how to modify rear window defrost?

[tom klein]

i find it most annoying that my 2002 legacy wagon always starts up with
the rear defroster OFF. no matter what the dimwit engineers think, if
i needed the defroster before i shut the engine off, chances are that
i'll still need it when i start again. like, winter usually lasts
for months at a time, right?

so i would like to modify the defroster circuit so that the dashboard
switch works just like a real switch: ON when i turn it on, and OFF
when i turn it off, and not before then.

has anybody done such a mod, and would they like to "share the experience"?


........ tom klein

ps. please respond via the newsgroup.

 

[Todd H.]

i find it most annoying that my 2002 legacy wagon always starts up
with the rear defroster OFF. no matter what the dimwit engineers
think, if i needed the defroster before i shut the engine off, chances
are that i'll still need it when i start again. like, winter
usually lasts for months at a time, right?

Yeah this is a little annoying coming from my Mazda or was it my Honda
that had a hard switch.

But the counterargument is "how many folks leave that thing on all
summer as a result?"

I do prefer having a hard switch though. The heated mirrors and the
seat warmers appear to operate this way, so they're not even
internally consistent on this. Hee hee...

so i would like to modify the defroster circuit so that the dashboard
switch works just like a real switch: ON when i turn it on, and OFF
when i turn it off, and not before then.

has anybody done such a mod, and would they like to "share the
experience"?

Haven't done it, but I can't imagine it'd be too hard. Before I'd
attempt it though, I'd be sure to get hold of the wiring diagram. Good
luck!

 

[John O]

But the counterargument is "how many folks leave that thing on all

summer as a result?"

It shuts itself off after ten minutes, doesn't it? That's usually long
enough to get the stuff off, and by then the inside of the car is warm and
thst heat does the job. Never thought about it before, otherwise.

-John

 

[mulder]

The switch is an input to the ECU, which in turn operates the
defroster through a relay. It's a momentary/toggle switch and there
isn't much you can do to change its operation. I suppose if you are
electronically inclined, or know someone who is, it would be possible
to build a circuit that would send a momentary pulse to that circuit
every time the car was started. But there are considerations such as
do you really want it to come on every time you stop and start the
car, adding logic to make it only work on the first cold start of the
day wouldn't be worth the effort. Another factor to consider is that
the defroster puts a heavy drain on the electrical system so it's best
to wait until the engine is running for a minute or two before turning
it on.

 

[John Rethorst]

Another factor to consider is that
the defroster puts a heavy drain on the electrical system so it's best
to wait until the engine is running for a minute or two before turning
it on.

Sorry if I sound like a newbie, athough I am - it makes good sense not to turn
the key to accessory, turn the defroster on, then start the engine, as you say.
But if the engine is running, how could the defroster load matter? I ask because
I generally use it in the morning, and I go in the order: start the engine, turn
on the df, set the heat, wait another half a minute for engine warmup, get in
gear and go.

 

[Todd H.]

The switch is an input to the ECU, which in turn operates the
defroster through a relay.

Oooh. If that's the case, I wonder if the ECU connection internally
is such that hacked ECU firmware could modify this behavior.
Muwuhahahahahaha.

Those folks who tweak ECU firmware for performance and all that jazz
might be enjoined in such a query

 

[T]

i find it most annoying that my 2002 legacy wagon always starts up with
the rear defroster OFF. no matter what the dimwit engineers think, if i
needed the defroster before i shut the engine off, chances are that
i'll still need it when i start again. like, winter usually lasts for
months at a time, right?

so i would like to modify the defroster circuit so that the dashboard
switch works just like a real switch: ON when i turn it on, and OFF
when i turn it off, and not before then.

has anybody done such a mod, and would they like to "share the experience"?


....... tom klein

ps. please respond via the newsgroup.

You want this but you really don't want this.

What things like this should be designed to do is to let go or disengage
when you turn the Ign off and remove the key.

Just hit the button each time you need it, there are worse things in life.


TBerk
need a better (actual) explanation why?

 

[tom]

It shuts itself off after ten minutes, doesn't it? That's usually long
enough to get the stuff off, and by then the inside of the car is warm
and thst heat does the job. Never thought about it before, otherwise.

-John

1) you don't live in canada. there are folks south of the border who
think they don't even need a rear defrosters - like, it's never cold
and damp there. yeah, right!

