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Sun Jan 23, 2011 7:28 pm

Hello, I'm trying to debug the rear window defroster on my '69. Since right now it's putting out 1.5v, I'm guessing it needs to be rewired.

So could anyone check the voltage on their working defroster for me and post it up here?

I'm looking at running voltage from the battery to a relay to the rear window, using the stock switch as the on/off switch for it. So basically just rewiring the stock one. I just wanna know if I can run battery voltage through it, or if I should get a voltage regulator and stick that in line with it.

Might be cool to add a variable one or something with a knob on the voltage regulator, so you don't have to have it a full power all the time, just say half or quarter power when cruising.

Sun Jan 23, 2011 8:28 pm

Are you referring to the wires that attach to the grid pattern on the rear window? Chances are that the grid itself has deteriorated to the point of no repair. The grid itself is nothing more than pattern of high resistive wires bonded onto the rear window. Over time those wires crack or get more resistive to the point of not working anymore. There are kits to repair some of the breaks but I'm guess that your grid is well beyond repair. If I recall, the Bentley manual had a procedure to measure the resistance of each leg of the grid as well as a repair procedure.

You may want to refer to the appropriate wiring diagram before making any modifications to your vehicle. Power to the rear defogger is already controlled by a relay. Here's a link to the wiring diagram http://www.thesamba.com/vw/archives/info/wiring/bug_68_69_usa.jpg

Fuse 1 in the main fuse panel provides power to the control signal to the relay. There is a separate iinline fuse (S2) for the actual power to the grid.
If the defrost light on your speedometer comes on the Fuse 1 is OK. Check the inline fuse (S2). That's a good place to check voltage.

Rather than a variable voltage it would be better to have the the system to come on to full power for a specific time period. That's how most modern cars work nowadays.

Sun Jan 23, 2011 9:17 pm

I'm referring to the wires that lead up to the defroster grid on the rear window. I'll have to go take a close look at the grid, but it appeared to be in decent order for being 41 years old. The light on the speedometer lights up green (thought it was the generator light at first, that's how I found this forum, then found out it was the defroster, and found the switch).

I'm just really hoping I don't have to run new terminals up to the window through the headliner. Can't replace that right now.

Judging by the schematic, that's also just a plain SPST relay with current directly from the main relay (C1?), so it would be safe to just leave the current system intact, and duplicate it and run that (To get it working/see if it works properly, then repair the original).

Thanks for posting that link by the way. I don't think I've seen that one yet, should come in handy.

Sun Jan 23, 2011 9:28 pm

First thing I would do is check the inline fuse (S2), then check pin "B" of the relay. Should read 12VDC when the switch is turned on (ignition on as well). You may also hear the clicking of the relay when you turn the switch on and off.

If you have 12VDC at pin B, then check voltage at the grid (White Wire). Also check and make sure the ground wire is good to ground (brown wire)

Depending where it is, even a microscopic crack in one of the traces would effect one or more of the traces.

Sun Jan 23, 2011 10:00 pm

Tested pins B and M on the relay. B got a varied 3v -7v, and M got 12v so my money is on a crappy connection / general corrosion. All main fuses are good, and I can try and find that secondary fuse sometime when it's not raining rather hard outside.

Measured resistance on the rear window grid, got 12MOhms. I'm probably just going to try and pull the connectors on everything that I can find and clean them with a wire brush.

Probably do that tomorrow when I don't have to hold a flashlight and two multimeter leads.

Sun Jan 23, 2011 10:36 pm

gtkid2002 wrote: Tested pins B and M on the relay. B got a varied 3v -7v, and M got 12v so my money is on a crappy connection / general corrosion. All main fuses are good, and I can try and find that secondary fuse sometime when it's not raining rather hard outside.

Measured resistance on the rear window grid, got 12MOhms. I'm probably just going to try and pull the connectors on everything that I can find and clean them with a wire brush.

Probably do that tomorrow when I don't have to hold a flashlight and two multimeter leads.
While you are at it, make sure the relay is properly mounted. According to the wiring diagram it gets it's coil ground through the relay case.

