OBC questions (3.8 typ, E30)

[Michiel van Wessem <jmvw@xxxxxxxxx>]

Hello,

I have two questions about my OBC:

- - What is the function of the rotary switch at the back of the OBC? The
switch has 4 positions, which are marked: 30, 60, CH30 and CH60. I suspect
it's there to change the sound of the "bong", the OBC alarm. Is this correct?

- - Does anyone have schematics of the OBC? The ETM just calls everything
electronic "solid state". 

I'm asking this last question because I'm trying to find out why the range
display on my OBC doesn't work. Months ago I thought I had it, but it turns
out it's not so simple. For some reason the OBC doesn't read the fuel tank
level correctly. Actually, I found out it doesn't read the level at all.

When the OBC has been disconnected for a while and is reconnected, it'll
display about 50 l. After driving for about half a tank, it'll settle down
to 12-16 l, which will never change anymore. 

I've cut the wires from the fuel gauge to the OBC (fuel tank level and fuel
tank empty) and the results are the same. 

I think that what is happening here is that when the program cannot read
the fuel tank level it sort of tries to replace that reading by first
assuming that the tank is full and then using the fuel rate from the
Motronics control unit to estimate how much fuel is left. This doesn't work
well.

I measured that the signal on these wires coming from the fuel gauge is
correct, so it's not a bad contact on that side. Also, I've measured that
there is some conductance on the two wires going into the OBC, so it
doesn't look like a problem with the OBC connector either. For some reason,
the OBC itself seems incapable of measuring the voltage. 

I'm hoping for schematics, or maybe someone knows about the input
circuitry. I have opened the OBC and I can see that it wasn't made to be
repaired (except for replacement of the keyboard and display), but I'm
hoping this is something relatively simple (like a broken capacitor).

Thank you very much,

Michiel ('87 325es <http://www.vramp.net/~jmvw/cars/BMW/> )

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