09-16-08, 01:10 AM
Hi, OK, hope someone can help, as this one is really kinda aggravating me!! I have a 1978 Fleetwood, and since I got it the gas gauge has never worked. The car is in great condition, with 58,2xx miles, little old lady, etc if that matters. It always reads all the way on "E", and there IS NO little red light for a low gas reminder. (Surprising, because this car is absolutely loaded otherwise) Anyway finally got around to getting it taken to the shop so that our mechanic could look at it. First, he tested the sending unit using that "ground method" I guess you could call it, where he grounded the wire near the gas tank and the gauge should read full. Well, he grounded it and it DID NOT change position at all. I watched him work on it for about a half hour and he couldn't get the gauge to move at all. Then he tells me it is possible that the actual gauge is bad, so I call up my "parts source" which is basically a guy that lives near me and his house if full of 1977-1979 Cadillac Parts (lucky me!) and he has a gas gauge unit from a 1978 Coupe DeVille with 30,000 miles that was hit in the side and totaled, everything was mint on the car, so this gauge should be good. OK, I open up the dash and was expecting to see a regular-type wiring harness, like the ones used on the back of the radio, etc. but instead it looks as if the gas gauge uses a type of "printed circuit board" connection, if you've taken the top dash pad off on your Caddy, you know what I mean. OK, so I get this gauge in, and it still does not work! Now, I have a manual for a 1979 Fleetwood, and I figured I would change the fuse that runs the gas gauge just for the hell of it even though it was not blown... According to the manual, that fuse is 20 AMPS and runs the gas gauge, transmission downshift something or other, BRAKES light, OIL PRESS light and FASTEN BELTS light. Now, in connection with the gas gauge not working, the FASTEN BELTS light never worked either, nor did the dinger that is supposed to sound when the key is first turned. I'm not sure if this is just a concidence though. While I had the dash open, I took a bulb off another light that I know worked and stuck it in the FASTEN BELTS slot, and the light still did not light up. So, the gas gauge and FASTEN BELTS light do not work, yet the OIL PRESS and BRAKES lights do, so does anyone know if there is anything else to check? Sorry this is so long, and yes there is power going to the sending unit. Sounds like a wiring problem still...
Thanks for reading
09-17-08, 01:36 AM
Wow, now that I read this I probably made this more confusing than it has to be. The real question is: Is it possible that there could be a problem with the actual printed circuit board that goes to the gas gauge? Like I said, that same printed circuit board splits off to each of the idiot lights, all of which work except the FASTEN BELTS light. I know of a place to get one, I might do that just to see if it works...
09-17-08, 06:17 AM
Somebody probably disabled that stupid seat belt light within the first year of ownership. People used to hate those things. It might be unplugged under the seat. I've always been intimidated by that circuit board stuff behind the dash. I believe power goes to the gauge and the sending unit completes the ground circuit. Maybe you could check to see if the gauge is getting voltage. If you had access to another gauge panel it seems like it would be easier to swap the whole panel. I haven't heard of any electrical failures in those printed circuits over the years so I'm not so sure you will find the problem there. Sorry I'm not more help. I wish I had a neighbor with lots of 77-79 parts because I need a rear bumper for my 77 Fleetwood.
09-17-08, 11:18 AM
Usually, when a gas gauge ceases to operate, it is a grounding issue. Sounds like your tech is aware of this, and it was the way that he was trying to fix it. So, we need to explore other options.
I really think that the tank needs to come down. It's not hard to do. Siphon most of the gas out, put a floor jack underneath, disconnect the straps, and lower. Check to see if there is something holding the filler neck in (can't remember). This way, you can closely inspect the wiring on top of the tank. Hopefully, you will see the problem right away. If not, out comes the sender. See if there is something broken. Check the arm and the float to make sure that it swings through its full range of motion. Next up, measure the resistance change from full to empty (spec should be in the FSM).
Keep us posted...
09-18-08, 12:18 PM
Thanks guys, yes we are going to lower the gas tank to check connections, it is the only thing left to do. That same guy has a couple good sending units laying around too (lucky me again) and if anything we can put a different one in. My mechanic also said it wouldn't be that hard to just wire in a new connection to the gas gauge if it is grounding out somewhere and we can't find it. By the way, could anyone tell me what wire is what on the harness that connects to the printed circuit board on the top dash? I don't have anything but the owners manual. I was thinking we could test the wires for power right before it gets to the printed circuit board and that would be helpful too.
Generally when a GM gas gauge stays on E it means no power to the sender, in other words the positive wire back to the sender is disconnected. What your mechanic did was verify that no power is getting to the sender by grounding the sender harness. A shorted sender wire would cause the gauge to go past full. So, you need to read your service manual and identify the colored wire that powers the gauge on the printed circuit board. you will see a multi wire connector that plugs into the circuit board. trace the copper line on the circuit board down to the plug. That will tell you which wire provides power to the gauge. with a test light, verify voltage to that wire. If the light turns on then the wire and fuse are good. You either have a crack in the printed circuit or the power wire to it is open (disconnected) or the wiring to the sender is open. If it were me, I'd run a wire from the positive post of the battery to the power wire to the sender. If it now registers, back trace the wiring to where you find it disconnected.