The fact that you have 12VDC on M would indicate that the fuse (S2) is OK. Getting less than 12VDC voltage out of pin B would indicate that the relay is bad. Most likely the Normally Open (NO) contacts inside the relay have corroded. I'd replace the relay.

You could try cleaning the outside contacts of the relay. Use the eraser on the end of a regular pencil. It's abrasive enough without creating metal filings. Never use a file or sandpaper on metal contacts

I don't remember offhand what the resistance per leg should be. Maybe someone with a Bentley manual could pipe in.

Once you have 12VDC to the grid you can try this procedure to track any breaks. http://www.popularmechanics.com/cars/how-to/maintenance/4221212

Mon Jan 24, 2011 6:56 pm

I pulled the relay and found that the ground tab was just totally rusted. Not much chance of me cleaning it off, and the copper contacts on the inside had begun to get all green and corroded as well. So I soldered a ground eye crimp to one of the coil leads on a new SPST, which I will mount in the same place as the old one, and plug the contacts into that. Gotta use a different mounting screw and clean the ground point up a bit before I mount it.

I'll check the voltages at the driver side mounting tab after I get the relay in and post them up and see if it's better or not.

Mon Jan 24, 2011 7:20 pm

Mon Jan 24, 2011 8:45 pm

I didn't catch this when I pulled the old fuse for some reason, but the secondary in line fuse holder had been broken. So it was getting 1.5v at the window because of the corrosion. Either way I'm glad I replaced the relay. Cut out the broken fuse holder and installed a 20amp blade type fuse in it, got 12v at the window on the driver side! :D

Couldn't test the other side because I can't step on the floor pan because it's about ready to drop out. I should be replacing the floor pans soon, and I will hopefully give it a road test tonight and see how it performs.

Thanks for the tips, it was greatly appreciated! :wink:

Tue Jan 25, 2011 12:41 am

gtkid2002 wrote: Measured resistance on the rear window grid, got 12MOhms.
Everything you do from now on is a complete waste of time. Your grid is shot and there's nothing you can do to fix it. Replace the window with a new one. $$$.

Tue Jan 25, 2011 1:02 am

The grid is supposed have some resistance, I just don't remember how much. 12 Meg does sound pretty high. I remember some old procedure where the actual parameters were given but that was 30 years ago. Might be in a Bentley Manual.

The grid can be replaced without replacing window. At one time you used to be able to run down to your local FLAPs and pick up a generic one. I haven't seen kits available in may years.

Here's a source where you can get a grid replacement kit. May not look OEM, but it would make it functional. http://www.frostfighter.com/
Looks like kits are available for less than $50

Tue Jan 25, 2011 4:44 am

Bruce wrote: gtkid2002 wrote: Measured resistance on the rear window grid, got 12MOhms.
Everything you do from now on is a complete waste of time. Your grid is shot and there's nothing you can do to fix it. Replace the window with a new one. $$$.

What should the resistance be then? I'd honestly expect it to be high. With more resistance generally comes more heat, unless it's just overkill. Then I think it starts going the other way after a point.

Tue Jan 25, 2011 5:24 am

gtkid2002 wrote: Bruce wrote: gtkid2002 wrote: Measured resistance on the rear window grid, got 12MOhms.
Everything you do from now on is a complete waste of time. Your grid is shot and there's nothing you can do to fix it. Replace the window with a new one. $$$.

What should the resistance be then? I'd honestly expect it to be high. With more resistance generally comes more heat, unless it's just overkill. Then I think it starts going the other way after a point.

The equations are those: U= R*I and P = U*I, where P = effect or power, U = voltage, I =current, R = resistance.

The power P = U*I, but you dont know what the current I is, so you use the first Equation to find it and get I = U/R. Put that into P = UI and get P = U*U/R. Put in the numbers. P = 6*6/ 12 000 000 Ohms = 0.000 003 Watts. Maybe enough to melt one snowflake an hour.

As said before, the grid is shot.

