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'85 318i Temperature Gauge Sending Unit Calibration

Does anyone know of a calibration (verification) procedure for the
temperature sending unit?  My gauge appears to function normally until a
little past the halfway mark.  After that, in the span of 10 seconds, it can
be at the 3/4 mark.  It never goes any further.  This can occur during
driving at a steady 75mph.  Then wham!, you can look down and the gauge will
be back at the 1/2 mark.  The gauge doesn't appear to be stuck (thumping it
doesn't work).

The same thing seems to apply if you stop, turn off the engine, let it sit
for 10 minutes, then start it again.  The guage will always be at the 3/4
mark.  That's not unusual, but what is is that the guage will linger at the
3/4 mark for about 30 seconds, drop to midway between 1/2 and 3/4, then wham!
drops immediately to 1/2.

(By the way this has been going on for a long time, its just gotten so lately
it is a bit aggravating)

I just installed a new copper core radiator (other one leaked at top tank,
imagine that!), so I know that the radiator shouldn't be contributing to
this.  It just seems that the calibration is off after the gauge hits the 1/2
mark.

I don't have a problem with replacing the sending unit if it is bad, but I
don't want to replace it only to find out it's okay ($75.00 Superior-BMW).
 Any help would be greatly appreciated.

RKB