Rear Defogger Renewal

[Jetaway]

Hi, all.

The rear defogger on my early 1972 Z, which has the older vertical grid, doesn't work. The illuminated switch lights up and I measure 12 volts at the body side connector both between connectors and + to body ground. One of the grid lines has an obvious break, but that should only affect that one line. I measured the resistance between the cleaned the hell out of (i.e., shiny now) grid side connectors and come up with 25,000 ohms, give or take 1,000. If I've done the math right, that works out to a current of .0005 amps, which wouldn't provide much in the way of heating power.

My working assumption is that while the grid appears intact (mostly, see above) to the naked eye, either abrasive wear from cleaning or close to 40 years of aging, or both, has deteriorated the gird, to the point that only an extremely tenuous electrical connection exists across the grid. Outright replacement of the window with a new one would be, if even possible, would be an expensive proposition.

Which seems to leave me two options. The first, and less desirable, is to use an anti-fog product and rig up a fan to blow air across the window. The second is to recreate / renew the existing grid with an aftermarket repair product. I would prefer a functioning grid over the fan and anti-fog option, but am wondering about the practically of recreating the entire grid. The posts that I have read here on grid repair deal with filling in relatively short breaks in the grid, not, in effect filling in the entire grid. The aftermarket products that I've seen also appear to be designed as gap fillers, not OEM replacements.

So, my questions are: 1) has anyone tried to recreate an old grid? If so, did it work and if yes, what product(s) did you use? 2) If you have experience with filling gaps, how much of the stuff would I need? I know, I'm asking for a guesstimate, but based on how much of the bottle you used for your repairs, how many bottles would I need? 3) Would it work? If no one has been successful recreating an old grid, I'm somewhat leery of spending a lot of time on what may be, in the end, an unsuccessful and possibly unattractive attempt. Why shouldn't it work? Well, my thought is that if the resistance of the repair product is significantly different from that of the original material, bridging short gaps may degrade performance somewhat compared to "new from the factory," but because it is used for only one, maybe two, percent of the grid, not be noticeable in use. However, this may not be the case when replacing, in effect, 100% of the original grid with the fix-it product. If, for example, the fix-it product has twice the resistance of the original, the current drops by half, which I suspect would reduce the heating power enough to be noticeable in daily use. OTOH, if the resistance is 1/2 the original, I'd get plenty of current, probably enough heating power and more than likely, one blown fuse after another.

Chris

[gnosez]

CircuitWriter is a very good option....

[d240zx2]

You might investigate these....

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How to Repair Your Defroster Grids - Popular Mechanics

Dec 18, 2009 ... These electrically conductive, heated grids on the rear window are literally painted onto the glass. They're reasonably tough, ...

www.popularmechanics.com/cars/how-to/maintenance/4221212 - Cached

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Rear Defroster Repair Tips

Although effective, rear window defroster grids are vulnerable to damage and ... This polymer bonds aggressively to glass and provides maximum electrical ...

www.glasslinks.com/tips/defrost1.htm - Cached - Similar

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How to Troubleshoot a Rear Window Defogger | eHow.com

All you need is a very basic understanding of electricity or the desire to ... You can buy a defogger repair kit to fix a broken rear window grid at most ...

www.ehow.com › ... › Repair Rear Window Defogger - Cached - Similar

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Amazon.com: Permatex 21351 Rear Window Defogger Tab Adhesive ...

Rated 2.0 out of 5.0

16 reviews - $9.77 - In stock

Permatex 15067 Quick Grid Rear Window Defogger Repair Kit by Permatex $11.50 .... I do a lot of board level electrical repair, but I wasn't familiar with ...

www.amazon.com › ... › Adhesives - Cached - Similar

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Frost Fighter Defroster Repair and Replacement Kits

Automotive rear window defrosters break. Things happen. Scratch the grid or snap ... tabs to the rear window defroster making the electrical and structural ...

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rear window defroster (Page 1) - Technical - Volkswebbin

7 posts - 3 authors - Last post: Jan 31, 2006

Re: rear window defroster. Swims: By now the electric grid on the glass is probably "broken" some place. There are kits to re-paint the grid ...

www.volkswebbin.net/viewtopic.php?id=81374 - Cached

Get more discussion results

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Rear Window Defroster Problems : Auto Repair Information Blog

Jun 28, 2010 ... The electrical rear window defogger is one of those systems that operate ... On many models full battery voltage is sent to the rear grid. ...

www.certifiedmastertech.com/.../rear-window-defroster-problems/ - Cached

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Caswell Inc. - Permatex® Sealants

Nov 11, 2010 ... Permatex® Electrically Conductive Rear Window Defogger Tab Adhesive. Quickly and easily bonds defogger tab to grid on rear window. ...

www.caswellplating.com › Permatex Products - Cached - Similar

[Arne]

I have repaired gaps in my vertical grid, and find that the repaired spots are MUCH more effective at de-fogging than the un-repaired areas. So I suspect you are correct - whether from age or whatever, the original grid has lost effectiveness over the years.

Bearing in mind the good results I have had with the Versa-chem epoxy repair product, I have considered using that to re-do the entire grid. If I do this, I will probably (shudder!) remove the original grid first, then apply the replacement. I haven't yet decided for certain that I want to do it, but I have picked up a couple spare packages of the epoxy product just in case. If I do it, I'll post the results here.

[Mikes Z car]

Arne,

Does the tube of Versa Chem have enough in it to do a whole window? I bought a tube of repair paint that looks like it would only repair a foot or so of bad spot.

Mike

[Arne]

Not sure. I remember there being quite a bit left over when I did the original repair, and since it is a two-part epoxy however much you don't use is wasted. I don't know if one kit would do it, but I think two might.

