Is Hatch Defroster Circuit Adequate for Heated Fan?

[rossiz]

my 78 280 rear hatch defroster doesn't work - all the horizontal filaments are compromised in one or more locations. there is power at the clips up on the ceiling and the console switch works fine. i drive the car every day and in seattle winters the hatch glass is constantly fogged. i upgraded to a honda blower fan, which will eventually de-fog the rear if i blast the defroster on high, but it's so slow getting the warm air back there that i often reach my destination before i can see adequately out back.

my idea was to mount a cheapo 12v defroster fan to the back panel that covers the tail lights, pointing up at the hatch glass and hopefully do a better job of keeping things clear. my concern is i don't want to melt the circuit and/or blow fuses. the add says it draws 150 watts - anyone know if this is doable?

[Zed Head]

I ran something like that on my defrost circuit.  The circuit has a 20 amp fuse.  The Google says 12.5 amps at 150 watts / 12 volts.  10.9 at 13.8 volts.

The most effective thing that I did for my back window fogging problem was to fix all of the cabin leaks and get everything dried out.  I also used Rain-X on the outside of the back window.  Sometimes it's the outside fogging that's the worst.

[Patcon]

the defrost lines can be repaired. There are videos on how to do it. 12.5 amps is a lot of current for the old harness wires

[Zed Head]

3 minutes ago, Patcon said:

the defrost lines can be repaired. There are videos on how to do it. 12.5 amps is a lot of current for the old harness wires

People say that but I haven't a Z car owner do it yet.  I think that you can repair breaks but the problem with the Z grids is that whole grid wears down.

There's a company that makes a replacement grid.  Last I saw that they had some new sizes that would work.

[Mark Maras]

Til you get it fixed, you might try Rain-X Interior Glass Anti-Fog. It's nerve-wracking driving with limited vision. I remember one cold winter morning when I was scraping frost on the inside glass. What a PITA.

[zKars]

Repairing a defroster grid, while very possible, is a bit of a test of wills.

Me, I'm just replacing the whole thing with a modern equivalent. scrape 'er off and start fresh.

http://www.frostfighter.com/clear-view-defrosters-list-12V-1.htm

These guys sell quality repair kits and total new systems. 

[rossiz]

i've looked at the frostfighter setup and it seems a pretty slick system. i'm thinking it's a better long-term solution for sure, but will require some time and patience to get it right. can't imagine a good installation working upside down in the hatch, so that means taking off the hatch and working on a bench - more effort than i have time for at present, so i thought i'd do a quick-n-easy temporary fix w/the $10 fan.

my hatch glass has some scratches on the outside from the PO installed louvers and i'd probably want to scrape off the old factory defroster lines to get best contact/adhesion with the glass - has anyone tried removing the glass and having it polished to get rid of scratches and the defroster lines? this is a bigger project (summer for sure) but it's what i had in mind for the permanent fix. it would also be the time to straighten out the bent brightwork and replace the hatch glass gasket...

[Captain Obvious]

I wouldn't plug that defroster into the cigarette lighter outlet, but if you cut the lighter adapter end off and splice it directly to the wires that go to the original defroster, you'll probably be OK. The reason for that suggestion is that the defroster circuit has it's own dedicated relay to drive the defroster (and only the defroster)

As Zed Head already suggested, the current draw from that defroster is going to be ten to fifteen Amps and the defroster circuit is fused for twenty. Where I come from, a factor of two on a safety device is the limit, so you'll be pushing that, but it's going to be for intermittent use. If anything, I would expect that the relay would go up first and not the wires. Just make sure you have good clean tight connections everywhere. Last thing you want to do is burn up any energy in the connection points. You want all the energy in the heating coils, not the connectors.

[Zed Head]

Using the defogger wires was the plan, I think.  It has a switch, and a relay and fuse.  Hardest part is getting fan rotation right.  If it spins backward for long you'll burn up the heater elements.

I mounted my heater/fan on a piece of wood so I could remove it easily in the summer, but it did slide around some on the hard corners.

 

I used the defogger circuit to run my fuel pump way back when my new AFM fuel pump contacts fouled.  Used the old car length piece of wire that a PO had installed to run it back when he/she had their own problems.  Great fun.  Switched fused fuel pump power, with an indicator light.

[rossiz]

thanks for the helpful replies - just what i was looking for. 

yes, i plan to simply crimp a set of female connectors onto the defogger fan (i'm sure it's too weak to actually defrost, and we don't get much of that here anyway) and use the existing defroster circuit and switch. i'm happy to see it uses a relay - at least the oem switch is safe. when the fan arrives i'll figure out a mounting scheme - probably as simple as some double-sided foam tape to the plastic tail light cover panel, as the fan comes with a "swivel base" which i'm hoping will allow adequate adjustment. i'll be sure to let you all know how it works out...

[Captain Obvious]

Sounds like a plan. Just remember... Good connections all around. 10A will drop an entire Volt on just 0.1 Ohms of resistance. It's the law.

Might want to pull the defroster relay out of it's socket, clean the contacts, and reseat? Same with the fuse?

I remember fogging on the inside of the glass from my first Z, but I must not be driving the current one enough because I haven't had that problem. Of course, being my summer car, most of the time my windows are down.

 

[rossiz]

got her mounted up over the weekend (thanks to Zed's help w/diagnosing a dead relay). it was a simple job - i cut off the cigarette lighter adaptor and crimped on a male spade connector to the hot wire, then tucked the wire up the window frame by the hatch glass gasket and taped it in place. the fan has a little swivel base which i fastened to the inside of the hatch panel by replacing the top center hatch panel screw with a longer one, then crimped a ring connector to the ground wire and used the same mounting screw to complete the circuit. done in 15 min.

 this morning seattle blessed me with an opportunity to test my defogger fan - pissing down rain for my commute.

the back hatch fogged up (as usual) as i sat in the driveway letting the engine warm up, so i activated my newly re-purposed "defogger" switch and enjoyed the pleasant blue glow of the indicator light while the little cheapo fan came to life. the output is tepid and weak, but even so, it cleared the hatch in just a few minutes and it's whisper quiet - i can barely hear it. best part is once i got the hatch clear i turned it off and the glass stayed clear for the rest of my 1/2 hr. commute.

its a little goofy looking, but simple, works just fine and completely reversible. the whole thing sits below the bottom of the hatch glass so you can't see it from the exterior and it's black so it doesn't call much attention to itself from the inside either.

if anyone is interested i can post a pic or two.