Okay, I took a look underneath the car today and there doesn't appear to be any problem with the brown wire from the sending unit. You can follow it up through the shelf in the trunk to where it goes with the rest of the rear end wiring up to the dash. My original thought was that when they installed my new exhaust system, they must of cut a wire or something as the gauge was working when I brought it to them, but no longer working after they were done. The circuit is very simple as you can see from the image below, which I copied from the service manual, including the misspelled GAGE. I did eliminate a lot of wiring, but the circuit is correct from an electrical standpoint. As this is a 4400, it is a single wire system with no fuse, unless the other idiot lights it's tied to are fused, I didn't trace it out, but they all work, so even if there is a fuse it would have to be good. Now for the questions: 1. If the long brown wire from the gauge to the sending unit was broken, would the gauge read full or empty? 2. The 4700-4800 series cars use a 3 wire sending unit, the black one goes directly to the frame ground. My (4400) sending unit only show 1 wire going to it, a brown one, which goes to the gauge. Can I assume the ground for the sending unit is obtained via the metal connection to the tank? FYI the gauge reads full all the time which I'm told indicates a loss of the ground connection. I hate not having a fuel gauge so any help would be appreciated.
If you disconnect the brown wire, it will go to full. Ground it and the "gage" should go to empty. If that test passes, the sender is most likely bad. You can measure the ohms from the sender. 90 ohms is generally full and zero ohms is empty. On a 61, the sender is easily acessible from an acess plate in the trunk. If your gage reads empty, a very likely suspect is the brass float. They leak and sink like a rock. I bet if you take the sender out you'll find your float is bad. A new float is readliy available from any early Impala vendor as it the cork gasket you'll need to replace.
Folks, please give the OP the courtesy of reading his post fully. The gauge is reading Full and he did check the Ground wire visually. Mosslack, get an ohmmeter and a jumper wire and test the wire for continuity. I think that wire popped off or broke.
Thanks Jason, that was just the info I was looking for. Now I can check it at the watertight connector near the right rear wheel. Is the access panel in the trunk held in with screws? Looks like some kind of caulk over where the screws should be. And I apologize for my snappy answer, to the previous poster, it's just been a long day.ou: I do appreciate everyone's help on all of the questions I ask. ---------- Post added at 11:56 PM ---------- Previous post was at 11:52 PM ---------- Thanks Bob, I guess it was just an odd coincidence that it stopped working right after they installed the exhaust. But I guess it just happens that way sometimes.
I apologize as I did read your post but, I missed the last part of this question. I think you hit the nail on the head here with the last sentence. Here is a good bit of information on trouble shooting gas gauges. Good Luck Dan [FONT=Arial, Helvetica][FONT=Verdana, Arial, Helvetica]Most gas tank units consist of two parts, the gauge mounted in the dash and a tank sender mounted in the tank.[/FONT][/FONT] [FONT=Arial, Helvetica][FONT=Verdana, Arial, Helvetica]The following trouble-shooting instructions apply to the AC-type fuel gauge, which is one of the most common type of units. Your service manual will give you any added instructions necessary for your specific type of fuel gauge.[/FONT][/FONT] [FONT=Arial, Helvetica][FONT=Verdana, Arial, Helvetica]The gas gauge, often located in the instrument cluster, consists of two small coils spaced 90 degrees apart with an armature and a needle placed at the intersection of the two coils. A dampener is also located on the armature to prevent excessive needle movement on rough roads.[/FONT][/FONT] [FONT=Arial, Helvetica][FONT=Verdana, Arial, Helvetica]The tank sender unit is mounted to the outside of the gas tank (with a float on the inside, of course) and is made up of a metal housing that contains a rheostat (which is simply a resistance unit), and a brush that comes in contact with the resistance unit. The opposite end of brush unit is attached to the float arm located inside of the gas tank. The movement of the float arm is controlled by the amount of fuel in the gas tank. The variations in the amount of fuel in the tank cause the arm to move. This changes the resistance of the tank unit, changing the amount of current at the gauge unit coils, which in turn moves the needle located between the coils.[/FONT][/FONT] [FONT=Arial, Helvetica][FONT=Verdana, Arial, Helvetica]Beginning around 1950, some vehicles have a small voltage regulator behind the dash that supplies a stable voltage supply for the instruments - something less than the battery voltage of six (or 12) volts. This isolates the instruments from the effects of varying battery and generator voltage. So if you are checking voltage at the dash gauges on these cars and you read something less than battery voltage, it may be normal. Follow the power wires from the instruments and you'll probably find the regulator.[/FONT][/FONT] [FONT=Arial, Helvetica][FONT=Verdana, Arial, Helvetica]The most common cause of gas gauge trouble is a poor ground, especially at the tank sender unit. It is important that all wiring connections are clean and tight, and free of dirt and corrosion. A poor ground or loose connection to a gas gauge is just as likely to cause problems as loose or dirty battery cables assuredly will with your starting system.[/FONT][/FONT] [FONT=Arial, Helvetica][FONT=Verdana, Arial, Helvetica]For example, if the gas gauge needle remains on empty when the ignition key is turned to the "on" position and you are sure the gas tank is partially full, battery current may not be reaching the gauge. To check further, connect a jumper wire between the ignition switch and the dash gauge. If the gauge now works, replace the defective wiring between the gauge and the ignition switch.