Here's a summary of the process to troubleshoot the gas gauge. Olds used a 0-30 ohm sending unit through the 1964 model year and a 0-90 ohm unit from 1965-on. In both cases, 0 ohms drives the gauge to EMPTY and the other end to FULL. Once you know that, you realize that if the gauge is pegged on EMPTY, there's a short to ground somewhere between the gauge and the sending unit in the tank. If the gauge goes past FULL like the one in the photo, that means there's either a bad/corroded connection or an open circuit (which is infinite resistance) somewhere between the gauge and the ground at the tank.

The hard short (pegged on EMPTY) is pretty easy to diagnose. Simply trace the wire from the gauge to the back of the car until you find the short. The tan wire runs from the dash harness to the ribbon cable that runs under the carpet to the rear lamp harness in the trunk (or in the rear load space on wagons). You'll find a rubber grommet at the back of the trunk where this harness connects to the sending unit lead-in harness. A short can be anywhere in this harness, including the part under the car from that grommet to the tank sender. Of course, if this is a new problem, look first at locations where you may have been doing other work recently.





If the problem is the needle is past FULL, the potential bad connection or open circuit locations are more varied. The easiest and first thing to check is the connector on the tan wire at the back of the trunk. Unplug that connector and use a jumper to ground the wire in the rear lamp harness that runs forward. If the gauge goes to EMPTY, the problem is from that point down. If it does not go to EMPTY, there is an open circuit between the connector and the gauge (like a bad pin on the back of the gauge cluster). If this test works correctly, reconnect the wires and go under the car. Clean the connection where the ground wire bolts to the floorpan. If that fixes the problem, you're done. If not, the tank has to come out to access the other connections or the sender or the sender itself. It's also possible that the ground wire where it is soldered to the sending unit has come loose, in addition to a bad spot in the sending unit itself. The bottom line is, don't assume you have to immediately replace the sender or even drop the tank until you do a little troubleshooting.

 

Thank you for posting this Joe. My gauges readings are wonky, I will study the sequence of errors mine has and try to match up to these guidelines.

 

Good info Joe, thanks for posting.

My gauge works fine on moderate to warm days, but when it gets cold, it takes on the "open" condition (reads past full) until the tank is below 1/2 full, then it seems to read OK. Thus, I suspect that the sender itself has become temperature sensitive and opens up when cold and more than 1/2 full. Its been like this for at least a decade. I'll eventually get to it, but as long as I get a reading when it gets low, I can suffer.

 

Thanks for the write up, Joe.

I had trouble when I installed a new fuel sending unit in my 71 98. The sender kept shorting to the bottom of the body. I had to re-engineer the fuel sending unit.

https://classicoldsmobile.com/forums...71-98-a-73750/

 

Yeah, oddball problems like that can be very difficult to diagnose, since traditional troubleshooting flowcharts don't necessarily take those types of problems into account. It can take careful insight and understanding of the system to successfully diagnose them. Well done.