Fuel gauge sending unit calebration

[N8]

This is for a car I am bringing back from the dead so I don't have any experience with it working properly. I put in a new fuel tank and sending unit. I have the sending unit out of the tank and even if I bypass the stop and have the arm touch the sock the fuel gauge reads 1/4 tank. When resting on the stop it also reads 1/4 tank. Full reads about 1/4 tank past full. With the sending wire disconnected it reads 1/4 tank past full also. Bending things to make the full read properly seems possible, but I'm not sure how to get empty to read properly. Could this be an issue with the gauge? I had to splice on a new sending unit connector because it was missing (there were a lot of broken wires on the underside that seemed like they got ripped off somehow). Could too much resistance in the wire be causing everything to read high?

Thanks for any help,

Nate

[Frank DuVal]

Open circuit on late model fuel gauges always puts the needle way past full. And typically when the gauge reads empty there are a few gallons left in the tank. It is odd that you cannot get the gauge to read empty by swinging the float arm to the bottom stop.

When you run a jumper wire from the sending unit's terminal to ground, what does the gauge read?

Is this a late model? Early and late sending units have different resistance. 30 Ohms early, 90 Ohms late.

[vairmech]

This doesn't matter what model it is.

Unhook the sending unit, turn the key on. The gauge should go to or past the full mark.

Now ground the wire from the car, turn the key on. The gauge should go all the way to empty and maybe a little past.

Make sure you have a GOOD ground outside of the car, use a jumper wire and maybe go to the door jamb courtesy light switch.

If the above works properly then the car system is working the way it should and points to a sender issue.

[N8]

Thank for the tips. I disconnected the sending unit and connected the sender wire the the ground strap in the engine bay with a long wire to get a good ground. When I turned the key off and on the fuel gauge was not consistent. 7 times out of 10 it read 1/4 tank, once it read empty and twice it read half full or more. I pulled the instrument cluster after that and am going to clean the contacts of everything because that was on the list anyway. Is there a way to bench test the gauge while it is out? If the gauge tests ok, I guess that means that there is a wiring issue.

[N8]

Here is what I found on bench testing:

1 ground the case

2 12 v Plus to one terminal..cannot tell you which one...

Gauge should read FULL or over

3 Ground other terminal... gauge should read Empty.

[Frank DuVal]

Ken is spot on!

[66vairguy]

Nate - I did not notice a mention what year your Corvair is! The advice given here has been good.

Running a LONG wire from the engine compartment for ground is not advised. Better to find a good body ground close to the gauge.

Usually the gauges are reliable, BUT after decades corrosion can affect the gauge. If you have a LM fuel gauge I can post picture of terminals and how to test it.

The EM gauge has an exposed wire wound resistor on the back that corrode at the terminals, or can be damaged when removing the instrument cluster. The resistor connectors are crimped to the resistance wire, once the connections are corroded a new resistor (easier to find a newer gauge) is required.

The LM gauge has the wire wound resistor inside the case. Sometimes dismantling the gauge and cleaning the contacts will restore the gauge to correct operation.

[N8]

I have a 63. I ended up taking the gauge out and it fell apart when it was removed. There was a broken rivet and the wires also broke when it came apart. Drilled out the rivet and replaced that and resoldered the wires for E and F. Works as it should now. Bench tested F and E both worked! My guess is that the E side wire had a bad connection inside the gauge because it was able to move because of the broken rivet. I'm going to clean and paint behind the instrument panel while it's out so I won't have everything back together today. I took care of cleaning the cluster while it was on the bench this afternoon.

Thanks again for pointing me in the right direction for the initial trouble shooting! I really appreciate the help.

[66vairguy]

Glad you found the problem. It takes a "skilled" hand to fix a gauge as they are fragile.

I found some pictures of a couple EM fuel gauges I worked on decades ago. They show two different designs. One used rivets to attach the rear resistor, the other does not.

Be careful installing the instrument cluster so the exposed wire wound resistor is not damaged.

Good luck with the car.

[vairmech]

If you want a good used gauge I have one. I'm going to Helen, GA in a couple of weeks.

[N8]

All back together and fully functional!

Thanks again.

[Frank DuVal]