fuel gauge help needed
 

71 firebird esprite with big clock idiot light cluster.When key is off the fuel gauge reads 1/2 full,when I turn on the ignition key the needle goes to 3 oclock instantly.It has always worked and after sitting 6 months it is acting up.Is this a sending unit or float or what ? I pulled the sending unit ground and cleaned but no change.Thanks for any help.

GM car gauges are designed using a pair wire wound 12 volt DC electro-magnetic coils. When the key is not turned on; any GM car or truck gauge (Fuel, oil pressure or coolant temperature) on any car built after 1965 (A body), 1967 (F body) can come to rest at any place on the dial. This does not mean anything. When you turn the key on 2 things happen. 12 volts at the back of the gauge on one threaded post. 12 volts battery + energizes both coils. At the tail end of one coil the current goes thru a 90 ohm resistor (mounted at the back of the gauge) to ground. The steel housing is used as part of a ground pathway to get the connection back into the PC and then to the plug. (On all 2nd Gen F body cars there is a thin black wire in the multi-plug) which is constant ground.

The other coil exits out a different post thru the printed circuit into that same multi-plug. The wire in that plug which relates to the fuel gauge is tan. The tan wire goes thru several more plug-in body connectors as it makes it's way back to the trunk area. Then it goes underneath the car to the fuel tank sender. On the edge of the sender there is a black wire that connects to ground. This is what gives the gauge it's reading. It is a reference to either more or less "ground" at the trunk floor pan underneath the car.

More ground = trending towards the E mark while less ground trends the gauge towards the Full mark. In ohms one or zero = E . 44 or 45 ohms = 1/2 scale and 89, 90 or 91 ohms resistance at your tank sender = Full. (This standard of ohms as it relates to the fuel tank gauge/ sender was used on every GM car made after and including the year 1965). Prior to that GM cars had a zero to 31 ohms scale range to the fuel tank sender.

If your pointer goes way past the Full mark when you turn the key on the first thing I would check is the tan wire.

Follow that wire all the way back to the trunk checking the Fisher body plug-ins as you go. The next thing to check would be the sender ground. Often the attachment of this wire will become corroded on cars that sit. The black wire should be intact. That wire is grounded all the time, (even if the car is not running.) Never EVER PUT POWER to the tan wire, you will damage the sender and perhaps cause a fire. The tan wire (key on) should have about 3 volts in it. You can measure the tan wire with a voltmeter to determine if your dash gauge is bad or your tank sender is bad.


I would say you have an open circuit somewhere in the tan wire string.

It could be dash area (printed circuit) a bad plug-in connection in the body harness or trunk compartment; that or it could be a bad gauge, or bad tank sender. Old tank senders do go back after 50 years. These parts were not designed to last forever.

Fuel gas issue
 

Hi Peter,
Thanks for the detailed info! I have a similar problem that both my brother and I have yet to solve and hoping you can steer us in the right direction. I’ll try to be brief but important I share all the areas we’ve ruled out.
68 Pontiac Lemans - New sender, new sender wire, new electrical harnesses throughout. Non rally gauge set, new gauge PCB but original fuel gauge. Fuel gauge reads accurate at key start position. Once you get the car running the gauge passes full and off scale. Grounding “S” ohm to ground on back of gauge sends needle to empty. Reading 75 ohms at back of gauge which is accurate to gas level in tank. Checked new sender before I installed and was putting out 90 ohms at full. Checked sender ground connection- good. Replaced voltage regulator and charging circuit with 13.8 volts. Checked various grounds and all good. All other indicators on gauge, such as the temp, oil, gen lights work fine. I have 3 supplementary gauges under dash, voltage, oil pressure, temp and they all work properly. Any ideas would be appreciated! Your thoughts?

Need to do testing at the back of the car.

Un-plug sending unit wire, ground it and see if guage goes to empty.

Ohm test sending unit wire from tank to check fuel level and see if sender is working.

Ohm testing back of guage gives reading on resistors inside of it.

Clay

Fuel gauge issue
 

Thanks Clay! I think you nailed it.

Un-plug sending unit wire, ground it and see if guage goes to empty.
Does not go to empty


Ohm test sending unit wire from tank to check fuel level and see if sender is working. Yes reads 76 ohms which is accurate for fuel in tank


Ohm testing back of guage gives reading on resistors inside of it. 0 ohms with sender wire disconnected
Still 0 ohms with sender wire connected

I have brand new harnesses from gas sender including the Mylar PCB. I’m suspecting the connection at the PCB or even the PCB itself?

I have continuity 0 ohms from trunk harness to connection under dash. However I have 16 ohms from under steering column connection to “S” connection on back of gauge. Trying to get the Mylar PCB connector off without removing the gauge cluster! Lol.

There are several makers of those new printed circuit boards. One is made using white mylar, and another makers version has a green tint to the clear. One thing I have noticed, especially on the one with the green tint is that (depending on how long) one of those has sat on a warehouse shelf in a plastic package the raw copper develops sort of a "film" on it. It may have even been left over from the manufacturing process. I can't say for sure. The cure is that you need to VERY carefully clean all the copper "footprints" on one of those new PCs before you install.

One more item which is very important is you MUST HAVE clean hardware to attach the threaded studs from the fuel gauge that connect power and seeker to the PC. You need to use brand new or cleaned & re-plated hardware. NEVER EVER re-use old corroded and/or rusted hardware on a new PC!!!!!

On all 68-thru 72 Pontiac A body cars there is no ground wire in the plug-in. The ground for the dash cluster is achieved by overlapping pieces of metal. The ground is the steel housing that the gauges and instrument lights are mounted into. So as a first thought; adding a black ground wire to the back of the gauge housing and running that wire up underneath the dash to a solid ground (bolted connection) would be a great way to ensure that you have good ground before you do anything else.

The threads on those mounting posts are 6-32. There is also a #6 flat flat washer on top of each copper "footprint" before you thread on the nut. One more thing I have seen is the thin "arched" copper tabs inside of the master plug will turn brown or tan when they get old. You should clean those tabs where they touch the PC using an eraser. They make special erasers for people who have model train layouts. (To clean the rails on the train track.) Those erasers work great for cleaning electrical touch points on car wiring. I have used those for years. To clean a new printed circuit I highly recommend that you buy the solid rubber eraser, NOT the one that has bits of abrasive grit molded into it! (Grit just like sandpaper.) They sell both kinds of erasers for people who have model train layouts so make sure you get the right one!!!

I can post photo of the one I use if anyone needs to see that; just ask.

Should have got some kind of reading from sender stud to ground. Printed circuit and connectors wouldn't change that reading unless you had a dead short some where.

Just tested a firebird fuel guage (S to ground) and got 97 ohms.

Clay

Thank you guys for the help! I bypassed the Mylar PCB and was able to determine the wiring from the back of the car was sound. I connected power and sensor (ohm signal) directly to the gauge without going through the Mylar PCB. What I found was the small white strip 90 ohm resistor connected from gauge studs 12 VDC to sensor stud was corroded. Of course it was the one item I forgot to disassemble and clean when I restored the original 68 dash! I cleaned off all the corrosion and re-installed, first without the Mylar PCB. Success! The fuel gauge now reads accurate (tank almost full) with ignition at start position and when engine running. Previously with engine running the needle would pass full and go off scale out of view. I think cleaning up the 52 year old resistor solved the problem. I need to reinstall the Mylar PCB and cross my fingers. I will take note of all your suggestions on using new hardware and clean connections so I don’t introduce a new problem.
Thank you again!