[Casey Lipe]

Does the fuel guage or the convertible top switch run through the printed circuit on the back of the guage cluster. I am having fits chasin down the juice for my fuel guage. Anybody help?

[Casey Lipe]

Does the fuel guage or the convertible top switch run through the printed circuit on the back of the guage cluster. I am having fits chasin down the juice for my fuel guage. Anybody help?

[pggto]

Check the sending unit ground by the gas tank. I have found over the years that most of my electrical problems are due to poor or bad grounds.

[peppergreen]

Casey,

The tan fuel guage wire runs directly from the sending unit to the printed circuit.

[78 GHOST]

Casey, the fuel gauge behavior will provide clues regarding exactly what is wrong.

Essentially, the mysterious Tan wire is the powered lead from the gauge to the sending unit. The sending unit acts as a potentiometer to regulate the amount of current permitted to find the (-)Ground (Black wire) of the circuit.

If memory serves, when the float of the sending unit is positioned at the lowest point, or the tank is empty, the resistance of the sending unit is 0 Ohms, permitting the maximum flow of current, driving the indicating needle towards EMPTY.

As the float rises, the resistance increases and this drives the indicator needle towards FULL. Unfortunately, if you have no ground or a poor ground, the fuel gauge will indicate FULL all the time the gauge is powered. This is very common.

When the gauge is not powered, the indicator needle will come to rest at any position. Therefore, you can stick to examining the powered side of the equation.

[Casey Lipe]

I checked/cleaned the ground wire from the sending unit and it is good. I went so far as to run an additional wire(for test) from the ground wire at the sending unit to the negative side of the battery. My guage still reads full! There are three terminals on the gas gauge itself. The two outsied terminals are conected to the printed ciruit. The one in the middle isn't connected to anything(on mine). Shouldn't I have 12v at one of these terminals?

[burgundygto]

Matt- If I remember right, it is the opposite. As the float moves up, you get less resistance. A full tank indication means no resistance, or the tan wire is shorted. When mine did this, I found that the tan wire connection on the tank has a plastic insulator on the stud to prevent it from grounding on the tank. This insulator deteriorated and my tan wire was shorted to the tank. I would try unplugging the tan wire at the trunk connector and see if the gage goes to empty. If it does, the sending unit is bad or the insulator is bad. If the gage stays full, there is a short between the gage and the trunk.

[78 GHOST]

Here's what the 1970 Pontiac Service Manual, Page 12-9 has to say:

An electric fuel gage is used at instrument panel (dash unit) and fuel tank on all models. The fuel gage indicates the quantity of fuel in tank only when ignition switch is turned on or to accessory position.

When ignition is turned off or to start, the pointer may come to rest at any position. The letters "E" and "F" on fuel gage are used to point out direction on indicator travel only.

Gage readings are made from five markings on gage face. The left-hand line indicates empty. The centerline half-full and right line full.

The dash unit is an electromagnetic instrument consisting of a permanent magnetic armature and spindle assembly mounted in a pocket in center of a plastic core with a quantity of silicone dampening fluid to restric pointer movement due to car motion. A pointer is attached to spindle. Around the core, and surrounding magnet armature, are three coils that produce a magnetic field which attracts the armature. The direction of the magnetic field of coils is determined by resistance of the tank unit rheostat.

Coil No. 1 is connected through the gage terminal to battery. The opposite end of coil No. 1 is a common point to coil No. 2 and rheostat. The common point is another gage terminal to which coils 1 and 2 are internally connected and rheostat is externally connected. A third coil is connected to opposite end of coil No. 2. The opposite end of coil No. 3 is connected to ground.

Coils 1 and 2 are wound in opposite directions so that in operation their magnetic fields oppose each other. A fixed calibration resistor is connected across coil No. 1.

Here's the part about resistance

When the fuel tank is empty, the tank unit rheostat resistance is approximately zero ohms. All current passes through coil No. 1 and the armature and pointer align themselves with the magnetic field of this coil. As the tank fills up, the rheostat resistance increases and current now flows in all three coils. The armature and pointer will align themseles with resultant magnetic field of all three coils. The exact position of armature and pointer depends on magnitude of each field produced by coils. The total angular travel of armature and pointer from empty to ful in 90 degrees.

