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924RACR · Posted: Mon Nov 08, 2004 12:41 am Post subject: weak rear defrost - fixed?

In playing with my turbo yesterday, got a number of things done... started out with a pre-winter oil change, figured out that I'd left my O2 sensor unplugged, and that my water pump has that nice little leak of death...

Anyway, I wanted to play with the defrost setup, as wihle it functioned, it was somewhat intermittent and not very effective all the time - very slow to heat.

I confirmed that I had a good 12V back on the feed side, the LH side. I was having trouble, however, with checking the ground. Finally, after having it on for a while, I accidentally figured out what it was that was heating up! Not the grid, but the hatch strut on the right!

See, the defrost grid grounds through the body of the left-hand hatch strut. That's why you have to buy a LH and a RH strut, why they're not the same. However, there was a lot of corrosion at the top of the RH strut, leading to a poor connection. The excessive amount of resistance resulted in generation of a lot of heat - nearly enough to burn on contact.

So, the long-term solution is probably that I'm going to have to replace the struts - note that you must replace them both at the same time; if you don't, you stand a very good chance of having the hatch glass separate from the frame, since a new strut will exert much more force than an old one, twisting the frame.

However, for the meantime (since I can't budget for that) I've just run another wire to ground, to hopefully let this work much better. We'll have to wait till a chilly work morning for results, to see if this has improved the efficiency of the defroster. Just thought I'd pass it on...
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

marky522 · Joined: 02 Nov 2002 Posts: 335 Location: Port Huron, MI

Dont worry, i dont think we'll have to wait long for one of those chilly mornings... lol

Mark

924RACR · Posted: Sat Nov 13, 2004 3:43 am Post subject:

Well, we've had plenty of chilly mornings, but none with any frost, till this AM. It wasn't a big frost, but I can say for sure that the defrost was kicking it's butt pretty well, and had it cleared within 5-10 minutes of starting driving (ambient temp was 30 deg). Not a 100% conclusive fix, but I must say a definite improvement, adding in that ground wire.

Hopefully I'll have another more distinctive update before too much longer...
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

Smoothie · Posted: Sat Nov 13, 2004 4:19 am Post subject:

Mine's working now too with a repair to the frayed ~10 gauge wire at the left side plus bypassing the shock there with an added length of 10 gauge wire running from the repaired connector up to the grid.

_________________
"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

Johno · Joined: 14 Oct 2003 Posts: 53 Location: Greendale, WI

I noticed that mine didn't seam to work very well or maybe didn't work. So I checked and would get 12 volts on the line going into the L strut but checking the volts out of the strut was intermitent. So I ran a line by passing the strut completely. So I have the 12 v line attched to my coiled cord which is plugged into the window.
When I hook the volt meter to the L & R I get 12 volts. We checked the ohms on the window (don't recall the reading but the circut was not open which would indicate a broken line in the window).
So one would assume that the window should clear? It was frosty here and it did not appear to defrost. Do these work really really slow?
_________________
79 924 NA

Smoothie · Posted: Sat Nov 13, 2004 4:42 am Post subject:

Until my most recent repair, I was getting zero defrost action, but always read 12v across the grid with a voltmeter. Had tried bypassing the struts several times in past years with no luck (still got the volts, but no defrosting). My problem turned out to be with the connector at the car body side of the left strut - the wire there was frayed, apparently limiting the current flow (12v went through, but the fray was like a bottleneck and limiting the current (amps)). You should be looking for some type of bottleneck like that somewhere. Either a frayed wire or bad, oxidized connection.
_________________
"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

924RACR · Posted: Sat Nov 13, 2004 11:57 pm Post subject:

We had a good frost this AM, and while I wasn't going in to work, I went out and ran a timed test at idle for the good of the board.

I should clarify, it was not merely fog but full-on frost, estimated ambient temp of about 30 degrees - a good test. Plenty enough frost to fully obscure the whole hatch

With the new ground, I saw the first signs of defrost happening in 1 minute, apparent in 2 minutes through the rearview (rather than directly). After about 5 minutes, visibility was noticably improved (could see somewhat through the hatch, though not clear). At about 7 minutes I would say it was clear enough to drive, though not 100% clear, and could've been cleared by the wiper if that were hooked up. I terminated the test at that point, but I suspect it would've been clear by around 10 minutes.

I expect that replacing both hatch struts would possibly yield even better results, since the power was still being supplied through the left strut, which may have had less than optimal efficiency.

Now I just have to fix the two broken defrost lines in the middle of the hatch. I noticed that the breaks in the lines were easier to see from the outside than inside.
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

924RACR · Posted: Sat Nov 13, 2004 11:59 pm Post subject:

Oh, yeah, the hatch defrost was well matched in pace by the windshield defrost; I could clear the windshield well enough with the wipers at 5 minutes (of idling) that I could safely drive.
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

Smoothie · Posted: Mon Nov 15, 2004 2:45 am Post subject:

We had some ice yesterday morning -
http://pg.photos.yahoo.com/ph/smoothies931pics/detail?.dir=7a86&.dnm=a2b0.jpg&.src=ph

with defrost on for 4 min., some sign of something happening -
http://pg.photos.yahoo.com/ph/smoothies931pics/detail?.dir=7a86&.dnm=b387.jpg&.src=ph

after 7 min. -
http://pg.photos.yahoo.com/ph/smoothies931pics/detail?.dir=7a86&.dnm=7bf5.jpg&.src=ph

12 minutes -
http://pg.photos.yahoo.com/ph/smoothies931pics/detail?.dir=7a86&.dnm=1d0a.jpg&.src=ph

this is after 19 minutes, but I was out and around and had already stopped at an ATM and shut the car off, so subtract maybe 4 minutes, so say we're at 15 minutes here (also been out of the shade, plus the interior's been warmed-up) -
http://pg.photos.yahoo.com/ph/smoothies931pics/detail?.dir=7a86&.dnm=f5a4.jpg&.src=ph

here at 21 minutes minus 4, so call it 17 min. -
http://pg.photos.yahoo.com/ph/smoothies931pics/detail?.dir=7a86&.dnm=a51d.jpg&.src=ph

completely clear at 30 - 4 = 26 minutes, but the knucklehead at McDonald's had forgotten my hash browns, so this pic was delayed a few minutes and the window was actually clear a few minutes earlier -
http://pg.photos.yahoo.com/ph/smoothies931pics/detail?.dir=7a86&.dnm=9eb7.jpg&.src=ph
_________________
"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox

Sleykin · Posted: Mon Nov 15, 2004 4:36 am Post subject:

I think you still have a problem with current path. On mine .. in the time it takes to scrape the windshield and side windows the back is nearly clear and the mirrors are clear.
_________________
Glenn Neff
Medford, OR
87' 924S

Smoothie · Posted: Mon Nov 15, 2004 8:10 am Post subject:

That reminds me I still have to check the mirror heaters..

As far as the hatch - it seems to be defrosting at about the same rate as it did last year. That was when I was using a half unzipped zip-cord connected directly to the tabs on the grid at one end and a cigarette lighter plug at the other - plugged in when I needed it. -So I wonder if there's a problem with the grid itself or the quality of the tab connections. Otherwise, since I already have the paint to repair it, I might try experimenting... and scratch that break in the grid again to take the top half of it out of circuit - like a previous owner had done. -
http://pg.photos.yahoo.com/ph/smoothies931pics/detail?.dir=7a86&.dnm=c693.jpg&.src=ph
_________________
"..it's made in Germany. You know the Germans always make good stuff."

'82 924T, US version, dark green metallic, 5 speed Audi 016G gearbox