[/FONT][/FONT] [FONT=Arial, Helvetica][FONT=Verdana, Arial, Helvetica]If the gauge needle remains stuck in one place, try turning the ignition switch off and on several times in succession. This will allow you to determine if it is the dash gauge or the tank sending unit that is defective. If turning the ignition switch off and on (a half-dozen times is a good test) does not seem to help, and you've verified that the dash gauge is receiving power, try the following troubleshooting steps:[/FONT][/FONT] [FONT=Arial, Helvetica][FONT=Verdana, Arial, Helvetica]A functioning sending unit will have a high resistance when the tank is full and a low resistance when the tank is empty. If the gauge reads higher than it should, make sure the wire attached to the electrical terminal on the tank sending unit is making a good ground connection. If necessary, clean the connection and reinstall the wire to the terminal, then check the gauge operation. If that didn't fix the problem, use a jumper wire to ground the electrical terminal on the tank sending unit to the tank unit housing (leave the wire to the gauge connected to the tank unit). Turn the ignition switch on. If the gauge reads empty or below empty, the sending unit is defective.[/FONT][/FONT] [FONT=Arial, Helvetica][FONT=Verdana, Arial, Helvetica]If the gauge still reads high, try grounding the tank unit electrical terminal to a clean portion of the frame. If this brings the gauge to empty or below, the tank unit is OK, but there is a bad ground between the gas tank and the body or chassis (or less likely, between the tank unit and the gas tank). Either way, find and correct the bad connection or merely run a permanent jumper wire from the frame to one of the screws holding the tank sending unit to the tank.[/FONT][/FONT] [FONT=Arial, Helvetica][FONT=Verdana, Arial, Helvetica]If the gauge still reads incorrectly, the wire from the tank unit to the gauge may be faulty. Check by using a jumper wire to ground the tank unit terminal on the gauge (the one not connected to the ignition). If the gauge now reads empty or below, the problem is in the wire leading to the tank (or its connection to the gauge). If grounding the tank terminal still doesn't cause the gauge reading to drop to empty or below, the gauge is faulty.[/FONT][/FONT] [FONT=Arial, Helvetica][FONT=Verdana, Arial, Helvetica]When the gauge reads lower than it should, check at the gauge by disconnecting the wiring from the tank unit. Turn the ignition on and if the needle reads full or above, the gauge unit is probably good and the tank unit or the wiring leading to it are probably the problem. Reconnect the tank unit wire to the gauge before proceeding.[/FONT][/FONT] [FONT=Arial, Helvetica][FONT=Verdana, Arial, Helvetica]The next trouble-shooting task begins by disconnecting the wire to the tank sending unit. If the gauge still reads full, there is likely a short to ground somewhere between the sending unit and the gauge. Look for worn insulation allowing the wire to contact the body or frame. If the gauge reads full or above with the wire disconnected, the problem is in the tank unit rather than the wiring. It could be an electrical fault, or the float may be stuck or may have sunk because of a hole caused by corrosion. Remove the tank sending unit and check it. If the float is bad, you'll likely hear gas sloshing around in it or see gas dripping out. If the float is OK and the float arm seems to be swinging freely through its full range, replace the tank unit.[/FONT][/FONT] [FONT=Arial, Helvetica][FONT=Verdana, Arial, Helvetica]Ground the gas tank terminal post of the tank sending unit using a jumper wire. If the dash gauge now reeds empty, the gas tank unit is defective. If the dash gauge needle fails t move, the dash gauge is defective.[/FONT][/FONT] [FONT=Arial, Helvetica][FONT=Verdana, Arial, Helvetica]If the gas gauge works, but reads lower than it should, check for defective wiring between the gas gauge and the tank sending unit. Also check for a poor ground at dash gauge and the sending unit by using a jumper wire.[/FONT][/FONT] [FONT=Arial, Helvetica][FONT=Verdana, Arial, Helvetica]If the dash gauge reads empty when the tank is actually half full or greater, the wiring between the dash gauge and the tank sending unit is defective. Disconnect the wire at the gas tank sending unit terminal. If the gas gauge reads still reads empty, the dash gauge unit is defective.[/FONT][/FONT]
Wow! Great stuff there Dan, thanks. I won't be able to get back to it until tomorrow, but I'm sure I have a pretty good understanding of how it works now. Did you get all that James? Sounds like you could use a bit of advice on yours as well.
The first place I would look is the the sending unit itself. As this is the ground I would take that apart and clean and reassemble. There is a lock ring and a O ring to seal. I would be willing to bet this is your problem. Your not getting a good ground.
I apologize for not reading 421-6speed's lengthy post, but I would get an Ohmmeter and check the resistance from the fuel tank to a known good ground. The resistance should be close to zero. If the resistance is much higher then zero - or even infinite, the ground is the problem.
Thanks John, measuring from the shiny metal on the actual sending unit to a good ground was just the same as touching the 2 probes together, .5 ohms. Measuring from the sending unit electrical terminal, where the brown wire attaches, to ground on the body was about 20 ohms. This was done with the key off of course. I also had a 12 volt test light and touching that to the sending unit terminal, with the key on, produced a very dim glow from the bulb. I would think that at least confirms the brown wire making it all the way to the dash as there is no other source of power going to that terminal as far as I can see. I can go back and check the voltage present at the terminal with the key on if anyone thinks that would be helpful, but at this point it looks to me like the gauge itself might be bad. What do you guys think?
Okay I went back and disconnected the brown wire from the sending unit. The resistance on the sending unit now reads about 30k ohms. That's right, 30,000 ohms, and grounding the brown wire does make the gauge go to empty as it should. It would be my guess that the sending unit is in fact defective. One possible senario is that while putting on the exhaust, they hit the tank hard enough to crack an old, old sending unit. Just my guess, it was old so any kind of severe vibration is probably all it took. Now the question is, where the heck to you find a replacement for something like this? I assume this design was used for several years. Anyone know the range of years and models?
Well, believe it or not, I did find a sending unit here for $79 plus shipping: http://www.classiccarautoparts.com/gastank_sending_units3.html At that price, it may have to wait awhile, but at least I know I can get it when I need it. Many thanks to everyone who helped in this matter, it was greatly appreciated.
