Rear defrost

[bakergtt]

Is there any way to make a rear defrost work better? mine seems to take forever to get the window de-iced or de-fogged.

Where should i start? what should i look at/for?

[RABID_SUB]

my guess is there is a relay that only lets so much power go to the coils in the glass. up the amps, and use heavier gauge wire. thats really all you can do.

[Superu264]

At least your's works. I bought mine in September and just had to use the rear defrost today.. its completely useless, it does nothing. Anyone ever have any trouble with theirs? Any suggestions before I go out and brave the cold to test everything out?

[Wiscon_Mark]

mine isn't very effective when its below 20 degrees (farenheit) or so, but its definitely on. Gil_ong81 did a thread about this, anyone who is having problems should check it out.

[Plays_with_Toys]

Wow. You guys have it rough. I don't even scrape off my rear glass. I turn the car on, put the rear defroster on, and by the time I'm done with the windshield the entire rear window has melted. Its ridiculously quick.

[RABID_SUB]

mine works fast to. maybe they get a hard freeze in the east that doesnt melt as quick.

Superu yours may need pluged in at the glass or the relay might be bad. Does the light on the switch light up?

[gil_ong81]

At least your's works. I bought mine in September and just had to use the rear defrost today.. its completely useless, it does nothing. Anyone ever have any trouble with theirs? Any suggestions before I go out and brave the cold to test everything out?

one of my tabs broke off.

https://sl-i.net/FORUM/viewtopic.php?t=1326

[Superu264]

yea the switch lights up and everything appears to be in order, but absolutely nothing happens, and yea we do get some nice deep freezes up in the NE

[RABID_SUB]

check those tabs, they snap all the time.

[Superu264]

what tabs are you talking about?

[Reason]

Wow. You guys have it rough. I don't even scrape off my rear glass. I turn the car on, put the rear defroster on, and by the time I'm done with the windshield the entire rear window has melted. Its ridiculously quick.

Mine works fast too, there must be something wrong with yours.

[gil_ong81]

what tabs are you talking about?

here it is AGAIN.

https://sl-i.net/FORUM/viewtopic.php?t=1326

[Wiscon_Mark]

don't get angry now Gil :lol:

Mine works good in decently cold weather, but when it gets brutal out there, the entire heating system is pretty ineffective (damn wagons).

[gil_ong81]

don't get angry now Gil :lol:
i'm not. cos oh, you'll knwo it when i'm angry. :D

just don't want him to have to needlessly suffer what i went through.

[RABID_SUB]

the wires in teh rear window run to 2 tabs that are soldered on for teh wires to clip on to. they are behind the trim panels on the c pillars :wink:

[Superu264]

thanks guys, at the moment 4 inches of snow is falling so ill have to start working on it maybe in a few days

[badbasser98]

my defroster sux balls as well.... Mark, as far as the heat/AC is concerned, I have noticed that at crusing speed 45-65MPH the heat and AC are awsome, but city driving 25-35MPH both are terrible.... Must have something to do with the engine RPM's spooling the compresser.

-BB98

[BAC5.2]

my guess is there is a relay that only lets so much power go to the coils in the glass. up the amps, and use heavier gauge wire. thats really all you can do.

That isn't a relay's job though, is it? A relay is just an electric switch. It creates an EMF inside the case, and pulls a contact via magnetism. It runs with an incredibly low current draw, so it is good for allowing you to switch high-draw from far away without increasing resistance due to excess wires.

Think about how a defroster works. They introduce resistance, by having long lengths of thin wire. Increased resistance means decreased amperage and increased heat.

You want all of the resistance in the defroster wires, right? You focus down the voltage drop to a smaller area. Think of it like a magnifying glass. Focus it just right, and you can catch an ant on fire.

So you don't want excess resistance anywhere in the circuit. You can't increase the voltage to the system, because that would potentially overload the circuit. You also want to remain within factory power delivery specs. The factory designed the system to perform under optimal conditions given the specs. Over time, terminals can corrode and increase resistance. You want to remove this as best as you can.

So you want to reduce resistance before and after the defroster wires. The less the resistance, the greater the current draw. That's why you can arc-weld a screwdriver to a starter solenoid if you accidentally ground the screwdriver while testing current draw on a starter motor. 160amps instantly grounding through some crude steel, and BAM. OK, OT, ADD, whatever.