Tue Jan 25, 2011 8:27 am

Yes, but everything you said is theoretical. The resistance of the grid changes when voltage and current are applied to it. Typical resistance in a GM seat heater element at rest is only 1 to 3 ohms, but they make your ass hot. It would be better to compare at rest resistance with that of a grid that is known to work. :wink: Another way to test it is to hook it all up, then use a test light GENTLY to touch each line in the grid. The test light should be very dim at one side of each line, and a bit brighter at the other side. If it is off completely at one side and very bright at the other, that line has an open in it. Do this test to each line to determine how many of the lines work. If you only have one or two lines with an open, there are kits you can buy to repair the line once you determine where the open is.

Tue Jan 25, 2011 3:29 pm

Gah. Cures you Ohm's law! :evil: Although yeah, resistance may not mean squat once you apply current to it. Kinda like a lightbulb. It changes, and I think it goes up I think.

Does anybody have a working rear defroster that they could measure and post the resistances up?

I'm replacing the rear window seal, so I have the window out right now, so I can actually check the resistances a little better than I could with it in the car, stretching over the back seat and trying not to put pressure on the floor pans that would probably give if I stood on them.

Tue Jan 25, 2011 6:22 pm

Mine doesn't have a rear defroster, but are you sure you measured 12M ohms? Did you touch the leads of the meter together first to make sure the meter is functioning properly?

Secondly, since you have the rear glass out, you could bench test it with a test light like I mentioned earlier; Just wire one terminal to the positive post of a charged battery, then wire the other terminal to negative. Hopefully you can at least get a spark. Then use a test light clamped to the negative post of the battery to check the lines in the grid. Hopefully also, someone with a functioning rear defrost can take a reading for you. By the way, if you decide to bench test your grid, use some decent size wire, like 12 or even 10 guage.

Wed Jan 26, 2011 12:35 am

Paul Windisch wrote:
Secondly, since you have the rear glass out, you could bench test it with a test light like I mentioned earlier; Just wire one terminal to the positive post of a charged battery, then wire the other terminal to negative. .
Waste of time. His window is SHOT!

It's been a while, but I seem to recall the resistance of a good glass grid is around 4 ohms. FOUR OHMS. Not twelve MILLION ohms.

Wed Jan 26, 2011 12:54 am

Turns out that old relay was worse off than I though. I actually pulled the window out, and measured it. The traces are there, but the metal sure isn't. Which means the 12M ohms was from the relay tripping out. I don't know if a trace kit could fix it, but I'm betting not, unless I bought a big trace kit. Thanks for all the help anyhow.

Wed Jan 26, 2011 1:23 am

gtkid2002 wrote: Gah. Cures you Ohm's law! :evil: Although yeah, resistance may not mean squat once you apply current to it. Kinda like a lightbulb. It changes, and I think it goes up I think.

Does anybody have a working rear defroster that they could measure and post the resistances up?

I'm replacing the rear window seal, so I have the window out right now, so I can actually check the resistances a little better than I could with it in the car, stretching over the back seat and trying not to put pressure on the floor pans that would probably give if I stood on them.

The point of using Ohms law was just to show that 12 MOhms is pretty much a broken connection. 4 - 10 Ohms sound more reasonable. 4 Ohms mean you get about 10 watts out of it, which sounds about right. In a lightbulb the resistance changes because the filament is almost melting, but for the rear screen strings I actually think old Ohm is about right. Anyway, to measure the resistance you need to disconnect the spade connector at one side so you don't include any other components. But, we are looking at very low Ohm numbers here. Low ohms makes more heat, not the opposite way around.

If you want to get a correct reading when voltage is applied, you need to put the Amp-meter in series with the rear screen grid, apply 6 volts to the screen and Amp-meter, read the current and do the math.

Thu Jan 27, 2011 12:36 am

allsidius wrote:
The point of using Ohms law was just to show that 12 MOhms is pretty much a broken connection. 4 - 10 Ohms sound more reasonable. 4 Ohms mean you get about 10 watts out of it,

.... apply 6 volts to the screen and ....
Did they sell heated rear window in 6V cars over there?

On a 12V car, if the resistance is 4 ohms, there will be 36 watts of heat put on the window.