Fuel sender or gauge?

[Doug Wilson]

I have a 1975 450 SL that the fuel gauges sometimes works and sometimes doesn't. I have taken the plug off the sender and checked voltage there and have 12 volts to one of the three terminals, the other two I have Ohm readings to ground. I have removed the sender and checked for Ohms from the terminal that would have power going into it to the other two terminals and get no reading. I do get an Ohms reading between the two terminals, however. Does anyone know if this is correct? I hate to spend $140.00 for a sender and then that not be the problem.
Thanks

[dpetryk]

Using a resistance subitution box apply a resistance across the appropriate terminals of the plug and you should be able to make the guage move. If that works, you need a new sender. However, I must say that they are easy to repair. Generally they stop making good electrical contact and that is easily corrected.

[Doug Wilson]

Thanks Dave. When you say they are easy to repair, do you mean the sender actually comes apart? How do I go about trying to repair the sender or does the cleaning come on the head where the electrical connections are?
Thanks

[dpetryk]

Once you have an understanding of how these things work you will understand how they fail and how to fix them.

Here is how they work:

The sensor functions as a simple variable resistance element. The resistance is determined by the length of a resistor wire. The length of this wire determines the resistance of the circuit. The length of the wire is changed by a floating shorting device which moved up and down over the length of the resistance wire. The length of the resistance circuit is determined by the position of the float. As the float moves along up and down the shorting device provides a short circuit thus changing the restisance at the terminals of the sensor. As this resistance changes the fuel guage moves as well.

Here is how they usually fail:

The shorting device on the float usually fails to make good contact with the resistance wires. When this happens we see the "dancing guage" phenomonen. The guage will bounce from empty to some other level. This problem is usually due to a deposit or small corrosion forming on the resistance wires. They can fail where the wires are broken and in this situation you cannot repair them. But in my limited experience, I have never seen that.

How to fix them:

Using steel wool (very fine grade) carefully remove any deposits that may be on the resistance wires. One must be carefull not to break the wires since they are very fragile.

In the case of the sensor shown here which its from a 1987 420SEL, the resistance varies from about 85 ohms when the tank is empty to 3.6 ohms when it is full. I assume that your sensor operates of the same principal and is probably very similar to mine.

So check your sensor with a good meter and substitute the sensor with a resistance source to check your guage and isolate the problem.

[dpetryk]

A picture of the sensor internal parts and the resistance measured at various float positions.

[Zeus]

Just wanted to say...great pics!

[Doug Wilson]

That does make sense. I will tear it apart and see what I can do. Thanks again for the help.
Doug

[Pete240D]

I have the problem you've discribed (tank full w/ gauge indicating 3/4, and bouncing needle near empty). The pics you loaded didn't come through, can you try loading them again or e-mail me at pas7212001@yahoo.com ? Thanks! Pete

Originally Posted by dpetryk

A picture of the sensor internal parts and the resistance measured at various float positions.

[Blue 72 250]

you could just remove the sender and spray some carb cleaner in the little hole at the top. Then rust will come pouring out. The rust is what causes the problem. I would not open the sender up myself, as the wires are very easy to break. That should get you about 1 year of functionality before you have to do it all over again.

[mbzr4ever]

Originally Posted by Pete240D

I have the problem you've discribed (tank full w/ gauge indicating 3/4, and bouncing needle near empty). The pics you loaded didn't come through, can you try loading them again or e-mail me at pas7212001@yahoo.com ? Thanks! Pete

What Pete said, pictures please?

[vox_incognita]

http://www.dvsegmbh.info/PDF/einbau/42007/83/15-60_TAUCHROHRGEBER%20MB.PDF

224-817-014-025R ,I think....
float stop upper limit (full)
3.2 ± 0.5 Ohms
float stop lower limit (empty)
69.4 ± 2.0 Ohms
Clearance: Sender lower edge to tank
bottom > 4 mm
Good luck!

[mbzr4ever]

Thanks for the diagrams.

Took the sender out, looked good, except for some black, sooty residue on the bottom and the sides.

I just wiped it off best I could, without using any solvent or touching the wires.

We'll see if it eliminates my "jumping gauge" incidents.