The defrost relay should be a single throw, single pole relay. You have 4 tabs. Switch tabs, with thin gauge wires running from the switch on the dash and a constant power. The switch can be on power or ground, it doesn't matter. Flip the switch, complete the circuit, energize the coil in the relay, draw the contacts together with magnetism. The two other terminals are to pass some current through. 12v constant in, to 12v out, or you could toss it on the ground side. Most switches are on positive side, simply to reduce the chance of a short-to-ground.

So you have a simple circuit. 12v to switch to relay to ground. 12v constant to relay, switched out. Flip the switch, complete the circuit. There is a fuse to, and it's before the switch on the positive side. It's a saftey. Any accidental short-to-ground will blow the fuse and disconnect the circuit. Fuse blows because the new ground reduces resistance. Decrease in ohms = increase in amps given constant voltage. 12v constant. 3 ohms = 4 amps (12v/3ohms; ohms law). 2 ohms = 6 amps.

When you complete the circuit, you will send current through the defrost wires and to ground.

What you need to do, is use a slightly heavier gauge wire (available at speaker stores or hell, even home depot), and as little wire as possible. Thicker wire has less resistance. Replace the relay->glass wire, and the glass->ground wire. This will bump the efficiency in the system, and you should notice a notable increase in defroster performance.

Compare with a multimeter, and you should see a significant nominal difference too. I'd guess the factory ground has a good .4v voltage drop, and factory feed has nearly the same. From a 12v feed, a .8v voltage drop on the feed and ground wires is a lot. The defroster wires only see 11.2v of the 12v. If you could drop that voltage drop from .4v to .2v, you would see an 11.6v voltage drop across the defrost wires. That's a SIGNIFICANT increase. A difference in battery voltage of 12.3v and 12.2v is a 62% state of charge, vs a 50% state (respectively). .4V is a significant amount.

When buying wires, you want the ones with as little resistance as possible. You may want to consider insulating the wires somewhat, as low-resistance wires can cause some radio interference if run near speakers (the generated EMF can create a buzzing sound).

Hope that helps explain what is going on a little better for you. Go trouble shoot and tell us what you find! I know I would be interested in knowning what you find out! Go clean the dust off that multimeter!

Give it a shot.

Phil

[gil_ong81]

the power cable looks like it might be a real pain, but the ground cable is a snap.

[Huffer]

Damn Phil, that makes total sense to me!

[Wiscon_Mark]

that would be the longest typed post in SLi history....which gives me an idea :D


Phil, even if you get a little OT, I always learn something more, don't sweat it :)

[badbasser98]

Nice, that will give me another use for the multimeter I built in Electronics class :-D

Actually works pretty damn good :wink:

[RABID_SUB]

Damn. you know quite abit of info Phil. it will be good to have this. I thought defrost systems had a relay that shut them off so they didn't heat up the glass to much. maybe I'm thinkin of somthin else.

[Wiscon_Mark]

some have a shutoff switch, but I don't think ours do.

[BAC5.2]

It can only get as hot as it is designed to.

The wires may not actually get a full 12v.

They don't continue to just get hotter and hotter, they will reach a point and kind of hover around that same temp. Doing what I said will probably increase the temp of the wires slightly. I don't know if it would make them SO hot that it cracks the glass though. If your defroster doesn't really work that well, it might be worth a shot to ground the defroster a little better.

I don't know of any temperature triggered relays. It could use a thermostat to break the circuit, in kind of a feedback loop. But a relay is simply a coil of wire, and a contact that is held open (or closed) by a spring. Apply power to the coil of wire, it creates an EMF and it closes the contact and completes the circuit.

[gil_ong81]

If your defroster doesn't really work that well, it might be worth a shot to ground the defroster a little better.
i think the easiest way to do this is simply to clean off the connectors and the bolt that secures the lug to the frame. should not take more than a few minutes once you remove the trim piece that covers the c-pillar.

of course, while you have everything apart, you might as well just go ahead and replace the cable liek you said.

[BAC5.2]

i think the easiest way to do this is simply to clean off the connectors and the bolt that secures the lug to the frame. should not take more than a few minutes once you remove the trim piece that covers the c-pillar.

of course, while you have everything apart, you might as well just go ahead and replace the cable liek you said.

