Ventilation for Rear Facing Seats

supratachophobia · May 11, 2015

I had an idea that might need quite a bit of fleshing out. I understand there would be inherent risks involved, but I'd like to discuss the merits of 'if it could work' rather than 'if it should be done'.

I've used the rear window defroster exactly 0 times so far. My thought was to discover if the lead that goes to the rear-window defroster is a 12v tap from it's control module. And if so, could that 12v be attached to something of equal or lower amperage, such as a fan, to provide air circulation to the rear-facing seats. The added benefit to this would be the following:
1. control from main touchscreen
2. solution to rear ventilation (perhaps even multiple fans if amperage allowed)
3. easily reversible depending on season, but having a subtle switch in the trunk to toggle between the two functions

I'm not sure of the conditions that need to be met for the rear-defroster to work. In other words, is there a signal that is sent back to the control module that says the resistance has reached a certain amount, it's time to turn off? Or perhaps a signal that asks "is there really a defroster connected before I turn on?" type of communications. The ideal answer would be that the rear-defroster either stays on until turned off or it stays on for a set amount of time.

If I recall correctly, that system is at least 15 amps. Most 12v fans are 3-5amps from what I've seen so far. All it would take is two and they could be positioned for either bringing air further back from the main cabin or blowing directly on the occupants.

caps04 · May 12, 2015

This sounds like a brilliant idea Hoping someone skilled enough can look into this!

Spacela · May 12, 2015

The rear defroster will turn itself off after 15 minutes of use, so you to would have keep cycling it back on.

Fezzik · May 12, 2015

I'm actually looking at making a blower that will go into the hole cutouts and blow air down into the area more.

supratachophobia · May 12, 2015

Spacela said:
The rear defroster will turn itself off after 15 minutes of use, so you to would have keep cycling it back on.

That's actually not the worst thing in the world. Presumably you'd just need to occasionally run it anyway or on demand.

How do I got about finding out how rear defrosters work?

Evbwcaer · May 12, 2015

I really like the idea. There is usually just a harness that slides over a fitting on the window for electric defrost, it should be really easy to splice the same male end onto a 12v fan. I assume you just need to pop one of the plastic shrouds off the perimeter of the rear glass to reveal the harness.

You could install a switch so either the fans or the defrost is getting power, so you would not have to do much work from summer to winter. If I have time this week I'll dig into my Model S and see what I find.

Spacela · May 12, 2015

supratachophobia said:
If I recall correctly, that system is at least 15 amps.

Just flipped through the manual. The rear defroster is powered by fuse #52, which is rated at 30 amps. So, plenty of power for fans. :smile:

supratachophobia · May 12, 2015

Spacela said:
Just flipped through the manual. The rear defroster is powered by fuse #52, which is rated at 30 amps. So, plenty of power for fans. :smile:

SWEET! Now to find out which fans to test with... They need to be asthetically pleasing while also being small enough to not interfere with getting in and out of the back.

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Fezzik said:
I'm actually looking at making a blower that will go into the hole cutouts and blow air down into the area more.

Well now you know how to power it. And what cutouts are you taking about? The ones in the fender wells?

Evbwcaer · May 16, 2015

I did a little digging and took some photos. I am not 100% sure what I am looking at, so speak up if you are more knowledgeable. I am having trouble with the image uploader, so there will be two posts. I removed the upper plastic trim piece, that crosses the length of the glass, this makes removing the side pieces possible and the relevant wiring is underneath them. I did damage one or two of the plastic buttons that hold things down, they will probably still work and are very easily and cheaply replaced. They are the three turquoise colored dots in the first photo.

Here you can see two "boxes" with wires that go to the glass. The mostly black box is for the antenna. I think the mostly white box is the defroster hardware, the wiring is fairly beefy, makes me think it is not a signal wire.

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Here is a close-up of the white box's labels, if anybody wants to do some research.
.

Then there are the guts of the white box, some circuit boards. Why would a defroster need a brain?

As you can see the white box has a grey and white wire going into it. I don't know which is positive or negative.

I also don't know if you would want to tap into the system at those wires or downstream of the circuit board but before the glass.

supratachophobia · May 17, 2015

Awesome start. Here is what i found. Generally speaking, the black two lead wire that goes to the heating element should be carrying a 12v signal. Now what dots that leave the control unit for? Is that a sophisticated timer/temp sensor that controls the auto off functionality? And is the white/grey pair the thing you actually want to tap into? I'd the latter test +/- 12v, I would use those avoid potentially damaging the circuit board, whatever it's function. Bit does that circuit board have any signal out that tells the center dash computer that it's ok and ready to be used?

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Ventilation for Rear Facing Seats

supratachophobia

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caps04

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Spacela

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Fezzik

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supratachophobia

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Evbwcaer

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Spacela

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