Basically, I'm trying to find out if mine works or not.

Should I see a voltage drop or feel obvious heat if it works and I touch it?

Seems like a silly question, but given the other electrical flaws I've had to install relays to get things to work properly (window motors, headlights and the like) I have to wonder if all the power for the defogger is running through its on/off switch.

Looking at the schematic, I see a "delay relay" (why a delay?) but nothing else.

Thanks,

John
1987 XJ-S V12

 

You should feel warmth on the glass yes. Or, visually, if fogged, the glass should clear.

"Delay" relay is odd nomenclature. Operation isn't delayed. However, the relay is also a timer that cuts the circuit after xx-minutes. That's how mine worked, at least.

Power for the rear defogger comes via the 35 amp fuse and thru the relay/timer. The relay/timer is triggered by the switch. If the dashboard indicator light comes on, the relay/timer is functioning, as the output to the window grid also powers-up the warning lamp

An easy place to start diagnosis is to turn the switch 'on' , and assuming the indicator lamp comes on, simply check for 12 volts at the grid on the window. You should be able to see the tabs at the edge of the glass where the grid attaches to the wires.

If the indicator light does not come on then you have a problem upstream: no voltage to the relay, relay not grounded, or the switch is defective or not grounded

Cheers
DD

 

Thanks for that Doug!

John