Rear Window Defroster Troubles

I've always been able to resolve any problem
with the grid type of rear window defroster.
Usually, its just one of the power leads that
has to be reattached. If a grid line develops
a crack it can easily be repaired using that
resin sold in most auto stores. It is mixed
with a conductive powder like nickel, copper,
or silver.

Recently, I checked out a defroster grid on a
1991 Taurus wagon. It suddenly stopped working.
There appears to be nothing wrong with it. All
voltage and resistance tests are nominal. The
power connectors are firmly attached and there
are no cracks in any of the grid lines.

A mechanic I've known for twenty years told me
it simply died of old age. He asked me if I
ever had an electric blanket, heating pad, or
space heater that failed to heat up after a
number of years. The nichrome wire in these
appliances produce less and less heat as they
age. He basically said the only solution is to
repace the entire rear window!

I seem to remember a kit I saw in a catalog many
years ago. You scrapped off the old grid and
replaced it with something that looked like a
big decal or auto pin stripping. Is there
anything available that can replace the defroster
grid, or is replacing the entire rear window
the only solution?

Holophote

In article .com>,
"Holophote" > wrote:

> I've always been able to resolve any problem
> with the grid type of rear window defroster.
> Usually, its just one of the power leads that
> has to be reattached. If a grid line develops
> a crack it can easily be repaired using that
> resin sold in most auto stores. It is mixed
> with a conductive powder like nickel, copper,
> or silver.
>
> Recently, I checked out a defroster grid on a
> 1991 Taurus wagon. It suddenly stopped working.
> There appears to be nothing wrong with it. All
> voltage and resistance tests are nominal. The
> power connectors are firmly attached and there
> are no cracks in any of the grid lines.

No *VISIBLE* cracks, or "The meter says that each line is intact when I
try to read it"? Sounds to me like there's a main "buss-bar" with a
crack that's visually "not there", but electrically, is the size of the
grand canyon.

>
> A mechanic I've known for twenty years told me
> it simply died of old age. He asked me if I
> ever had an electric blanket, heating pad, or
> space heater that failed to heat up after a
> number of years. The nichrome wire in these
> appliances produce less and less heat as they
> age.

Baloney. Those items stop working because the resistive element has
failed, usually by physically breaking. They don't "die of old age",
they die of "trauma" (wire/item physically broken) or the heating
element burning out. (IME, usually but not always, at the junction
between the resistive element and the wire that's supposed to feed it)
In the case of an electric blanket, the failure cause is almost always
metal fatigue, and almost always at the point where a wire enters one of
the "capsules". The wires inside are metal, just like any other wire,
and just like any metal other than mercury, if you flex it at the same
spot often enough, it gets brittle, then eventually cracks. Particularly
true for copper/copper-containing-alloys that "work harden", but all
metals are susceptible to some degree. Position a (relatively) solid
edge, like the junction between the "very flexible" wire, and the
"almost totally non-flexible" "capsule" to put the flex point in a fixed
position, and the wire will break in a surprisingly short amount of time.

> I seem to remember a kit I saw in a catalog many
> years ago. You scrapped off the old grid and
> replaced it with something that looked like a
> big decal or auto pin stripping. Is there
> anything available that can replace the defroster
> grid, or is replacing the entire rear window
> the only solution?

Seems to me I remember the same kit (or something very similar) myself.
I also recall filing it under the heading "just like the stick-on,
do-it-yourself window tint" - which basically translates as "nothing
I'll ever spend any money on." I simply don't trust ANY adhesive to
stick to glass for an extended period.

I suspect that you've got a crack in one of your buss-bars. Pull the
wire off both grid terminals, and clip a lead from a meter set to low
ohms to one of them. Use the other lead of the meter to slowly work your
way up the "bar" from the terminal until you lose continuity. Your crack
is between the terminal and the probe. Pinning it down is then just a
matter of moving the probe around until you establish the exact
location. Fixing it is, as I'm sure you know, nothing more than painting
on some of that conductive epoxy across the break.

Something of a long-shot possibility, but one of the lower lines may be
shorted to ground by moisture/contaminants - give the window a good
cleaning on the "grid" side and see if that makes difference.

--
Don Bruder - - New Email policy in effect as of Feb. 21, 2004.
Short form: I'm trashing EVERY E-mail that doesn't contain a password in the
subject unless it comes from a "whitelisted" (pre-approved by me) address.
See <http://www.sonic.net/~dakidd/main/contact.html> for full details.

Hello Don.

Electronics was my hobby when I was a kid. As
an adult it became my profession. Mercifully, I
never got involved in auto electronics!

90% of what you said I was already thinking some-
where in the deep recesses of my tired brain.

