1957 Gas gauge

[Theodore K.]

Gas gauge has not worked corretly for some time. Recently removed the sending unit from the tank and cleaned the resistance wire with scotchbrite and re assembled. I checked out the action of the gauge and carefully measured resistance at empty, full and about 1/2 of range. I then set up and checked the gauge by using a potentionmeter set to the same three resistance values. The gauge basically read okay. It worked correctly for some time. However after running for a time the gauge will drift up and read full. Shutting the engine down and turning the key back on gives a correct reading again but If I drive it the same thing happens. I suspect a ground problem in the instrument circuit but I am not sure. Any suggestions as to what may be wrong?

Thanks in advance.

[Roy B.]

Re: 1957 Gas gauge

Most problems are always a grounding thing

[Jack H.]

Re: 1957 Gas gauge

Yes, you might have a grounding issue with one or more wire contacts being corroded and presenting too high of an ohmic contact resistance profile that heats and increases...

But, another variable in your description is the car's charging system. When the engine isn't running (or running at an RPM level high enough to exceeed the generator's 'cut through'), there's no charging system contribution and the basic supply rail to the system is whatever the battery's current state of charge is.

When you drive and get the engine reved up, the voltage regulator should cause the generator to kick in and drive the battery ABOVE its current charge level to replenish lost charge. Therefore, the system's supply rail is changing (one level without the generator and another, higher, level with the generator active).

Given fixed resistance levels in the gas gauge and tank sending unit, you can expect branch circuit current through the gauge to be higher while you're driving the car than it is sitting stopped or idling. The gas guage is simply reporting the amount of current flowing through the tank sender to ground....

[Theodore K.]

Re: 1957 Gas gauge

Thanks Jack. That is kind of what I suspected. When it is running and charged, the gauge is reading about full but occasionaly I can see it bounce alittle.
Ted

[Theodore K.]

Re: 1957 Gas gauge

Roy & Jack,
Thanks for your comments. Finally got around to cleaning the ground under the dash and on the VR. One or both cured the problem, at least for now.

Roy,
I noticed in a later thread you discussed cleaning the slider in the tank. That is what I did. Instead of sandpaper, just used scotchbright. Also, used a DVM and measured resistance at three different positions and then checked calibration on gauge using a 50 ohm pot and those three readings. Ground problem did not show itself until afterwards when the engine was running.

Thank you both for you good comments.
Ted von Kampen

[Art A.]

Re: 1957 Gas gauge

Ted, By any chance are you related to a Bill von Kampen who worked at Chevrolet Engineering?

Art Armstrong

[Theodore K.]

Re: 1957 Gas gauge

He is my brother. I also worked there two summers as in intern in 1962 & 1963.
Ted