1997 Saab 900 Rear Defogger Grid Repair Help Needed

[hursst]

Hello,

    I have a '97 Saab 900 on which I'm trying to get everything working.  The electric rear defogger grid works with the top 9 rows, but not with the rest.  I took a test light to each grid to make sure this was the case.  I have also found that the test light on the driver's side of the grid (on the 9 working rows) is very strong, but on the passenger side, it is very weak.  

 

    I found that towards the far side of the 9th row, there is no light at all and any grid lines below that will not light up at all.  My assumption is that there is a break in the grid towards the edge of the 9th row.  

 

   I have not found any obvious breaks in this area, but a few areas that have some minor scratches that do not appear to go thru the grid line, but I don't know for sure.  I tried a repair kit on these areas with no luck.

 

   My question to any electrical pros are:

 

1) Why would the working 9 rows show a very bright test light response on the driver's side, but very weak on the passenger side?

 

2) Given what I've mentioned above, does anyone think the problem is simply a break in the grid, or could this be another problem?  Since I do not see any obvious breaks in the grid, what else might this be?

 

Any tips or help is greatly appreciated!

 

-Chris

[Rusty_OToole]

The heater works by resistance. If the power is applied to the driver's side and grounds on the pass side it is logical you would get a more powerful signal on the driver's side.

 

There must be a break where the power does not get through. Usually it is visible although you may have to look very closely to see it. My guess is the power is applied above the 9th row and there is a break in the vertical main before it even gets to the horizontal lines.

[hursst]

Rusty,  I took a look at the horizontal grid line with a magnifying glass, but I didn't look too hard at the vertical sides, so I will do that next.  Thanks!

[pfloro]

3 hours ago, Rusty_OToole said:

My guess is the power is applied above the 9th row and there is a break in the vertical main before it even gets to the horizontal lines.

Rusty is "on to it".  

 

With power off, make a resistance check on each vertical strip just above & below the 9th grid line.  The "open" side will read infinite ohms. The "good" side should read closer to zero ohms. 

 

From pictures of the backlight I found, the power connections are in the middle of the grid.

 

Power flow is parallel from the positive "feed" side to the "grounded" side.

 

Grid repair kits are available but I don't have any experience in this type of repair.

 

Keep us posted...

 

Paul

Edited March 24, 2023 by pfloro (see edit history)

[hursst]

Thanks.  I'll try these tips as well when I get a little more time.  I've had good experience with repair kits in the past, but that's when I've seen an obvious break in the grid.  

[pfloro]

On 3/23/2023 at 7:03 PM, pfloro said:

 

With power off, make a resistance check on each vertical strip just above & below the 9th grid line.  The "open" side will read infinite ohms.

I just realized that my above  logic was not totally correct.  Correction: The "open" side (vertical area probed)  will indicate higher resistance in the "broken" area as opposed to a proper conducting area.  Since this is a grid, there are multiple current flow "paths"... 

 

Paul

Edited March 25, 2023 by pfloro (see edit history)

[hursst]

I retested and tried some different things and found that there is a vertical strip that runs underneath one of the hatch glass trim pieces.  If I stick the probe far enough under the trim within any vertical area between the 9 lines that are working, the probe will light up.  Not so for grid line #10 and beyond...although I did discover that the very last grid line at the very bottom of the glass is working.  I'm still not fully sure how all of this works, but this testing seems to imply that the vertical connection between grid line #10 and all the rest (except the last one) have been severed.  What I'm not fully sure of is if one part of the vertical connection is broken, wouldn't all subsequent grid lines not work?  This seems to be the case, but the last grid line working is throwing me off.  Tomorrow I'll try to remove the trim and dig around a little.  Unfortunately, there was a glass leak repair done with a bunch of windshield sealant, so this will cover up much of the vertical grid structure and may be difficult to remove.

 

Thanks for all the tips! I think I'm headed in the right direction, at least.

[hursst]

I'm making progress, but haven't quite figured it out.   I tested out the far side vertical connection and noticed there was no vertical connection under the 9th row from the top. So, I took off the trim and discovered a wire connector right at that point (Photo 1). Sure enough, if I disconnected it, then no grid lines work. There is power going to the wire at the connector, so everything seems to be going fine here. I then took off the trim on the far side (passenger side) and found another connector (Photo 2). I unplugged the connector and all grid lines now worked on the rear glass and there was power to the wire that leads into the connector. When I plug the connector back in, the same grid lines (most below #9) go dead again.

My question to the electric pros on the forum is, would the connector on the driver's side (black wire leading back towards the rear of the hatchback) be the ground wire, I'm assuming? Why would the lower grid lines lose power when I disconnect the connector with the black wire? What might be wrong here? Obviously, this should be plugged in. It's also obvious I'm missing something simple, but I'm not much of an electrical guy. Main fuse is good, everything else seems good.

Any help is greatly appreciated, as usual! -Chris

[pfloro]

I'm a bit confused too but here is how I would proceed:

 

1) Disconnect both female connectors from the grid "lugs".

2) Turn on defroster power.

3) Determine which one is hot.  From your findings, I'm wondering if both feed the grid (not logical)...  Measure from the disconnected connector to ground with your test light.

 

To avoid confusion, refer to "the sides" as either "driver" or "passenger".

 

Is there a chance you can get an electrical wiring diagram for your specific year & model...?

 

Paul

 

 

 

 

 

[hursst]

Paul,

   I unplugged both connectors and turned on the defogger.  I am getting power thru the main wiring harness to the connector on the driver's side, but there is no power on the "tab" connected to the driver's side grid, no power to the "tab" on the passenger side grid, and no power to the ground wire.  If I plug in just the driver's side connector, I get full power to ALL grid lines, but (I'm learning this as I go), there is no resistance, as my test light will light up if I touch a grid line anywhere on any line.  My understanding is that I should get a bright light on the driver's side, but it should fade as it goes across the grid to the passenger side.  If I plug in the passenger side connector, at this point, I will get the proper resistance from the 9 upper grid lines, but nothing from the other grid lines (except the very bottom one).

 

   I did find an electrical wiring diagram in my factory shop manuals for this car, but I'm not the best at reading it.  It is attached.  I figured out that the connector on the passenger side is simply a ground wire, but it leads thru a large part of the rear wiring harness and ends up in the trunk.  I found the ground attachment and cleaned it up, but there was no change to the situation.  On the wiring diagram, there is an item called "J150" towards the lower center of the diagram.  It leads to "G3," which is the ground that I found.  I don't know what "J150" would be, I don't know what that symbol is, so I wonder if this has anything to do with the situation.  

 

   It also looks like there is a relay to all of this under inside the lower dash, so I'll have to find that and look at it.  It seems the more I learn, the more complicated this thing gets.  I may have to try to find a professional auto electrician to figure this one out.  Very frustrating.

 

   Thanks for trying to help me out with this.  -Chris

[Oldtech]

J 150 will just be a plug. J for junction. The relay will be for the power so it is working. It seems to boil down to a break in the trace on the window somewhere between the 9th and 10th lines. It seems on the GROUND side (passenger) as you say there is voltage everywhere with the ground disconnected. 

[hursst]

Thanks, I think I am getting down to what the problem is, but now have to find the exact place where it is broken.