I completely agree with you about all of these
film and sheet type products that people add to
their car windows. If they're lucky they can simply
peel them off when become hazy, pucker up, or
simply fall apart. Sometimes, removing the film
and/or adhesive results in a badly scratched window
or damaged window caulk and moldings.

I'll definitely do a slow and careful resistance check
on the defroster grid as you suggested.

Holophote

"Holophote" > writes in article .com> dated 20 Jan 2005 07:45:25 -0800:
>Recently, I checked out a defroster grid on a
>1991 Taurus wagon. It suddenly stopped working.
>There appears to be nothing wrong with it. All
>voltage and resistance tests are nominal. The
>power connectors are firmly attached and there
>are no cracks in any of the grid lines.
>
>A mechanic I've known for twenty years told me
>it simply died of old age. He asked me if I
>ever had an electric blanket, heating pad, or
>space heater that failed to heat up after a
>number of years. The nichrome wire in these
>appliances produce less and less heat as they
>age. He basically said the only solution is to
>repace the entire rear window!

That diagnosis does not fit with your statement that "voltage and resistance
tests are nominal". If you apply a nominal DC voltage to a nominal
resistance you're going to get a nominal power output. Unless the grid has
mutated to produce X-rays or some other radiation, this output will appear
in the form of heat. Also, what he describes is a gradual process, not a
sudden one.

I would suggest a re-test. First test the voltage with the defroster "on".
Then test the resistance with (at least) one side disconnected. Don't trust
"off" to disconnect it for you; the relay/switch might also short it, i.e.
connect both sides to ground, which will give you a very low resistance
reading even if the grid itself is an open circuit.

Output power is V^2/R, so if you have a 10 Ohm resistance that's 14.4 Watts,
almost reasonable.

-- spud_demon -at- thundermaker.net
The above may not (yet) represent the opinions of my employer.

I fixed the defroster grid!

Without Don Bruder's suggestions I would have floundered
around and accomplished nothing. Thank you very much
Don!

I keep a cheap analog and digital meter in my garage. If
they are damaged its really no big deal since anything
I can do without paying a mechanic more than pays for
another cheap meter.

Before I started checking for any lack of continuity or
change in resistance between each bus and its connector,
(as Don suggested) I ask my neighbor (its his Taurus) if
he had ever repaired the defroster grid in the past. He told
me that one of the connectors broke loose a few years ago.
The repair he made seemed very solid. I couldn't detect any
play when I wiggled the connector.

My cheapie meters showed no lack of continuity or
significant resistance change when I tested the positive
and negative sides of the bus. It didn't seem like there
was a crack or fissure to be found.

Because I have access to lab grade instruments at home
and at work I decided to try one of my DVMs with data logging
capabilities. This instrument can detect extremely brief
intermittent changes in continuity, resistance, voltage, etc.,
and keep a record of each change that can be displayed on
your PC.

With the data logger hooked up I clipped one lead to a
connector and taped the other lead to one of the bus strips.
Again, I wiggled the connector for about one minute and
plugged the DVM into my Desktop. WOW! I couldn't believe
what I was looking at. Hundreds, maybe thousands of
intermittent breaks, that could never be detected by my
cheap meters.

At this point I had to make a decision. The connector
was solidly attached to the bus. I could have pried it
loose but there was a real good chance that a big chunk
of the bus would be break off with the connector. I decided
to clean the connector with a tarnish and oxidation removing
chemical without removing it from the bus. It worked fine.
I then bridged across the connector with a silver based
adhesive (made by CAIG) and flowed the resin onto the
bus. Problem solved.

One more thing that you may find difficult to believe. I visited
three major auto stores looking for the conductvie adhesive.
I'm glad I never carry sharp objects in my pockets because
by the time I visited the third auto store I was ready to chew
broken glass!

These guys didn't seem to understand words like "conductive,
defroster, or grid." In one case I spoke to the manager of an
AutoZone. He told me that he had never heard of anyone
having to repair a rear window defroster! I drove home feeling
like I had just taken a trip through the Twightlight Zone. I
couldn't acccept that no one in the three auto stores I visited
knew what I was talking about. I finally decided to drive down
to my office. I walked into one of the tech labs and picked up a
tube of CAIG silver based adhesive. All CAIG products are first
rate. It sealed and bonded the connector perfectly.

I just want to thank Don again. Without his guidance my
neighbor would have been disappointed, and I would have
felt really bad since he's done many favors for me in the past.

I mentioned in my original posting that I remembered seeing a
kit many years ago that allowed you to completely replace the
defroster grid. Don said that he seemed to remember similar
kits, and that in all likelihood they were junk. I completely
agreed. Well, I did a one minute Google search and found
these guys at http://www.frostfighter.com. They sell a complete
line of defroster repair and replacement kits. Who knows if
they're any good.

Holophote