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Running heated windscreen from heated rear window circuit?


Filbee

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Right, that's Christmas done and dusted! Thoughts now turn to some Defender fettling over the holidays 😊 

The next task on my list is wiring up my heated windscreen. I've got a Red Bison controller, TD5 switch and wiring, now I just need somewhere to run the live input from. I know I could run it straight from the battery, but I'd rather have it fed from an ignition switched live. 

My vehicle is an ex-utility TD5 110 hardtop, so it hasn't got a heated rear window, but it does appear to have the wires running to the HRW fuse position in the fuse panel above the transmission tunnel in the cab. 

Is there any reason I shouldn't run my heated windscreen from the HRW fuse position provided I use a 30A fuse instead of the 20A that would have been used for the HRw if it had one?

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I don’t know if that feed will be heavy enough for a front screen - they draw considerably more than rear screens, especially that little rear screen on a Defender.  I think you’d be wise to run a new feed, or at least update the wire to the fuse in the standard fuse box - overloading wiring is not a chance worth taking.

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If a significantly higher current draw is involved one option would be to keep the existing wiring to the existing fuse location but run the output from the fuse to a new 40 amp relay, suitably wired, and let that feed the heated screen.  Apart from ensuring the wiring is up to the job you also reduce the risk of burning out the HRW switch with a higher than design duty.

Mike

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If the kit has the suitable relay (probably a timed relay) with the switch, then you would connect the main circuit feed for the relay to the battery and the switch feed to an ignition switched circuit in the fuse box, usually white wiring.  That way, the switch can only send a signal to the relay when the ignition is live.

Don’t feed the relay from a switched live, as it’ll be drawing all that current through the ignition switch.

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6 hours ago, Snagger said:

If the kit has the suitable relay (probably a timed relay) with the switch, then you would connect the main circuit feed for the relay to the battery and the switch feed to an ignition switched circuit in the fuse box, usually white wiring.  That way, the switch can only send a signal to the relay when the ignition is live.

Don’t feed the relay from a switched live, as it’ll be drawing all that current through the ignition switch.

This is the way to do it.

I'd want to know how many amps it draws though - if you don't know that you're on dodgy ground.

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I have seen instructions with 60a relays.  I measured the draw on the two front screens on my 109 and they were under 20a, so I imagine a Defender screen would be about 30, but may take a surge that demands a bigger relay.  You don’t want to be running on the limit of any relay as you could weld the contacts shut, causing big problems when you can’t switch the screen off.

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On 12/26/2022 at 9:26 AM, Filbee said:

Is there any reason I shouldn't run my heated windscreen from the HRW fuse position provided I use a 30A fuse instead of the 20A that would have been used for the HRw if it had one?

Yes, by putting the 30A fuse in there the weakest link (which was the fuse) is now your wiring, which is coated in lovely plastic which gets hot and drippy and eventually bursts into flames taking your truck with it, because your fuse won't blow in time.

Good enough reason?  :D

 

 

 

 

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1 hour ago, western said:

My heated screen has a 70amp relay to power it via the 10 minute GLR10 Timer. 

My wiring diagram (wire colours are my choice, not LR factory) in page 5 in 

 

Thanks Western 👍

As it seems the wiring for the rear screen isn't up to the job of carrying 30A, I'll add a suitably beefy busbar behind the left hand dash fed from the battery via a 30A fuse and power the screen via a 60 or 70A relay triggered from an existing ignition switched live I have running to a secondary fuse box behind the dash.

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1 hour ago, Bowie69 said:

Yes, by putting the 30A fuse in there the weakest link (which was the fuse) is now your wiring, which is coated in lovely plastic which gets hot and drippy and eventually bursts into flames taking your truck with it, because your fuse won't blow in time.

Good enough reason?  :D

 

 

 

 

Thanks, yes I'm fully au fait with what happens when you overload wiring having nearly had my old Corrado go up in smoke on the side of the M5 many years ago due to a badly fitted alarm at some point in it's past!

I just wasn't sure if the HRW wiring was beefy enough to carry 30amps. It looks roughly the same diameter as the feed in the kit I have but I didn't want to make any assumptions...

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I have a heated front screen with a split circuit, so three tags. Each side has a fused power relay and is fed through 4.0mm2 cable. These relays are switched by a single timer relay, triggered on (and off if necessary) with a momentary Carling switch. I think I used 6.0mm2 for the common 0V return to a bulkhead earth point.

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Thanks for the help and advice all. I've ordered a selection of cables, terminals,  a heavy duty relay, a couple of fuse holders and a 100A busbar.

My plan is to use some chunky 16mm2 cable from the battery to the busbar via a 70A fuse, then run the power to the relay via 4.5mm cable from the busbar. The heated window controller can then run from the relay via the supplied cable, which seems to be 4mm2, via it's own 30A fuse as supplied. 

The relay will be triggered by the TD5 heated windscreen switch, which I'll run from my existing ignition switched live.

Not sure where to put the busbar yet, either behind the passenger side dash, driver's seat box or under the bonnet. I can then also run the power to my relay switched headlights and any other power-hungry accessories I may fit in the future. At least with the busbar I won't need to keep adding more wires directly to the battery...

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I haven't, but the heated window power circuit will be protected by a 30A fuse as it is in the OE set up, and the cables powering it will be 4.5mm2.

The 16mm2 cable and 70A fuse will be supplying the busbar that I will take the power from for my heated screen, headlights and any other high current drawing accessories I might add over time. 

Each of the circuits coming off the busbar will be protected by appropriately sized fuses.

I hope that makes a bit more sense!

Edited by Filbee
Bloody predictive text error correction!
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On 12/29/2022 at 4:30 PM, FridgeFreezer said:

Have you actually found out how much current the screen really draws yet? The screens in my Series are very low power (~10A), so sticking 70A wiring & fuses round them would be a recipe for a fuse that never trips in a fault condition...

It’s a fair point, and there is no benefit in massively overrating the electrical circuit.  A screen is very unlikely to develop an internal short that could endanger the car, though - it’d be the wiring or terminals that short, and that would blow the fuse if the wiring is rated above the 70A of the relay and fuse.

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  • 2 weeks later...

Well I've not made much progress on this yet! I fitted the busbar behind the dash and identifed a suitable hole in the bulkhead to run the cable through from the battery/seatbox in the behind-dash area. There's a 12.5mm hole with a blanking grommet on the left hand side between the heater box and inner wing (next to where the pipe to the washer jet goes through the bulkhead) that will do nicely.

I'm also going with 10mm2 cable, not 16mm2. I got some 16mm2 cable and it's massive! I've chosen 10mm2 because according to the workshop manual, that's the size of cable that runs from the battery to the main 100 amp fuse under the driver's seat that basically seems to run all the vehicle systems apart from the starter motor. That would include the OEM heated windscreen, seats, mirrors etc. So if 10mm2 is beefy enough for that job, then my 10mm2 cable should be able to run the heated windscreen, headlights and possibly a couple of other lower power accessories in the future.

I'll stick some pics up when it's all done 👍

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