Heated rear screen wiring?

[mattyguk1]

Hi all, I'm in the process of fitting a heated rear screen to my 1970 saloon. What is the safest and easiest way to wire it in, including adding a warning light or similar to save forgetting it's on ?

[neilmorey]

There's a good thread from years ago. With these discussions you need to read it all to get all the info on wiring diagram, relay, cable size, fuses but it is there.

Until reading it I hadn't considered a relay could fail closed so having the tell tale on the heated window circuit may be best.

viewtopic.php?t=50470

[mattyguk1]

Thanks for that , looks like what I need

[geoberni]

Until reading it I hadn't considered a relay could fail closed so having the tell tale on the heated window circuit may be best.

Failing closed is a rare failure in a properly designed circuit, but it can happen, usually from the contact surfaces inside becoming pitted with arching damage and basically sticking together.
So if for example someone is doing DIY wiring and installs a relay that is not of sufficient load capability, or of poor construction as we see with so much of the cheap rubbish currently (no pun intended) sold online, the excess current through the contacts basically welds them together.

[mogbob]

One option for you to consider is a timer relay that will switch off automatically after a certain time. You will need
a " beefy" to cope with the electrical load of a rear windscreen.
Here's what I had in mind : -

https://www.autoelectricsupplies.co.uk/product/836 . I've used the Company for years. Other suppliers are available but this the sort of thing to look for.
As others have pointed out the main supply wiring should be 17.5 amp 3.3mm 28 / 0.30 wire whilst the switch wiring will need 8.75 amp 2.6mm 14/0.30.
Bob

[mattyguk1]

Thanks Bob that sounds like a good idea and I will definitely be adding it to the system. Now I’m adding the heater rear screen I’ll be changing from dynamo to alternator and adding a few other bits, i.e electronic ignition and electric washers to start with. Would it be best to add another fuse box?

[geoberni]

Thanks Bob that sounds like a good idea and I will definitely be adding it to the system. Now I’m adding the heater rear screen I’ll be changing from dynamo to alternator and adding a few other bits, i.e electronic ignition and electric washers to start with. Would it be best to add another fuse box?

I'd certainly add some more fuses with all that extra load on the system.
Do a search on here for words such as 'Alternator' 'Fuses' because there are several topics on such. I seem to recall one fairly recent post where someone used the redundant Voltage Regulator as a fuse box.

[Sleeper]

viewtopic.php?f=11&t=71953

John ;-)

[mogbob]

It's a plus 1 for an extra fused box.
My mate wanted " just one or two " additions to the electrical system when we re-wired his Moggie I put a secondary fuse box inside the car under the dash. Count up how many feeds you need and allow for a least a couple of "spares " positions.
My modern car has 3 fuse boxes , engine bay , glove box area and in the boot.I've never stopped to count how many fuses !
Bob

[mattyguk1]

That should keep me busy for a while, thanks all