Electrical Puzzler - Backlight Demister

[Mike Wattam <MikeWattam@domain.elided>]

Hi Peter

The heated backlight problem is fairly common on all cars.  It occurs when
there is damage caused by window cleaning (an essential job) or striking
with some object, or maybe just old age.  It seems to be that there is
often enough conductivity for a continuity test to tell you all is OK, but
when the backlight is turned on there is not enough current flow through
these 'elements' to enable them to heat and clear the window.

So, what you have to do is find the damage points.  This is best done by
steaming up the window (bowl of near boiling water?) and then turning the
backlight switch on.  For those 'wires' which are not working, on each you
should see develop one small round area which does clear - this is the area
of high resistance and damage.  Carefully mark the position(s) of damage.

Sorting it out is theoretically simple.  From DIY motor stores it is
possible to buy repair paint - made by Comma if I remember correctly.  This
is painted over the damaged area and all should then return to normal.  The
difficulty is that it is almost impossible to apply without the result
showing clearly - masking off each side of the element does not work too
well as the paint peels off with the masking tape!

The other difficulty is that when you find and rectify one break, another
may still be present and you will then need to go all through the same
palaver over again.  If this happens you can assume your demister is dying
of old age and is ready to be thrown away.

Mike Wattam
Triumph Stag Register