Fuel level gauge and reserve light

[s class]

Interesting one - I have a 280S here, and one of its little niggles is that the triangular reserve fuel light glows continuously, regardless of the fuel level.  The actual level indicating needle appears to work correctly.

As I understand it, both the needle and the light derive their signals from the same pair of wires coming from the level sender at the tank.  The level sender appears to be a simple device, basically it is a resistance that varies depending on the fuel level. 

Does anyone know how the reserve light is actuated?  I suspect it may involve a resistance comparison with a fixed reference resistor?

[koan]

I suspect it may involve a resistance comparison with a fixed reference resistor?

Nothing that clever. One end of variable resistance and one side of switch are connected to ground at the tank. Two wires run up front from the sender, one is gauge, other is switch for light.

koan

[s class]

Ah, so in that case almost certainly the problem is at the level sender, not in the instrument cluster.  I guess I'll pull the sender out tomorrow. 

[koan]

Ah, so in that case almost certainly the problem is at the level sender, not in the instrument cluster.  I guess I'll pull the sender out tomorrow. 

Pull the wires off the sender first, if the light stays on it's a short to chassis in the wiring or in the cluster.

koan

[s class]

I pulled out the sender, and checked everything out.  The problem lies with the sender.  It has two outputs - the first is for the level gauge, and it is in the form of a resistance that varies between about 80 ohms (empty tank) and 2 or 3 ohms (full tank).  THe second is for the reserve light.  The base of the float has a metal disc that forms the contact of a switch to bridge two pins at the bottom of the sender when the tank is empty. 

I went through 4 units at the scrapyard, all of them had non-functional reserve light circuitry.  I selected one that I feel will be OK once cleaned up and serviced though.