Edited December 25, 2010 by Arne
typo repair

[Jetaway]

Thanks for the replies. I think I'll try to find some information on what is the resistance per foot of a repair product and use that to calculate to estimate the resistance of a renewed defroster grid. In the meantime it was relatively easy to hook up a computer fan I had lying around and mount it to blow on the back window. I suspect it won't be strong enough, but haven't been able to test it as I am apparently living in the only spot in California not being hit regularly with rain.

Merry Christmas!

Chris

[bpilati]

This a old, old question that has never really been answered or addressed. These grids have had problems for many years. It would be interesting to hear how they performed when brand new. If they were like the blower fans, well use your imagination. Considering the switch is not very substantial, and I don't believe there is a relay, the amperage was likely low to begin with. Upgrading the defogger would likely require addition of a relay to supply higher current to the defogger grid. Here is an aftermarket product, but not sure how to use it vertically. They have complete kits.

http://www.frostfighter.com/

[dbcjmc]

Hi guys! This string prompted me to finally fix our completely non functioning defrost on the 280Z. It's kind of an expensive fix.... it took 7 bottles of the Permatex repair stuff at $13each. Yikes!

All told, it's working again and clears up the window in about 5 minutes.

[Jetaway]

Thanks for the replies. I think I'll try to find some information on what is the resistance per foot of a repair product and use that to calculate to estimate the resistance of a renewed defroster grid. In the meantime it was relatively easy to hook up a computer fan I had lying around and mount it to blow on the back window. I suspect it won't be strong enough, but haven't been able to test it as I am apparently living in the only spot in California not being hit regularly with rain.

Merry Christmas!

Chris

An update:

To collaborate the working figures in my head, I measured the resistance of the functioning defogger grid on my other car. Measured out as .8 ohms, working out to 15 amps, which falls into my working range.

I'm going to show my work here because I'm not 100% (or even 95%) confident in my electrical circuit abilities.

The PERMATEX repair product datasheet lists the minimum electrical resistance of the applied product as .001 ohms / cm.

My 1972 240Z has the pre-1972 vertical grid which consists of 20 traces, each 26.5 inches long. For applying the formula to calculate resistance in a parallel circuit, I concluded that electrically it consisted of 10 traces, each 53 inches long (and assumed 0 ohms for the thick connector across the bottom) or 135 centimeters. So each run would have .001 X 135 = .135 ohms. Plugging and chugging with:

RTOT = 1 / (1/R1 + 1/R2 + ...)

= 1 / (7.4 X (10))

= 1 / 74

= .0135 ohms.

Indicating a current of:

Amps = 12 / .0135

= 888.8

= Ain't going to work.

The FROSTFIGHTER defroster kit may be a viable alternative. If I do go this route, I will definitely contact their technical staff. My own preference regarding modifying my Z is to do so in a way that it could be changed back to its original state. For best results, FROSTFIGHTER recommends removing the old grid, noting that it is likely that a red line, now part of the glass will remain despite any reasonable attempts at removal. So, in the oft-chance that a miracle repair kit is invented, I'd rather keep the existing grid in place. Running the FROSTFIGHTER grid as intended, that is horizontally, would make a checkerboard pattern -- definitely not desirable. An alternative, which I would think would work, would be to run the replacement grid bars parallel and right next to the existing grid lines. It might look a little odd, but not as odd as a checkerboard pattern. I suspect to keep the current draw reasonable you'd need to replicate the existing electrical layout (i.e., ten 53 inch traces) and not simply rotate one of their kits 90 degrees (creating twenty 26 and 1/2 inch traces). Which is why I would contact their technical staff.

Just BLOWING AIR on the rear window.

In the earlier post I wrote that I hooked up a computer fan to blow on the rear window. A few days later I was able to give it a good defogging test. Two people, 40 miles to get it nice and toasty inside, then a stop for groceries on a 42 degree, 100% humidity afternoon. Complete fog-out upon return to the car. Using only the smaller fan, the results were ... better than nothing, but not that good. In the five miles / 12 minutes back to home it was able to clear about 1/3rd of the window, but at least it was a useful part of the window. I managed to find a second, larger fan, and installed it. Haven't had an opportunity to test it yet, but I am very optimistic that with the much greater airflow and cross-firing pattern of the fans that I will be able to drive fog-free this winter. I wouldn't expect them to be of great use in deicing, but that is not currently a concern. Very quiet too, not even noticeable with the engine running.

I've attached a pair of photos of the fans, the smaller is a little less than 2 and 1/2 inch square and the larger 3 and 3/4 inch on the square. For the wiring, I simply used the existing defogger connections. I've attached them to my speakers, a simple box or even just a panel set-up mounted on the shock towers should work just about as well.

Have a happy New, all.

Chris

[Jetaway]

Hi guys! This string prompted me to finally fix our completely non functioning defrost on the 280Z. It's kind of an expensive fix.... it took 7 bottles of the Permatex repair stuff at $13each. Yikes!

All told, it's working again and clears up the window in about 5 minutes.

Well, I'll be a monkey's uncle.

Could you field a couple of questions for me?

1) How many coats did you use?

2) If you have an ohmmeter, what is the total resistance of the grid?

Thanks.

Chris

[dbcjmc]

Well, I'll be a monkey's uncle.

Could you field a couple of questions for me?

1) How many coats did you use?

2) If you have an ohmmeter, what is the total resistance of the grid?

Thanks.

Chris

Hey Chris!

No problem!

My son and I did about 2 coats each. We shined the flashlight thru from the underside to double check the areas that needed extra coverage.

No ohmmeter. Just used a continuity tester to see if we had a connection. The continuity tester I bought at OSH for $4.99 had a light in it. It seems the better the connection, the brighter the light. We did notice that the connection (light) got much better after 12hours of drying time.

Cheers, and Happy New Year!!

db