Absolutely you should clean up the contacts.

Do a combonation of what we have said, and you should have a snow-meltin window! At the very very very least, clean the contacts and you should see a huge improvement!

[jmgtp]

My rear defrost works ok... takes a bit long for it to clear the glass when its 20* at 6am.

A bit off topic but still rear defrost related... does any one else have severe radio (FM) interference when they turn the rear defrost on? Mine makes FM all static. It is definetly due to the radio antenna being in the rear glass and interference from the defroster since its so close. Has anyone found a fix? I tried a signal booster in another car that had reception problems and it didn't do anything - I think those are probably just snake oil. Grounding issue somewhere perhaps? My b on the semi-thread-hijacking.

[Wiscon_Mark]

what MY Legacy? I don't remember any 2nd gens have an antenna in the glass...

[jmgtp]

it is an 01, i prolly should have mentioned that 1st time around :oops:

[BAC5.2]

What Subaru has the antenna IN the rear glass? The new ones have them in the FRONT glass.

Lots of resistance causes stronger electro magnetic fields, and I suppose that could cause interference. But on the FM channel, that's odd.

How is the signal otherwise? Maybe a bad antenna ground?

That's really odd. A little more info might be helpful.

Phil

[jmgtp]

With the rear defrost off the signal is good - just like you would expect it to be. With the rear defrost on the signal is definetly weakened. If the radio is on and getting good reception and I turn the rear defrost on it instantly gets some static, sometimes depending on signal strength it goes to all static and I'm either forced to listen to another station or switch to CD. There is a direct correlation between rear defrost and radio reception in my car. The antenna line is at the top of the rear glass. If I recall correctly it isn't just a straight line but has a 'T' shape in the middle. I can take a pic if there is some question here.

[Huffer]

I don't know of any temperature triggered relays. It could use a thermostat to break the circuit, in kind of a feedback loop.

I'm not sure how it works, but I know my girlfriend's 2003 Mazda 6 has an auto shutoff for the rear defroster. It's great, you just turn it on when you start the car, then start clearing the side windows...by the time you get to the back, the ice is sliding off and by the time I get in the car and down the road, the defroster is off again.

[scottzg]

Wrap the defroster power wire (not the relay wire) around a magnet as close to the fusebox as possible.

Goin' on a hunch.

[BAC5.2]

I don't know of any temperature triggered relays. It could use a thermostat to break the circuit, in kind of a feedback loop.

I'm not sure how it works, but I know my girlfriend's 2003 Mazda 6 has an auto shutoff for the rear defroster. It's great, you just turn it on when you start the car, then start clearing the side windows...by the time you get to the back, the ice is sliding off and by the time I get in the car and down the road, the defroster is off again.

Probably a temp sensor or something.

[RABID_SUB]

I'm guessing a timer. next time you use it time it and see if it is consistant

[Superu264]

I FINALLY took out the seats and plastic to only find that BOTH tabs have completely fallen off, soldering will take place tomorrow, its already too dark/cold now.

Just a question though, has anyone soldered these back on? I want to use my soldering iron but Im afraid to harm the glass. Any input is appreciated. :D

[Wiscon_Mark]

I believe Gil did some work on that...check the other thread he gave you a link to for info.

I know mine haven't fallen off (it'd be pretty obvious in a wagon), they're just not very effective :?

[badbasser98]

What Subaru has the antenna IN the rear glass?

Mine...

[gil_ong81]

Just a question though, has anyone soldered these back on? I want to use my soldering iron but Im afraid to harm the glass. Any input is appreciated. :D

yeah, you have to be really careful. if you heat things up TOO MUCH, that woven strip will actually separate from the glass. my advice is to use lots of flux if you have it.

and of course, after putting everything back together, give the glass a good wipe-down since the flux and solder give off fumes that cling. along those same lines, keep you doors or window open to allow plenty of ventilation.

[Superu264]

MY BABY WORKS

Left work early today and took it to my dads firehouse so I could work on it in a nice warm room (no room to work in my garage). So while among the firetrucks I just soldered both tabs on, fogged up the windows, turned it on and what do ya know? Cleared up instantly! Thanks for all the advice guys, I wish I had some knowledge for yous but Im still learning. :roll: