Rear window defrost wiring

[jaltman]

I have a 73 240z. Given our recent spat of cold weather (and an intervening sunny day above 40) I have decided to dig into why the read defrost is not working. I have studied all the various wiring diagrams and most of the ones i have dont even show the defrost. The grid looks good. One side's wire goes to ground, there is about 5k resistance across the grid, continuity generally checks good everywhere i checked it.

There appears to be a relay in the actual wiring that does not appear in any of my wiring diagrams. A very nice 14v appears at the hot side wire to the grid when I turn the switch, but it then cuts off and goes to zero. I can hear the relay, but can't tell which one is clicking by touch. If I power cycle the switch power returns again at the grid for a moment.

So, I suspect the relay has intermittent contact. Its kind of a PITA to get the relay board out as it partly blocked by the A/C added in, in front of it. Anyone know which relay is the culprit?

As a lesser question, I have the XenonS30 FSM and various wiring diagrams I have found on the internet, but is there a wiring diagram that actually shows all the wiring, including the relays etc? 2 out of 3 I have dont even show the rear defrost, none show the light attached to the switch, none show the actual colors of the wires to the switch before/after the connector behind the switch on the fuse panel cover.

[pwd]

My 1973 FSM shows a Rear Window Defroster Relay in the Body Electrical (BE) Section, pages BE-5 and BE-6, for Manual and Automatic Transmission, respectively.

[SteveJ]

You could bring the car over to my house tomorrow or next weekend. and we could dig it out.

Look at page BE-3. The connector is the one on the far right of the dash. There is an arrow going to #10. If you trace that wire, you'll see it go to the relay on the right side of the board.

Send me a PM if you want some help.

[jaltman]

Thanks to both of you. I see whats going on now. I have also found that the fuse is an inline behind/under the radio, so I think first thing is to dig it out and clean and retension those connections.

Steve, thanks for the offer, wont work this weekend. I am "on-call" overnight tonight and, it being Saturday night, there will be lots of bad boys out and about so my phone will ring often all night. Then my wife has a few things for me once I can get some sleep. I'd like to get together and meet, you have kindly been helpful before. I'll pm you once I see how next weekend pulls together.

[jaltman]

Apparent epilogue: The fuse in the in-line holder was a fuse too physically small for the holder, contact was poor and there was apparent heat damage to the fuse, although it was not blown. The switch also exhibited signs of intermittent contact, which with a lil exercise was healed. I have a spare if needed later. So, the voltage is now stable at the grid, but I will need a day where its use is needed to be sure. The weatherman predicts suitable testing weather in the upcoming week.

Edited February 6, 2011 by jaltman