Fuel sending Unit>>> No Low Level light... - PeachParts Mercedes-Benz Forum

fred98050 · #1 03-16-2010, 10:50 PM

Removed the fuel sending unit today, cleaned it up, 3 fine wires in place.
The level gauge works fine but I don't get the LowLevel light in the instrument cluster (checked the light).

Saw a small resistance in the Sending Unit, could this be part of the "low level" light system?

ah-kay · #2 03-16-2010, 11:57 PM

Not sure about the 300D and 300SD. In the 190D, at the bottom of the sender unit are 2 metal contact points. The float will come down and make the connection to turn on the low level warning light. There may be a metallic disk at the bottom of the float. Normally they are cover in muck and grime. See how the float comes down and what it touches, clean those and it should be OK.

If you can trace the wire back to the connector and short the 2 wires at the connector then you can see the light comes on without actually installing the sending unit.

Good luck.

Jeremy5848 · #3 03-17-2010, 12:57 AM

Correct. The low level switch should have near-zero resistance. If the contact points are dirty or corroded there will be too much resistance to light the bulb.

300dOwner · #4 03-17-2010, 09:24 AM

Plenty of posts on how to clean the sending unit...driver, start your search engine!

My 300D had the identical problem, all due to a dirty sending unit. Sediment builds up at the bottom of the sending unit and insulates the contact points. If you clean out the unit pay special attention (and care) to the very thin nichrome wires and if you disassemble the float system to clean it, be certain the tiny wipers that run along those wires are correctly situated. Wanna ask me how I know??

Good luck, the force of the forum is behind you!

gerkebi · #5 03-17-2010, 05:57 PM

Also, the low fuel light should light when you start the car for a few seconds, just to make sure the bulb is good.

fred98050 · #6 03-18-2010, 05:45 AM

Quote:

Originally Posted by 300dOwner

Plenty of posts on how to clean the sending unit...driver, start your search engine!

My 300D had the identical problem, all due to a dirty sending unit. Sediment builds up at the bottom of the sending unit and insulates the contact points. If you clean out the unit pay special attention (and care) to the very thin nichrome wires and if you disassemble the float system to clean it, be certain the tiny wipers that run along those wires are correctly situated. Wanna ask me how I know??

Good luck, the force of the forum is behind you!

I thought that I had cleaned the unit pretty well. I guess I'll take it out again and check more closely.

b2948kevin · #7 03-18-2010, 12:07 PM

I'll confirm what Ah-Kay said. There are 2 contacts at the bottom tha you need to spend some time with. Those are the lights that trip the low level light.

Have you checked the dash bulb?

fred98050 · #8 03-18-2010, 01:26 PM

Quote:

Originally Posted by b2948kevin

I'll confirm what Ah-Kay said. There are 2 contacts at the bottom tha you need to spend some time with. Those are the lights that trip the low level light.

Have you checked the dash bulb?

Yes, Thanks

To make sure, I swapped the (working) Highbeam socket with the low fuel light socket.

When I get back home I'll have to clean these 2 contacts really well and see.I need to test the gauge itself and see if it is sending the "low light signal" or not (might have a problem somewhereelse).

fred98050 · #9 03-21-2010, 01:46 PM

So I removed the unit once again. All contacts are clean at the bottom and the metal disk under the float is clean as well. I then connected the unit (without putting it in the tank) and the gauge is working fine, however even when the float is at the bottom making contact with the 2 lower contact points the light does not come on.....

Also, when you turn the ignition on, I understand the light should come regarless of float position. I am trying to find out if it comes on as a test of the sending unit or as a test for the light????

I wonder if anyone would know how to test the gauge for proper operation using a voltmeter.

Thanks

bustedbenz · #10 03-21-2010, 04:13 PM

Quote:

Originally Posted by fred98050

So I removed the unit once again. All contacts are clean at the bottom and the metal disk under the float is clean as well. I then connected the unit (without putting it in the tank) and the gauge is working fine, however even when the float is at the bottom making contact with the 2 lower contact points the light does not come on.....

Also, when you turn the ignition on, I understand the light should come regarless of float position. I am trying to find out if it comes on as a test of the sending unit or as a test for the light????

I wonder if anyone would know how to test the gauge for proper operation using a voltmeter.

Thanks

Test for the light as far as I know. Could be wrong.