holds true if your using Electrolytic cleaning or an acid based cleaner. Let the method do the job - you don't need to assist. When all of the rust has been removed, there may be a few small bits that remain on the surface. These will brush off easily when rinsing.
Once the rust is gone, rinse the parts thoroughly under hot tap water ONLY and blow dry with an air hose or let air dry. Do not use a heat gun to dry the parts as it can contribute to flash rust. Do not spray the unit with WD-40 to keep it from rusting. Unless you live at the beach or in a rain forest, it won't rust right away. Spraying anything on it now will only contaminate the wiring and surface metal making repairs more difficult.
After leaving the units in ER overnight, we are now rusts-free. A little soft wire wheel cleaning on exterior surfaces will remove any remaining 'dead' rust particles and discoloration. (It even looks like the one that was so badly rusted is now ready for restoration.)
Pay special attention to the 'hot' post. Use an X-Acto knife, dental pick or any other appropriate tool to chip away and remove all of the old sealant around this area. You want it clean as you'll be resealing this area later.
Repairing a broken winding wire
We now have clean and rust free sending units and can proceed with repairs. (Note: If your sending units winding wire to the hot post is intact, you can skip ahead to 'Sealing the hot post'. However, I suggest that you 'tin' that wire to re-enforce it.)
If the wire from your winding is broken off at the hot post (and you still have enough wire to span the gap), simply re-solder the wire to the post. However, if the wire is too short, use a dental pick or similar tool to unwrap one length of wire from the winding and re-solder the end to the post. On the vary rare occasion that the winding wire is found to be broken in the middle of the board, you have two choices for repair.
1. Providing that the break isn't along the path of the armature, you can unwrap the winding to the point of the break and re-solder it. To remove the winding from the holder, gently bend the two small metal 'points' that hold the winding board into the units holder. Only a small 'release' on these points is needed so, be careful. The winding will then lift out. Unwrap the wire to the point of the break, make your repair, re-wrap and reinstall the winding. Reset the 'points' to secure the winding in it's original position. A small dab of sealant on theses points (used in the 'Sealing the hot post' section) is good insurance. As mentioned above, if the break is along the path of the armature, cut the wire a bit shorter so the repair won't interfere with the armature.
2. Re-wrap the entire armature with new wire. This is probably the least desirable of the two options due to the following: I consulted with the owner of WireTronic Inc. to determine the type of wire used in these windings. After much discussion and testing, the wire was found to be Nichrome, a nickle and chromium based wire, approx. 33 ga. with a Pyre-M.L. insulating enamel coating or, NICR-33 HML. Sourcing this wire in 2' lengths is difficult. If a vendor had a spool in stock, the cost for a 2' length is only about $3.00. However, if a spool had to be ordered, the cost is over $500 and they would probably want to re-sell the entire spool.
If you're unable to locate a source for small sections of this wire, and your only alternative is to re-wrap your winding, it would probably be best to toss your sending unit and buy a good used one from eBay or other source.
Testing rheostat arm tension and ohms
Now that the winding has been repaired, the next step will be to ensure that the rheostat arm is making proper contact with the winding. To do this, first clean the underside or, 'contact' edge, of the rheostat arm with a small, fine piece of wet/dry sandpaper or Emery cloth. Slide a piece under the contact edge and gently burnish that edge from the top of the arm to the bottom. This will ensure that there is no corrosion on the arm that will interfere with a proper reading. Be careful not to apply unnecessary 'outward' pressure on the arm as you might bend it. Blow the area clean with an air hose.
Next, move the float arm to its 'empty' position. Using a small slip of paper (e.g. from a sheet of printer paper), gently slide it between the rheostat arm and winding as shown in the photo. When you do this, you'll want to feel some resistance and drag as the paper pushes though the contact points. Now, remove the paper, move the float arm up a bit and repeat the process. Repeat across the length of the winding.
The desired results are that you've felt resistance from the paper as you've pushed it between the rheostat arm and winding over the entire length of the winding. If so, you're good to go on to the next step. If these were not your results, do the following:
The rheostat arm needs to be adjusted so that it applies a bit more pressure on the winding. To adjust the rheostat arm, back off the Phillips headed screw a little so that the barrel can be moved across the float arm shaft. Insert the tip of a small, flat-bladed screwdriver between the right side of the barrel and the units frame and move the barrel slightly to the left. As you can see, this will increase the tension of the rheostat arm against the winding. Re-set the Phillips screw. Now, use the 'paper test' again to determine if you have achieved the correct tension. Re-adjust until you get it right.
