fluel gauge

[piddler]

The fuel on our 1967 230SL was sporadic for a while and then it went to empty and stayed there. Does anyone know the ohmic of the sender at empty and full? Is there a problem with the fuel gauge that is fairly common to these cars?

I have worked on cars as a hobby for a lot of years and I have learned that there are peculiarities to each of them. This is my first Mercedes so I will be asking a lot of questions.

Thanks, Pete

[DaveB]

There's some information on this in the site's technical manual:

http://www.sl113.org/wiki/Fuel/SenderUnit

[alchemist]

You may have to remove the sender cylindrical unit which is under the trunk mat. Disassemble it and clean it, use DW40 and rubbing alcohol. Sometime gunk in this unit causes your problem. Be careful not to damage the thin electrodes when you disassemble the sender.

[piddler]

I removed the sending unit (later type) and checked with an ohmmeter. With the sending unit inverted and the float at the top it read 67 ohms. With the sending unit upright it read 6 ohms. When I reinstalled the unit in the tank it read 30 ohms. I think this would mean that the sending unit is working properly. Am I correct in this assumption?

Is there something else in the circuit that is a common trouble spot?

[piddler]

I removed the gauge and checked the wiring from the sender to the gauge. everything seems to be intact. Is there any way to check the gauge itself?

I don't know how much voltage should be applied to operate the gauge. Would a 9 volt battery cause some movement? Would the full 12 volts be to much?

The other thing that I found was that there was no bulb in the hole for the reserve warning light. I could not find a connection for a bulb. Is there something simple that I am missing?

[piddler]

OK, I found the bulb for the low fuel indicator. Sometimes I just need to see what I am looking at.

Going to make some voltage checks when I get back to the car.

[hands_aus]

Hi,
I see you have a later 230SL.

There were a couple of different types of fuel sender and gauge combinations based on the model of the Pagoda and the fuel senders matched the fuel gauge for the different models.

The impedence of the sender for a 230SL and gauge were matched but would be different to the matched impedence for the later model sender/gauge combination.

The original sender for the 230SL was a cantilevered floating arm with a contact that rubs on a wire wound ceramic block.

The later sender you have may be correct for your car because often MB introduced changes in later years of a model.

This has been discussed on the forum with a solution being to change the fuel gauge to match the sender.

I have often thought there must be a way to attach a variable resistor to the circuit and adjust the impedence until the gauge is reading correctly.
However I am not a electronics technician.

I think the different impedences were/ are available in the Technical Manual  or in forum discussions.

cheers

[piddler]

Thanks for all the input. The problem is resolved.

 I am not sure what I did to fix it. I took the sender out and cleaned it up. Removed the gauge assy. and checked it out as best I could and reassembled everything. I did check the gauge after I cleaned the sender so I don't think that was the problem.

I did get a chance to replace the bulbs in the center dash unit. I hope I don't need to take that unit out again.