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Defrost Windshield - E30, possibly others


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Hey,

 

After having my heated grid fail at AMP, I went looking for other solutions.

 

 

I found one - however it is a bit more expensive. 

 

heated@2x.jpg?sfvrsn=993c1609_0

 

I found a supplier for E30 glass with two grids embedded in the glass, one for each side of the windshield.  Each grid takes a 25A power/ground (10ga quality wire) and will increase the temp of the glass significantly.  The wire output is at the bottom of the glass.

 

See the OEM version here.  This will look similar - very OEM pro look.

 

 

Looking to see if any other teams would be interested, to help the cost be reasonable for me, and to not be left with a pile of windshields to store...

If enough people wanted a different car, I could inquire about that.  I think I would need 10 or so interested.

 

Expect about $500 each, picked up in Raleigh or Delivered to a event.

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33 minutes ago, Crank Yankers Racing said:

Did you find out why your grid failed?  Was is a power issue? Break in the grid issue?

Mine failed because it just didn't work at clearing the fog...  it got warm.  It just sucked.

 

- relevant to your question because I believe it was the same type of defroster grid...

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39 minutes ago, wvumtnbkr said:

Mine failed because it just didn't work at clearing the fog...  it got warm.  It just sucked.

 

- relevant to your question because I believe it was the same type of defroster grid...

That's annoying... I installed the same grid last winter. I wonder if it's a power supply issue... and perhaps the grid is capable of more amperage than the controller is sending.... and maybe it can be 'tweaked' to increase the heating....get that summbich hot enough and it won't fog over anymore.

 

@Hi_Im_Will knows electronics and resistors and stuff....

Edited by enginerd
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51 minutes ago, wvumtnbkr said:

Mine failed because it just didn't work at clearing the fog...  it got warm.  It just sucked.

 

- relevant to your question because I believe it was the same type of defroster grid...

 

I have had zero issues with mine.  We used it last year and this year......just interesting on what is failing.  I know I had a break in mine which I fixed with some of their copper stuff and it works perfectly fine.

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  • Technical Advisory Committee

 

 

Mine still "Works" but not good enough to get the fog off. It pulled 10 amps at AMP when I was troubleshooting it.

I had a few "loose" connections between the grids and the bus-bars.  When pushed on, I could get current draw up to 14 amps.

 

The grid is pretty simple.  The problem is that it is made of copper (low resistance).  Since the copper is such a good conductor, the resistor grid people had to devise a way to increase the resistance to get more heat output, and they did so by running back and fourth across the windshield in series.

 

For fixed voltage, increasing current or decreasing resistance = more heat. 

P=V^2/R or P=I^2*R

 

That applies until a certain point, when your materials "melt" and then you just have an open circuit where no current flows.

If the copper defrost grid was just a big group of 12 grids in parallel, it would have a very low resistance, draw a very high current (like a short across the battery terminals), and likely get TOO hot and destroy itself.  12 wires of 4 ohms each = .33 ohms resistance, or basically the same as 200 feet of 12ga wire.  From V=I*R, we can guess that circuit will pull 42 amps.

 

The actual grid is 4 groups of 4 wires (If you paid for the overpriced Bimmerworld grid, its 4 groups of 3 wires), each "group" with 1 ohm of resistance (since they are all connected permanently, you cant measure the resistance of just one wire, but you can theoretically guess based on the formula that each wire is 4 ohms) .  The ground and hot are on the same side.  Because each group is in series, the whole grids resistance is ~4 ohms.

Basically the current path is this:

 

^

|________<

 >________|

|________<

>________|

|

^

The manufacturer suggests my grid should pull between 14 and 20 amps, based on length (34-50, longer = lower current due to higher resistance).

 

My grid is narrower at the top (40), and fatter at the bottom (44), which likely contributes to my temperature gradient.  If I average that, at 42, it happens to be dead in half of their range, so I should be seeing 17 amps or so to my defroster.

 

They do not specify a temperature spec.

 

If you measure your actual cars rear defroster ( I did a EK civic), OEM solutions use a pre-printed silver paste (Dupont 788x for example) to make the grid.  The civic is one current path, with 12 or so wires, making the grid.  The resistance across the whole thing is 4 ohms.  They can do it this way because the silver paste has a higher resistance than the copper strips.  Based on the formula, you can guess that each wire has a resistance of 48 ohms (12x the resistance!)

 

If one part of the grid fails, the overall resistance of the unit does not become affected nearly as much.  In this way, the grid will still work even if its resistance increases slightly from 1 or two grids failing.  11 wires at 48 ohms each = 4.36 ohms total.

 

If one or 2 of my wires fail (which I suspect 2 of the 4 top wires did), the resistance halves across the first group, meaning the two remaining wires get hot, however the 4 wires in each of the lower three sets are passing much less current, and do not get nearly as hot.

2 wires at 4 ohms each is 2 ohms across the first grid, but still 1 ohms across the next three, meaning the first grid gets significantly hotter and the next grids don't.  (we have fixed voltage, but variable current here - the 1st grid is now carrying ~2x the current as the 3 grids downstream)

 

My measurements showed at ambient in my shop, the first 2 wires got to 120F, whereas the other 3 grids only got to 79F in a 72F shop. 

Unfortunately, the 2 wires that were working were behind the damn RVA GFX banner.. whoops.

 

How hot is too hot?  I don't know, but obviously 120F is OK (thats +38 over ambient) and 79F is not hot enough (+7 over ambient).

 

I could go modifying my existing stick on grid, which I suppose I will if this idea doesn't pan out, but my first step was to look for an engineered OEM solution, which I found, albeit at a steep price. 

 

 

 

 

Edited by Huggy
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I remember watching some in car camera action during the Sonoma race in February where it rained a lot.

 

There was a Datsun 240/260/280Z {one of those} that ran and whatever system they used worked REAL well.

 Sorry I don't remember the car number.

 

 

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