Testing the unit
Using a digital ohm meter, test the unit with the float at both its 'full' and 'empty' position. Set your digital meter on the lowest setting for ohms. (My meters lowest setting is 200 so, I use that.) Attach your meters leads to the hot and ground post of the unit. They need to be attached firmly as ANY movement will cause the ohms reading to fluctuate. (When measuring ohms, polarity isn't an issue so, it doesn't matter which leads go to which connector.) With the leads firmly attached, move the float to the 'full' position. Your meter should read around 10 to 15 ohms. (Mine reads 6 ohms and my gauge works fine.) Move the float to the 'empty' position. Your meter should read around 65 to 75 Ohms. (Mine reads 79.) Your unit should read somewhere around these numbers.
When these units are assembled at the factory, a final 'ohms' adjustment is made using the hole and toothed plate you see in the photo below. Adjusting this plate 1/16" or so at a time in either direction causes the 'full' and 'empty' ohms readings to increase or decrease in unison. I've never found it necessary to adjust this plate. Once it's set at the factory, that's it. I would advise against moving it unless your very sure that it was tampered with by a PO.
If you find that your readings are far from those stated above, do this:
1. Inspect the float arm and its 'stop tabs'. Make sure they haven't been bent out of position. Use the photos as reference.
2. Inspect the rheostat arm and ensure that the Phillips screw is holding it firmly in position on the float arm. Use the photos to reference the correct 'far left' and 'far right' position of the arm as it rests on the winding.
3. Inspect the winding for a broken wire. Test it by placing your meter leads against the left and right leads (or near them) and look for a solid short. If you don't see one, the winding wire is broken somewhere on the board.
If, after checking the above, you're still having readings that aren't close to those stated above, toss you unit and pick up a good used one from eBay or other source.
Sealing the units hot post
If your sending unit wasn't leaking prior to cleaning and repairs, you might be thinking, "Why should I seal it? It wasn't leaking before." Well, that's a good question and my response would be, "Have you heard of Murphys Law?" Instead of chancing it, buy some good quality fuel resistant sealer and do it. 'Seal All' works well. It's only a few dollars and is available at most U.S. auto parts stores, hardware stores and on-line. Available in the UK as well.
Prior to applying the sealer, I cleaned the interior hot post and the surrounding area to ensure a good bond. To do this, I found that a .22 cal. bore brush attached to the end of a cordless drill works very well.
With the area cleaned, I applied two coats to the post and over the mounting plate as well. I reasoned that if the seal was leaking, it could also leak under the mounting plate.
Run the sealant all the way around.
Seal the top of the post as well. This area came from the factory with an epoxy coating on it but, I don't believe it was a fuel sealant, rather it was sealed to prevent corrosion of the connection. Just for cosmetics, I masked of the area, applied the sealant, then pulled the masking tape. This stuff sets up very quickly. You only have about one minute before it skins over. Because the exterior surface (top) of the sending unit is now missing its Cadmium coating, it's going to rust. I suggest masking off the male tabs and giving the top of the unit a couple of coats of Rust-Oleum clear to seal it.
Put the cover back on and your done!
Reinstalling the sending unit into the tank
Now that you're done with the cleaning, repairs and sealing of the unit, you'll want to use equal care in its re-installation. First, make sure that the surface around the tanks sending unit opening is free of paint and corrosion. Clean and smooth the area with some Scotch-Brite or other abrasive pad or paper right down to shiny metal. Now, inspect your locking ring. The bottom surface should be flat and true. If it isn't, your new seal is going to leak when reinstalled. If you can't fix yours, pick up a used one from another member or eBay.
Next, coat your new rubber seal with a thin film of light oil. Then, place the seal on the tank followed by your sending unit with the float pointing towards the front. I also suggest that you put a light film of oil on the mating surfaces of the ring and sending unit so that they'll move easier against one other during reassembly. If you wish, you can place the seal on dry but, never use sealant. Make sure your rubber seal is correctly seated within the depression of the sending unit. With the sending unit and gasket in place, drop your locking ring over the top. Now, while exerting downward pressure on the unit, turn the locking ring until the tabs of the ring and the tank connect. You probably won't be able to turn it past this point by hand.
With the screwdriver and hammer that you used to remove the ring, do the same thing but, in reverse order. Alternately, tap each one of the upright tabs on the ring, turning the locking ring clockwise until the arrows on the ring and tank are aligned. Note: If your seal leaks after assembly, it's due to one of three things:
1. Your seal wasn't aligned properly under the sending unit.
2. The contact surface of your sending unit and/or the tank wasn't properly prepared.
3. The locking ring isn't exerting enough downward pressure on the seal. To adjust this, bend the tanks locking tabs a bit so they'll exert greater downward pressure on the ring.
Next, clean the female connectors of the units wiring harness and install the ground lead. Prior to installing the hot lead, hold the male tab of the sending unit with a pair of needle nose pliers to support it, then slide on the female connector. It would be a good idea to inject some Di-electric grease into the hot connection to keep out dirt and corrosion.
Miller time!
