Hi world,
I need some help with the fuel gauge in my 74. The car is a 6 cylinder conversion but I don't think that this factor contributes to the problem.
The issue is that the gauge shows that the tank is 1/4 full even though it is full to the top.
I have two other gauges that I tried and both show the same 1/4 tank level. In my book this would indicate that the sending unit is the culprit. Couldn't be that all three gauges have the same defect. However, measuring the voltage at the gauge, I get 12.4VDC. I think this means that the sending unit is sending the correct signal. So where is the problem???
Has anyone had this problem? Any ideas?
Any and all help is appreciated.
Thanks,
Bill.

Remove the sending unit, take it apart, and clean the fireman's pole that the float sides up and down upon. An easy fix, the pole gets corroded and the float will not slide up and down

dr914, OK thanks.
So you are saying that as the float goes up, the resistor should send less volts?
Essentially empty = 12V, and full = somewhat less then 12V ?
Thanks,
Bill.

0-90 ohms resistance to the gauge, When it gets stuck nine times out of ten it is the sliding float, once in a while the wire breaks and then you can solder it back together

The sender is basically a variable resistor, so you could measure the resistance (in ohms) while moving the float up and down. As noted, it should read from 0 to 90 ohms(or there about).

Some of your terminology is off, so hopefully this helps:

The gauge should have battery voltage to it, and the sending unit provides the ground circuit. It has a float that rides up and down on a guide rod. The float has contacts (wipers) that run on resistor wires in the sending unit.
When the fuel level is high, the resistance is low. More current flows through the fuel gauge, and it shows a high fuel level. When the fuel level is low, there is more resistance, so more less current flows through the gauge, and it shows a low level.

Like George said, the most likely problem with a gauge reading low is the float stuck on the guide rod. Mine would stick on 3/4 full until I hit a bump, then it would read correctly for a while.

These cars can sit, and condensation in the fuel tank causes the rod to rust and keep the float from moving freely.

Take it apart and clean it. Just be careful of the resistor wires. They are very fragile. And replace the gaskets.

Hi guys,
I removed the sending unit from a spare tank and am ready to take it apart even dough the float seams loose and free inside. So how does one disassemble this thing? Is it the little nut on the bottom?
Thanks,
Bill.

Click to view attachment

Click to view attachment

Anybody?
I removed the small nut on the bottom of the sender but it is not coming apart.
What am I doing wrong?
Any suggestions?
Thanks,
Bill.

Yes. remove the little nut on the bottom, carefully slide the tube off. you will see the "firemans pole" George spoke of and two VERY fine wires. do not put any pressure on these wires they will stretch easily and sag or worse, break. you can clean them with carb cleaner and a q-tip. then use fine sandpaper on the center pole.

did I say CAREFULLY enough ?

mine would hang up in different spots and this fixed it.

I have not been able to take it apart yet. I have taken the small nut out at the bottom but the tube still will not slide off. I guess I need undo the crimped area that holds the tube to the plastic part at the bottom.
I did measure the resistance at the pins and it goes from 54 Ohms with the flow at the top, to 70 Ohms with the float at the bottom of the sender.
I know that the Dr. mentioned 0 to 90 Ohms. If that is the target this sender is way off.
Bill.

You can get a brand new OEM Bosch unit for about $150 on the internet.

I just had mine apart, and I don't remember having to un-crimp the bottom. I think if you gently wiggle the tube back and forth, it separated at the top. It has been on there for 40+ years!

BTW, my sending unit had 66.9 ohms empty and 9.2 ohms full.

I bought a new one from Sunset Porsche. It’s s nice to know how much fuel you have especially the way these /6 cars consume it.

Hi again guys,
Last knight I was just being an idiot but I did finally figure out that the tube separated from the top mounting.
Attached are some photos of the inner part of the sending unit. To me it looked absolutely clean. The center pole was shiny like polished stainless, and the three wires were intact.
The float moves freely and when I measured the resistance again now it read 70 Ohms at empty and 6.4 Ohms in the full tank position.
The thing is that this sending unit was also showing 1/4 tank even when full. I am beginning to think that the wires are in the wrong position at the gas gauge.
Can anyone confirm the correct position of the wires at the gouge? I looked at the factory shop manual electrical diagram but it does not show which prong each wire connects to.
Thanks,
Bill.

Click to view attachment

Click to view attachment

Your connectors should be in a plug that only plugs in one way

Cabinetmaker,
Yes, that is the plug at the sending unit, thanks. But I would like confirmation of the wire positions at the fuel gauge.
Bill.

Here you go.

Click to view attachment

Thank you Mike.
Bill.

Hi Guys,

My connector at the sending unit was broken.. My gauge sits on E. I will be pulling the sender as above. Can someone give me the correct wire locations / colors for the connector so I can verify wiring at the sender end?

Hi all,
fuel gauge problem solved. As several of you suggested it was indeed the sending unit.
The spare sending unit did the trick.
Thank you all for the help.
Bill.