1972 Cadillac Eldorado gas gauge problems

[Scot Minesinger]

Just about finished with this car and I cannot get the gas gauge working.

About two months ago it stopped working.  Replaced the Mylar printed circuit board and it worked, then after a week stopped working.  The Mylar circuit board replacement had delaminated. 

The sender tested good in that it had resistance and it varied with tank level as read at back of tank.  Nonetheless I knew it was original and did not really trust the sock and had no idea what the tank looked like inside.  So, I dropped the tank, replaced the sender and sock, cleaned tank, and reinstalled.  The fuel pump and rubber fuel lines have been replaced.  the fuel pump to carb metal line was replaced (old had been spliced with rubber sections).  Just wanted that question about tank to be resolved positively.  The ground from sender to frame is good.

Got another Mylar circuit board.  Then before installing it gauge worked again, then stopped.  I replaced the Mylar circuit board with a real good one and gauge does not work.  I have of course replaced the gas gauge itself too to no avail. 

Maybe it is the connection point where the wires connect to the printed circuit board?  Does anyone have any ideas or suffered this problem before?

["Cadillac Kid" Greg Surfas 15364]

Sounds like you have a poor connection or frayed wire in the harness.
Greg Surfas

[m-mman]

You describe changing the circuit board and that they keep failing. . . . ?

The material of the 'circuit board' must endure a very harsh environment inside the tank. I believe that it is something more than just a 'circuit board' material. The wires that wrap around these are extremely fragile and the number of turns and spacing of the wires greatly affects the accuracy of the gauge. . . . .
I have never had luck trying to repair or replace these. Where are you getting your parts from?

I found a new reproduction sending unit for my 1971 on ebay about 6 months ago. It wasn't a Cad parts dealer and I could not find a supplier who offered them. I have no idea who made it, but it fit and worked fine -- After removing the metal wire mesh sock which was too fine to allow fuel flow when the engine was operating above idle. (I added a filter to the rubber line near the pump)
After 20 years of running dry at 1/4 tank, The gauge finally works like it should.

[kkarrer]

Ok, I know it may either be obvious or may not apply to 72, but this 74 Eldo I've been working on has had a similar problem and I found that there's a fuse that also has to do with the kick down operation of the transmission and the associated micro switch on the carb linkage.  If this is also the case for 72 you may want to check that transmission circuit, the board, and the fuse there as well.  It's supposed to be 25 or 30 amp and needs to be in that range or it will blow.  I also just had an instrument panel pcb out in a 67 helping with a repair for a friend at Grand Nationals(clock and instrument face plate) and noticed that those boards are very susceptible to corrosion and age.  Maybe Honest John in Denton, Texas can help you out with a really good one. Ironically the fuel gauge in my 55 just got stuck on "F" on the way back from GN in Milwaukee.  As with yours the unit, ground, and tank are all less than a year old and those gauges are rarely at fault.  When I'm done with this 74 I'll have to tackle it.
Good luck and keep us posted. 
Ken Karrer CenTex Regional Dir.
1955 convertible coupe

[bcroe]

Just about finished with this car and I cannot get the gas gauge working.

About two months ago it stopped working.  Replaced the Mylar printed circuit board and it worked, then after a week stopped working.  The Mylar circuit board replacement had delaminated. 

I dropped the tank, replaced the sender and sock, cleaned tank, and reinstalled. 

Got another Mylar circuit board.  Then before installing it gauge worked again, then stopped.  I replaced the Mylar circuit board with a real good one and gauge does not work.  I have of course replaced the gas gauge itself too to no avail. 

Maybe it is the connection point where the wires connect to the printed circuit board?  Does anyone have any ideas or suffered this problem before?   

I have had lots of problems with gauges; sometimes they quit, sometimes they are not
accurate.  I assume you are talking about board the gauge mounts on.  A typical board
has copper traces, a gauge is mounted with iron self threading nuts on a brass pin.  Of
course oxidation between the different metals will ruin the contact in time.  Sometimes
the heat of the bulbs causes distortion so that contacts are no longer reliable. 

I solder across the connections, not relying on the original friction contacts.  Sometimes
I solder a wire onto the board in parallel to the original (now unreliable) path.  Usually
the gauges themselves are good.  2 short wires soldered from the snap in bulb holder
to the board cure any intermittent bulbs; don't turn them around and short the circuit. 
LEDs are better; soldered in, very little heat.  There are some pictures on my
PHOTOBUCKET, album SPEEDOMETERS and GAUGES. 
     http://s93.photobucket.com/albums/L71/bcroe/        click on an Album
  click on a picture to enlarge + description       good luck,   Bruce Roe