1980 300TD fuel light not working - PeachParts Mercedes-Benz Forum

turbozeke418 · #1 03-24-2012, 09:27 PM

I have a 1980 300TD with 190,000 miles. Runs great. I have never seen the low fuel light work. It has a bulb in the in the back of the cluster for it and I replaced it with a new bulb. The tank is almost empty right now and the light still doesn't illuminate. Is this a common thing or is there a possible easy fix that I'm not aware of. I don't NEED the light. I just would like it to work since it's there.

kerry · #2 03-24-2012, 10:53 PM

Tank has to be very low for it to work. Have you tried running the tank out of fuel to see if it comes on? Sender gets dirty. Not that difficult to remove and clean if that is the cause.

turbozeke418 · #3 03-24-2012, 11:19 PM

I've run it down to the point that it was bone dry. and it never came on. The car sat for about 4 years without moving. When I first started driving it 2 years ago the fuel gauge would work from full to 3/4 then go right to empty. Now it goes all the down to less than 3/4 but it gets jumpy and pegs back and forth between completely empty and the actual fuel level. The light never comes on though.

tee51397 · #4 03-25-2012, 08:05 AM

Pull the sending unit, take it apart and clean the contacts on the bottom. They can get covered in a black film that prevents the light from coming on.

Also if the tank is low you can pull it up a little and plug it back in to simulate a totally empty tank and see if the light comes on.

Alternatively, your post above sounds like a earth or connection issue. I'd start with a cleaning / inspection of the sending unit and go from there.

Jeremy5848 · #5 03-25-2012, 09:59 AM

When you turn the car on in pre-glow, all of the dash lights should come on, right? The low fuel light should also come on, then go out with the rest of the lights when the engine starts. If the low fuel light doesn't come on with the rest then there is a problem in the instrument cluster. Otherwise it is a sender problem as others have described or (small possibility) a wiring problem between the sender and the instrument cluster.

Jeremy

79Mercy · #6 03-25-2012, 07:14 PM

^ not on a 1980, the low fuel light only comes on when its low on fuel, this feature didnt come into play until late 81 or 82.

Unless he has a newer style gauge cluster in the car.

tee51397 · #7 03-25-2012, 07:59 PM

^ Yes, I can confirm the above is accurate.

Yak · #8 03-25-2012, 10:15 PM

The above descriptions of the "self-test" feature of the fuel low light are correct. That was a change in 81 or 82. Prior to that there is no way to tell if the bulb works or not until you run the fuel down that far.

And if the fuel is down that far, but the sending unit is gunked up, then the electrical connection to turn the light on might not get made since the float is sitting on top of gunk instead of metal to metal contact.

It's an easy repair on the sedan & coupe, probably similar on the TD. See the pics in post #22 in this thread for an indication of how bad the senders can get: W123 300D Fuel tank sender question

It's also a possibility that the contacts may have corroded away entirely. I've seen one like that in a JY. In addition to Tee's tips above, you could possibly check the function of the bulb by jumpering some pins in the connector.

I'd just pull the sender and carefully clean it, but that's without knowing the nuance of the TD tank layout. The TD tank is a bit different, with a "tank within a tank" capacity and that may also affect the function of the light in some way. See the drawings here for info on the tank screen: How does the tank screen work on a 123 wagon

turbozeke418 · #9 03-27-2012, 02:23 AM

I removed the fuel sender tonight and cleaned it all up. I plugged it in and started the car up without it inside the tank. When I tip it upside down the needle goes right to full. When I hold it rightside up, the needle goes down to empty and the yellow light comes on so I know the light works. I reinstalled everything even though its showing empty (with a little space left under E when you shut the car off) the light isn't on. Inside the tank from looking through the opening when I had the unit out there appeared to be a few inches of fuel in the tank. I guess i'll drive it till it runs out again and see what happens this time.

turbozeke418 · #10 03-29-2012, 01:29 AM

Ok I removed it again and took the entire unit apart (didn't know you could dissasemble it) and I cleaned it all out. The light works and I made it to the gas station and filled it up. The light went off and the gauge doesn't move at all. I removed the sender again and I tapped it against the housing and the needle wood spike every once in awhile. This made me think that when I put the connector back on I might of had a wire crossed? My connector is broken and when I took it off the sender the first time the wires came out. I put them where it looks like they were originally but I could have gotten it wrong. Does anyone have a schematic for which color wire goes to which terminal on the sender? One of the resistored wires is currently attached to the G on the top of the sender. The other resistor wire isn't connected.

charmalu · #11 03-29-2012, 11:24 AM

I posted this on ARMY`s thread where you asked this question.

The Brown wire is your ground.

Blue/Green wire is for the low fuel indicator.

Blue/Black wire is for the Fuel gauge sender.

This is as per my Electrical Trouble Shooting FSM 83-85 model yr.

Charlie

turbozeke418 · #12 03-29-2012, 04:48 PM

thanks

Yak · #13 03-29-2012, 07:50 PM

Same thing Charlie said, but with a picture.

I had forgotten about the compensating resistor due to the TD's tank.

turbozeke418 · #14 03-29-2012, 08:11 PM

I just readjusted everything and the fuel gauge still reads empty. If I touch it to the "T" it pegs at full. If I touch the blue/Green wire to the "T" terminal the reserve light comes on so the "t" would be the ground. I attached the brown wire to it and then the Blue/green wire to the "W" and the blue/black wire to the "G". The blue/black wire also has a resistor going off of it to another plug, but there are only 3 terminals on the sender for 4 plugs.

The guage worked perfectly fine before I disassembled the sending unit, but the reserve light didn't. i cleaned it out really good. Now the gauge doesn't register at all and the reserve light works. What could I have done wrong?

turbozeke418 · #15 04-01-2012, 08:18 AM

I took the sending unit out again and I noticed that the 2 thin wires that worked to measure the height of the float were not centered between the 2 metal tabs on the either side of the float. I reguided the wire between the tabs and put it back together carefully without jarring it around like last time and all works perfectly. Thanks for everyone's input!