2) if i want the sucker on, i put the little switch in the "on"
position. and leave it in the "on" position until i decide that it
should be moved to the "off" position. chances are, tomorrow's
weather will be a lot like today's weather. no timers, no bells and
whistles, and no "you've had enough, so i'm going to shut it off for
silly little old you".

i'm also pissed that subaru decided that i can't run the windshield
defog (front heater) on inside air (the little light goes off when you
move from lower vents to defog) - obviously, the news flash hasn't
percolated through that you can heat up inside air faster than outside
air, when the temperature outside is -30. it's the heat on the
glass, and not the humidity, that matters. (is anybody from
engineering LISTENING?)

3) if i'm stupid enough to ignore a lighted switched for the whole two
weeks that are called "summer" up here, i deserve whatever the
consequences might be of using a defroster unnecessarily. (20 lashes?
public ridicule? guest appearance on letterman?)


........ tom klein

 

[tom]

The switch is an input to the ECU, which in turn operates the
defroster through a relay. It's a momentary/toggle switch and there
isn't much you can do to change its operation. I suppose if you are
electronically inclined, or know someone who is, it would be possible
to build a circuit that would send a momentary pulse to that circuit
every time the car was started. But there are considerations such as
do you really want it to come on every time you stop and start the
car, adding logic to make it only work on the first cold start of the
day wouldn't be worth the effort. Another factor to consider is that
the defroster puts a heavy drain on the electrical system so it's best
to wait until the engine is running for a minute or two before turning
it on.

wow! hacking the ecu might actually be fun, and a lot safer than
chopping into the wiring harness!

the simplest approach i can think of is to replace the relay, if it's
socketed, with a "dud" relay that's always on, wired to an spst switch
in series. pop in a good relay, and everything's back to factory spec.
and just to make the engine-ears happy, of course i would opt for a
lighted switch, too.

where do i find information on the ecu and/or the car's wiring (2002
legacy wagon)?



........ tom klein

 

[John O]

1) you don't live in canada. there are folks south of the border who

think they don't even need a rear defrosters - like, it's never cold and
damp there. yeah, right!

Close, though. And I been there lots and lots of times, once driving from
Saskatoon to Moose Jaw and the dang car never got warm. -30 that day. The AC
plugs in all the parking spaces at the hotel were pretty interesting. Why
the hell was I in Saskatoon in February anyway? Dumb idea.

2) if i want the sucker on, i put the little switch in the "on" position.
and leave it in the "on" position until i decide that it should be moved
to the "off" position. chances are, tomorrow's weather will be a lot
like today's weather. no timers, no bells and whistles, and no "you've
had enough, so i'm going to shut it off for silly little old you".

Well, there's something to be said for that.

i'm also pissed that subaru decided that i can't run the windshield defog
(front heater) on inside air (the little light goes off when you move from
lower vents to defog) - obviously, the news flash hasn't percolated
through that you can heat up inside air faster than outside air, when the
temperature outside is -30. it's the heat on the glass, and not the
humidity, that matters. (is anybody from engineering LISTENING?)

Warmer, humid air hitting cold glass condenses pretty fast in my experience.
I only had an interior fog-up once, in my old Galant. PITA, that was.

3) if i'm stupid enough to ignore a lighted switched for the whole two
weeks that are called "summer" up here,

OK, well, just hardwire that sucker and be done with it. With only two weeks
of summer, you deserve it.

-John O

 

[John Rethorst]

i'm also pissed that subaru decided that i can't run the windshield
defog (front heater) on inside air (the little light goes off when you
move from lower vents to defog) - obviously, the news flash hasn't
percolated through that you can heat up inside air faster than outside
air, when the temperature outside is -30. it's the heat on the
glass, and not the humidity, that matters. (is anybody from
engineering LISTENING?)

This doesn't happen on my '01 OBW.

 

[Cam Penner]

i'm also pissed that subaru decided that i can't run the windshield
defog (front heater) on inside air (the little light goes off when you
move from lower vents to defog) - obviously, the news flash hasn't
percolated through that you can heat up inside air faster than outside
air, when the temperature outside is -30. it's the heat on the
glass, and not the humidity, that matters. (is anybody from
engineering LISTENING?)

Is that why cars never fog up on the inside in Florida?