How to test rear windshield defrost/defog? - PeachParts Mercedes-Benz Forum

WDBCB20 · #1 02-18-2020, 01:55 PM

The dreaded w126 rear windshield milky white/ delamination syndrome is manifesting itself on the '85SD passenger side rear corner. Only the upper 25% of the glass heats up - the wires show corrosion where the discoloration is seen.

Located a good seeming replacement at the pick and pull. How to test it before pulling?
Also, will any year fit?

Diesel911 · #2 02-19-2020, 09:35 PM

You can use your volt/ohm meter to see if there is continuity and that could tell you if it has a open circuit/dead (a test light that has a battery would also work) but that won't tell you if all of the grids are function.

I have not been to a junkyard out here for a long time but it used to be you could purchase a warranty on the part you take of. However, the warranty only covered you getting another used part.

Clemson88 · #3 02-20-2020, 09:32 AM

Unscrew the leads, one on each side edge of the glass and test the resistance between them.

jay_bob · #4 02-20-2020, 09:59 AM

Unfortunately all that will tell you is that you have at least one good wire.

The only real way to check is power up and do an IR scan. Unfortunately not everyone has one of those in their back pocket...maybe soon the price on those will come down to where they are commonly affordable.

Only other thing I can think of is that you will get a magnetic field around each conductor, and only the lines with continuity will pass current and thus generate a field about that line. May have to do an experiment with a compass and see if it is capable of detecting the field in each grid line and resolve to tell if a line is carrying or not. Or maybe a Hall effect sensor.

Diesel911 · #5 02-20-2020, 10:09 AM

Quote:

Originally Posted by jay_bob

Unfortunately all that will tell you is that you have at least one good wire.

The only real way to check is power up and do an IR scan. Unfortunately not everyone has one of those in their back pocket...maybe soon the price on those will come down to where they are commonly affordable.

Only other thing I can think of is that you will get a magnetic field around each conductor, and only the lines with continuity will pass current and thus generate a field about that line. May have to do an experiment with a compass and see if it is capable of detecting the field in each grid line and resolve to tell if a line is carrying or not. Or maybe a Hall effect sensor.

That is what I was trying to say.

SD Blue · #6 02-20-2020, 06:24 PM

Hmm... the pocket compass idea has some merit. I'm betting even a small battery, connected to the terminals, might be sufficient to cause a fluctuation of the needle when you scan it from top to bottom. It's worth a shot!

WDBCB20 · #7 02-20-2020, 07:05 PM

Quote:

Originally Posted by jay_bob

Unfortunately all that will tell you is that you have at least one good wire.

The only real way to check is power up and do an IR scan. Unfortunately not everyone has one of those in their back pocket...maybe soon the price on those will come down to where they are commonly affordable.

Only other thing I can think of is that you will get a magnetic field around each conductor, and only the lines with continuity will pass current and thus generate a field about that line. May have to do an experiment with a compass and see if it is capable of detecting the field in each grid line and resolve to tell if a line is carrying or not. Or maybe a Hall effect sensor.

How about icing up the glass with dry ice and seeing if it clears all up and down the window in that way typical of an operating defroster?

funola · #8 02-20-2020, 11:12 PM

Measure the current consumption!

jay_bob · #9 02-21-2020, 07:07 AM

That is plausible, except that you won't be able to detect that you have a few open wires that way.

Same reason you can't reliably flush out a parallel flow condenser or a radiator.

funola · #10 02-21-2020, 09:26 AM

If you measure one with no open wires, then measure another and get the same amperage, it also has no open wires.

barry12345 · #11 02-21-2020, 10:51 AM

Get a can of cold spray. Put a battery or booster cables on. Turn the rear defroster on. Spray a vertical pattern down through all the strips after letting them warm for a minute. You should see if all are heating or not.

Unless breaks are always visable to the eye you have to power it up. A hand held spot temperature reader would also do it. Read the strips from the inside though.

vwnate1 · #12 02-21-2020, 03:17 PM

Go to Harbor Fright and buy the $19 infra red thermometer and take it to the junkyard with some crap zip cord, connect the window to a junk battery and use the IR tool to see if the wires heat up equally .

It's not rocket science .

Someone here should have the FSM and be able to post the proper resistance for meter testing .

Diesel911 · #13 02-21-2020, 07:49 PM

Quote:

Originally Posted by funola

Measure the current consumption!

Yes with an ammeter but you would need a known good rear window to get a reading from and then you would need to take a Car batter with you to the Junk Yard so you could take a reading there.\
The tough word there is Junk Yard.

jay_bob · #14 02-22-2020, 01:31 PM

Ok so I did some real world testing on my W211 this morning while it was in the garage.

I started it and ran it for a few minutes and then got out my trusty HF infrared thermometer.

Turns out it is possible if you hold the scanner directly against the glass, over the line, to see if it is heating up.

Get a baseline first before you energize it (take temperatures over the glass in a few places and mentally average them).

Then give it a few moments to get it to start warming up. You will probably have to work quickly especially if you have a drill battery to work with as it won't last long with the kind of draw the window has.

I was able to discern the glass getting warmer over each individual line. Granted this is a modern window with the painted lines but should be the same principle for the micro wires in the W126.

As proof I went to the top of the glass and shot the first 4 or 5 rows. These are part of the FM radio antenna and do not heat. These were as expected the same temperature as the glass before heating.

vwnate1 · #15 02-23-2020, 11:55 PM

Quote:

Originally Posted by Diesel911

Yes with an ammeter but you would need a known good rear window to get a reading from and then you would need to take a Car batter with you to the Junk Yard so you could take a reading there.\
The tough word there is Junk Yard.

Why "tough" ? you're in Long beach where there are lots of junk yards and one of the larger LKQ Pick-A-Part ones with usually lots of Mercedes W123's & W126's....

They're the cheapest in town too ! I just bought a 6 disc CD changer for $18 + tax .