1960s Gauges not accurate or out of calibration

[gkhashem]

I have replaced my temperature sending unit with a correct one from Lectric Ltd. I have also replaced my fuel sending unit.

The temperature gauge when the car is off reads 1/4 and when started and warmed up goes to 1/2.
Seems like the gauge reads 1/4 to high.  We tested the gauge by grounding the connector and it moved from C to H and had full motion. Before the sending unit was replaced the needle was stuck at 1/4.

The fuel sending unit was replace today and goes to E when off but at first start showed 1/4 full but it should have shown 1/2. Filled the tank and it went to 3/4 full. So this gauge reads 1/4 too low. When I first got the car the gauge was erratic and actually got a but jumpy on the first fill up, then stopped moving and was stuck at 1/8 to 1/4.

Is this a ground issue at the instrument cluster?

[J. Skelly]

Was the sending unit tested at different voltages?  Is it a new, universal sending unit, a factory NOS, or one from another car? 

I had the rusty gas tank replaced on my Riviera and the gauge is now stuck between 1/4 and 1/2.  Different voltages applied to the sending unit resulted in no change to the fuel level display.  My only options are using a universal sending unit or finding an NOS one.   

[gkhashem]

This one is a reproduction made for a 1959-1964 Cadillac. It was about a match to the one in the tank except the pick up screen part.

https://www.ebay.com/itm/233538051561

Most likely as you say probably not an exact fit. If I fill the tank up to the top I get 7/8th full. I also cleaned off the ground wire again and got a good clean ground.

Not too bad of a reading but a bit aggravating.

[J. Skelly]

George,

Have you tried adjusting the float arm?  I know on many newer (last 30 years thereabouts) cars, the sending unit can be accessed from the trunk.  I'm guessing you have to drop the tank to access your sending unit.  I've read that if the float arm is slightly off, that can affect the reading as well.

[gkhashem]

I would have to drop the tank again.

I do not think the arm is adjustable.

[bcroe]

I have had plenty of problems with tank unit
readings.  The rivits, etc that hold the
wiring together can come loose, causing extra
resistance.  On mine they read 0 ohms empty,
90 ohms full.  If you remove the unit and it
does not read 0 with the float down, the bad
connection(s) can be found and fixed. 

The next problem is with motion of the float. 
It should just touch the top and bottom of the
tank at extreme motion.  Check the vertical
range with a ruler, compared to tank depth.  If
wrong the float arm may need change of length
and angle.  I used to do this with every car,
otherwise it would just read full till the tank
was nearly half emptied. 

The temp gauge sounds like a mismatch between the
sender and the gauge.  There is the possibility
of a poor connection where the gauge is mounted,
I have fixed some of these.  good luck, Bruce Roe

[35-709]

Bend the float arm down a little so the float can rise just a little further before hitting the top of the tank.  It does indeed require dropping the tank again which is a pain.  Repops can be expected to be off a little.  What we deal with today.

[dadscad]

I attached the procedure to test the fuel gage and adjust the tank sending unit from my 63 factory shop manual. Hopefully, this will be the same for your car. Or, if you have a manual for your car look in the electrical section for the instruments specific procedure related to your car. Hope this helps.

[gkhashem]

Bend the float arm down a little so the float can rise just a little further before hitting the top of the tank.  It does indeed require dropping the tank again which is a pain.  Repops can be expected to be off a little.  What we deal with today.

Thank guys I suspect you are correct also thanks to David too.

Unfortunately I think you are right and bending the arm down a little sounds like the solution and this should be checked before you put the tank back.

I suspect if I bought another unit that would read different than the one I have.