1958 Gas Gauge

[Jon S]

Gas gauge goes to "E" when ignition off.  When running it goes beyond "F" when filled up and shows 3/4 when close to empty.  Bad sending unit or something else I should check e.g. wiring connections?  Any simple tests?

[savemy67]

Hello Jon,

The shop manual for my '67 has a couple of simple tests to determine whether the gauge or sending unit is at fault.  One test checks the sending unit wire at the connection near the tank.  This may be located under the carpet in the trunk.  A couple of test jumpers and a multi-meter came in handy.

To test the gauge involved removing the dash pad, which seemed like more work.  My tests indicate my sending unit is bad, so I will have to remove my gas tank (even more work).  Based on what I read in my '67 shop manual, your symptoms sound more likely that your problem is the sending unit and or wiring, not the gauge.  The '58 shop manual should describe the tests which, in the case of my '67,  I did not find difficult to accomplish.  Good luck.

Christopher Winter

[Jon S]

Christopher -

The Shop Manual suggests using an AC Gasoline Gauge Tester P/N 1516000. Obviously I don't have one. Ignition off, the gauge registers Empty correctly. Ignition on and tank full, gauge goes beyond the full mark. At 1/2 tank it drops a little below the normal Full mark. So it is changing with the level of gasoline, just not in correct correlation. I had the dash pod out a while back. Maybe the printed board connector is not on properly or the obvious - the float is defective. I'm hoping for the former.

If the float (original) was bad everything I'm reading implies the gauge would read empty. So maybe it's a connector issue.

[savemy67]

Hello Jon,

I wonder how much an AC Gas Gauge Tester cost.  Do you have a picture?

From my '67 shop manual (you may be able to apply this to your '58):

1.  Ignition off
2.  Disconnect the wire running from the sending unit to the gas gauge in the dash.  In '67 there is a connection behind the rear license plate that is also accessible from inside the trunk under the trunk liner/carpet
3.  Turn on ignition switch (but do not start engine)
4.  With wire disconnected, gauge pointer in dash should read above full
5.  Connect the wire coming from the gauge to a ground (this is for a '67, so be careful with your '58 - check the wiring diagrams in the '58 shop manual).  With wire grounded, the gauge pointer should read below empty.

If your test results are as above, the sending unit is bad (again, that is what my '67 shop manual says, your '58 may be different).  Your gauge appears to be working but the level is indicated incorrectly.  My research indicates that this is usually a function of a failing resistance circuit in the sending unit.  Since the level on the gauge changes, the float appears to be working properly, although it could be getting stuck.  Unfortunately, it looks like a fix involves getting the sending unit out of the car.  Good luck.

Christopher Winter

[bcroe]

Hello Jon,

I wonder how much an AC Gas Gauge Tester cost.  Do you have a picture?

From my '67 shop manual (you may be able to apply this to your '58):

1.  Ignition off
2.  Disconnect the wire running from the sending unit to the gas gauge in the dash.  In '67 there is a connection behind the rear license plate that is also accessible from inside the trunk under the trunk liner/carpet
3.  Turn on ignition switch (but do not start engine)
4.  With wire disconnected, gauge pointer in dash should read above full
5.  Connect the wire coming from the gauge to a ground (this is for a '67, so be careful with your '58 - check the wiring diagrams in the '58 shop manual).  With wire grounded, the gauge pointer should read below empty.

If your test results are as above, the sending unit is bad (again, that is what my '67 shop manual says, your '58 may be different).  Your gauge appears to be working but the level is indicated incorrectly.  My research indicates that this is usually a function of a failing resistance circuit in the sending unit.  Since the level on the gauge changes, the float appears to be working properly, although it could be getting stuck.  Unfortunately, it looks like a fix involves getting the sending unit out of the car.  Good luck.    Christopher Winter     

This is the test that works for me.  My most common issue was that the ground wire from the tank
sending unit to the frame had a high resistance connection.  You can check the frame end easily; the
tank sender end needs the tank out (but not opened up).  Bruce Roe

[55 cadi]

I had a thread a few months back on this subject,

It turned out that the sending unit was not grounded good to the frame/trunk floor, to fix I have wire connected at the sending unit and the trunk floor, fixed the problem of fuel gage going from empty when off to pegged full when key on.

I replaced  the fuel tank and put in rubber between the tank and trunk floor to prevent rubbing and noise, and doing that the grounding is gone that the sending unit needed.

Jason

[walt chomosh #23510]

Ditto 55 cadi's fix on my 55 CDV....to test,one could simply hook a grd wire to a screw holding the sender to the tank and go to a good grd.(frame) All the tank unit does is vary the circuit resistance as the float travels up and down...there must be a ground...walt...tulsa,ok

[TJ Hopland]

I was just helping a friend with some odd gauge issues.   Problem in his case turned out to be the gauge cluster itself was not well grounded.  In his case it was a combination ammeter and coolant temp,  coolant temp was not responding correctly and we were pretty stumped because all the testing seemed to more or less check out.  We pulled the cluster and on the bench it didn't work at all which made no sense.  We then noticed a ground symbol on the diagram connected to the case as soon as we did that on the bench both gauges seemed to respond correctly.   We then cleaned up the back of the cluster and where it sat in the dash and got it all back and it worked.    I'm not sure why it had to be grounded since the coolant temp works like the gas where the sending unit grounds it.   I wonder if it was interaction with the other gauge in the cluster or some other way the gauge works where it needs a constant ground to properly reference what it should be doing?   

Don't know if yours works like that or not, just figured I would mention what we found just in case it helps you.