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Rear demister

P

Paul Fraser

New member
Seems to have stopped working. Output voltage at terminals at back is 0.12 V rather than the 12V I was expecting. What is the voltage meant to be? Resistance of loom seems to only be a couple of ohms so probably not 12V then... Fuse ok, as demister switch lights up, have reseated relay. Can anyone check the output voltage with an avometer please?
 
There might be a clue in the 2 ohms. Measured from the near side terminal, that is too much, and you should substitute another relay to see if the resistance changes. It is only just less than the hot resistance of a headlight lamp and much too high for circuit before it gets to the load. If my Ohms Law is correct, at that resistance, a 7 amp fuse wouldn't blow as only 6 amps would flow, if you shorted the cable to earth. Measured from the right-hand side it is about right for the complete circuit including the screen element. The earth to the right hand strut hasn't broken off has it? This is quite common. The HRW earths through the strut which has a short length of sleeve that lifts the sliding contact from the rod when the tailgate is closed.
 
You've confused me.
The voltage from the two terminals was 0.12V
The resistance of the rear demister screen is about 1.5-2.0 ohms (at 12V this would be about 8A, and the fuse is 16A). FYI I measured the resistance of the rear screen from metal strip on one side to the metal strip on the other side.
The earth strut is still there.
I suspect it is the relay as you say, but wanted some feedback before getting another.
 
Well after faffing about with this on and off today, sorted it in five minutes at half past eleven. There is another fuse, #4 on the auxillary fuse board, 25 ohms. Now have 10.5-11V at the rear.
 
What I was trying to say was with a resistance similar to a dipped headlight filament in the circuit BEFORE the load (i.e. the screen element) there must be something amiss. All there is up to that point is the supply via the relay and fuse, so I would expect the resistance to be fractions of an ohm.
I am not expert on 944s but based on the similarity of the electrical systems in a number of areas to the 924 I had taken the fuse you mentioned to be the no. 4 fuse on the short board up and to the left.
 
Yes the oval dash nomenclature explains it better, i.e. heated mirror and rear element control wire. Probably not an exact quote.
 
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