The fuel level sending unit in the tank (early 1985 R100RT) feeds 7L and 4L lights in the instrument cluster.. The 7L light comes on when I still have 11 Liters in the tank.(Verified when refueling). The system is not very accurate.
Is the sending unit adjustable or should I replace the unit?
Any suggestions?
Richard1

Hi
rightly or wrongly I just bent mine down a bit like a toilet ball float of days gone past. Seems to work although I still don't trust it that much.Someone will put you right, and me too I'm guessing.
Cheers RT

This explanation comes from a document I have in my K100 library. Refer to this schematic:


There are two thermistors housed in the fuel sender unit which is located in the fuel tank. The thermistors are positioned at two different levels corresponding to 7 and 4 litres. When the thermistor is exposed to the air in the tank, the appropriate light in the instrument cluster is illuminated.

Each thermistor is supplied with 12 volts that is current limited to 95mA, this current limit is controlled by some circuitry, including a variable resistor R3 (for 7L) and R4 (for 4L). With the applied voltage the thermistors self heat. When they are immersed in fuel the heat is dissipated and their resistance is approximately 600 ohms. While the thermistor is immersed in fuel, the inverting input of the sensing op-amp has 12 volts applied to it which is above the 4.7 volt reference and thus the output of the op-amp is not enough to illuminate the lamp. When the thermistor is above the fuel level and in air, the self heating causes its resistance to drop below 53 ohms, and the voltage at the inverting input of the sensing op-amp drops below the 4.7 volt reference level . This causes the output of the op-amp to illuminate the lamp. Adjusting R3 should adjust the current limiting circuitry and vary the point at which the op-amp switches.

I've not seen the circuit board carrying this componetry (because I have a float with fuel meter) so I can't be of much assistance with helping you adjust the variable resistors, should that be something you wish to persue.

Thanks to both of you for your response.
robmack, your explaination was excellent. However, I could not read the schematic drawing due to size (too small). I could however understand your explaination of the system and how it works. There seem to be two posibilities to correct the inaccuracies in the gauge.

1. Can the level of the thermistors be adjusted within the fuel sending unit itself? I don't know since I have never actually seem one outside the tank.
If possible, that would be one way.
2. Adjusting the variable resistor R3 as you suggest. Since I don't know very much about resistors, is that something that is physically adjustable(if I could find it) or does that mean replacing the resistor.
Again, thanks for the response.
Richard1

eichard ...if you right click on the circuit diagram and "save as"

you can then open the picture up from windows explorer and view it larger

its a bit fuzzy but you get the idea

nice book you have there rob .......would love to get my hands on it as well .....cheers guys

Check this previous thread.

CF