The fuel gage tank unit consists of a float with linkage conecting it to a movable contact arm and rheostat. As the float rises with filling tank, the contact arm moves over the rheostat which increases resistance into the dash unit electrical circuit, and provides more current into coils 2 and 3, and causes greater dash unit pointer movement toward FULL position.

First off, DO NOT connect unregulated 12V to any terminal on the gauge cluster, unless you want to blow $80 for a new printed circuit.

Just because there is an "open" terminal on the back of a gage, doesn't mean is needs you to connect it to anything. Chances are very great that it is used on the inside of the gage canister to connect the coils, or to mount the face. To connect is to 12V is to destroy the gage.

Secondly, if your gage indicates FULL, with the tank less than full of fuel, you have essentially a few "usual suspects."

1. Poor Ground connection.
2. Faulty in-tank sending unit
3. Faulty in-dash indicating gage

Burgundy, in your case the plastic insulator situation permitted infinate resistance, as it was shorted to ground, driving the needle off scale high on the gage. Remember what it says about the system only working if the ignition was on or in accessory, disconnecting the tan wire would be the same as turning the key off, depowering the system, rendering any gage indications meaningless.

Here's my advice to Casey:

1. With a Multimeter, and key ON or Accessory, measure the voltage supplied at the tank by the tan wire. You should be seeing a few volts. The value isn't that critical, we're just seeking to ensure the sending unit is getting power. If you don't have any meaningful voltage at the end of the tan wire, you can assume the wire isn't powered for some reason, rendering anything downstream of the gage moot.

One thing to consider is that over the last 35 years, it is possible for the tan wire to be broken or shorting out somewhere. It's a fairly low voltage system, so it might not smoke a fuse. This would be a problem with the ribbon that runs under the carpet to the rear of the vehicle.

You must ensure you're getting voltage to the sending unit, then ensure the ground is adequate. Your jumper lead from the negative side of the sending unit to the negative terminal of the battery leads me to believe that you are probably fighting a bad sending unit.

Here's what you can do.. Over time, water condenses in the fuel tank, eventually causing rust on the sending unit, fouling it's operation. I've had good luck in removing the sending unit, carefully cleaning it with a good electrical cleaner, and bench testing.

On the bench, you connect your multimeter to each lead, measuring Ohms, and move the float. If it's working correctly, the Ohms will vary with float position.

The sending unit in my truck was having issues with this and I managed to save a bad sending unit merely by cleaning it thoroughly.

[78 GHOST]

Hopefully this helps. I wanted to determine the readings you should look for. I can't vouch for this 100% as I had to make two repairs to the unit. This photo illustrates the Ohms with the float bottomed out.

The readings were around 1.4 Ohms.

[78 GHOST]

Here, the float is pinned in the FULL position.

Also, I made two repairs to this unit. If you look closely at the "powered" lead from the pin where the tan wire attaches, just below the red lead to the multimeter, there is a small red circle. The press-on nut which attaches the lead to the rheostat was loose, preventing a connection to the rheostat.

Also, in the rheostat itself, I had to solder up the tiny wire to restore continuity to the case of the rheostat (ground.)

As I had to resolder the small rheostat wire, the readings might be off a slight amount from a perfect NOS unit.

[78 GHOST]

Here's a dissected look at the innards of the rheostat. Essentially, there is a small spring-loaded shoe which rides on the tiny wire wrapped around a non-conductive material. I apologize for the fuzzy pics. You can barely make out a stamping outside the rheostat canister indicating "90 OHMS."

[Casey Lipe]

Thanks for the info, Guys. I have 8 volts at the trunk connector and the guage doesn't move when I disconnected it. I really feel like I have the sending unit grounded. So, it looks like sending unit or guage problems or maybe tan wire short. I'll let you know what I find.