W126 gas gauge / reserve light
 

How does the sending unit work? My gauge has been a train wreck from day one. And today, with well over half a tank indicated, my reserve light started to come on/off for a few miles until it was steady. Then my needle dropped to a quarter tank. A few minutes later, it showed over half a tank again. But the reserve light stayed on.

So should I remove the sending unit for a cleaning. I just ran B12 through the car. Or is it the dash gauge that is bad - which is what I think it is.

Most likely the floating unit in the tank.

Simply a variable resistor (full about 5 ohms & empty about 85 ohms).

If so, how does the reserve light work then? Is it on a separate float?

If it's like the W115 I took apart the reserve light is run by a pair of contacts which touch a brass ring on the bottom of the float. When the float hits its lower limit it triggers the warning lamp.

-J

Here is the diagram
http://www.xs4all.nl/~pruijt/Fuel.jpg

Rob

There are some threads here and at benworld that talk about a set from factory variable potentiometer. If you do a search you might find putting a radio shack fluid on it to complete the connection to help with the low voltage.

I read about that when I was looking into cleaning the float, the bit about eh contact fluid.

I guess from what I read here, it would have to be the gauge. If it was the float then they reserve light would not be on when the gauge reads a half tank. I put in just $20 today in gas and the reserve light it out but the gauge did not move from its spot.

there is a 99% chance that your sending unit in the tank is broken with one or more of the very fine wires off at one end,replace it.there is a 1% chance that the gauge is bad.

I guess I need to better understand the system. Either way, the sending unit is easy to get to so I will start there. I am VERY glad this car does not have the tank underneath. And I love the external, yet rather loud, fuel pump.

I will read up on cleaning it and report back.

Well I cleaned the sending unit VERY thoroughly with carb cleaner followed by electric parts clean (to not leave any residue). I then slowly rocked it back and forth out of the tank and watched the needle. Above a half tank, as before, it was not too bad. But it would never, ever get below half a tank. At reserve the contacts worked fine and the light comes on.

So I am back to either a corroded connection that is not reading the very, very low voltages at below a half tank or the gauge needs that Radio Shack stuff.

At least I know the sending unit "seems" ok. Wish I had another to verify. I also know my tank is very, very clean inside and no signs of rust from water contamination.