How to test power getting to Heated Windshield
 

Just discovered I have a heated windshield. I put a thermometer on the windshield and am trying to see if it works. Trying to turn on the rear defrost (which may also activate the windshield heater) AND twisting the same knob to the right. In either position, the temperature of windshield low down (the wires only go up about 6") does not go up.

Can I test to see if the fuse labeled for heated windshield (#5) is getting power? Can you actually test a circuit via the fuse box? If not, any suggestions on how to test?

I tried and did not see any voltage through the fuse - but not really sure if this is a viable test.

Any input appreciated. Same question applies for any circuit, I believe - so input overall would be valuable.

You can test that voltage is present at the input (top of the fuse) and then at the outlet (bottom of the fuse) in all ignition key positions. If it is there, then you just follow the wires...

If it is fuseblock 1 fuse 5 then it is (according to my Haynes Manual) powered by the X bus (when the key is in the ON and RUN positions).

There it probably goes to the Heater Control Unit and the Heater Control Relay.

Here is a circuit diagram I found on this site. Note it has power starting at a different fuse than 5. This is for a 1982.

http://www.pelicanparts.com/911/911_Parts/Electrical/911_electrical_82SC_defogger.jpg

This shows it powered off fuse 17 (16amp fuse) through a medium gauge red wire, through its own relay, to the windshield. T1d is a connection where the wire color changes from red to black.

The relay itself is triggered by the black wire coming from the defog switch terminal III through the relay and then back to terminal 31 to ground.

So you can measure voltage at the bottom of the fuse 17, the back of the switch at terminal III, the input to the relay (when the switch is in the proper position to activate), and the output of the relay at terminal 87 and that should check most of the circuit. From there it is a single wire to the windshield.