Rear defogger has me stumped

Lckent48

Original poster
Member
Dec 4, 2011
43
Rear window defogger not working. I have read a zillion posts and am appearantly to dense to figure it out. I have tested with multimeter & have 12v power at driver's side tab when dash button it turned on with ground to tail gate. I have 12v testing from tab to tab with switch on. I have continuity accros the grid, testing from tab to tab with the power switch off. I have ~12v across the grid testing all lines at several spots each.

Seems like it should be working, what am I missing?
 

coolasice

Member
Oct 27, 2013
1,019
Northern Maine
Rear window defogger not working. I have read a zillion posts and am appearantly to dense to figure it out. I have tested with multimeter & have 12v power at driver's side tab when dash button it turned on with ground to tail gate. I have 12v testing from tab to tab with switch on. I have continuity accros the grid, testing from tab to tab with the power switch off. I have ~12v across the grid testing all lines at several spots each.

Seems like it should be working, what am I missing?
check for continuity to ground to the vehicle body.. could be a broken ground in the hatch-body boot.
 

Mooseman

Moderator
Dec 4, 2011
25,262
Ottawa, ON
The grid itself is likely at fault. These trucks are notorious for wearing through the grid by the weatherstripping. Check out this post:

 

coolasice

Member
Oct 27, 2013
1,019
Northern Maine
Here is the service manual for diagnosing the defrost... look at the component testing section near the bottom
 

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Mooseman

Moderator
Dec 4, 2011
25,262
Ottawa, ON
If they're broken under the bakelite like in my picture, not much you can do there. Shine a light or look through in sunlight and you'll see if they're broken there. I even tried running a new line using that paint. It worked for a bit until the paint peeled off. Just a poor design.
 

Lckent48

Original poster
Member
Dec 4, 2011
43
I have been all over every line on the grid and tab to tab, there is no place I do not get power, if switch turned on, or continuity if the switch is off. Shouldn't it be working?
 

Mooseman

Moderator
Dec 4, 2011
25,262
Ottawa, ON
As you test the grids, with it on, in the middle of each grid line, it should read about 6V. If the voltage or ground stays steady 12V or 0V across the whole or partial grid line, it's broken.

This video explains it well using a test light.

 

Chickenhawk

Member
Dec 6, 2011
779
You really need to slowly examine every grid line with a strong magnifying glass or a phone with a magnifier app.
 

budwich

Member
Jun 16, 2013
2,027
kanata
1.8 ohms. I have continuity and voltage from tab to tab and across all lines.
OK.... so just to ensure that you are going "craxy", you need to find the fuse for the defrost. Pull it, then on the side that IS NOT the power feed... check that with a meter for voltage... then on the opposite side, again measure impedance (to a known ground). This reading should be the same as you just posted.... ensure that the defroster has been activated / pushed.... and it will be measuring the "entire circuit" as the fuse / power "sees" it. Go from the result.
 
Last edited:

Lckent48

Original poster
Member
Dec 4, 2011
43
Testing from ground tab across the grid, from component test instructions, "If the test lamp remains off at the middle of the grid line, test for an open between the test point and the voltage supply side of the grid."

This is the condition I have, in fact, the test light does not come on anywhere on the grid when testing from the ground tab with power on. So this means that no power is getting to the grid if I am interpreting this correctly.

But I do have power at the left tab, 12.7 volts tested at several grounds including the ground tab. So this would mean that there is a break between the power tab and the grid, correct? But if I test from the power tab to the grid, I get continuity and approximately 12v everywhere on the grid, which would mean that there are no breaks between the power tab and the grid, right?

None of this makes any sense to me, fortunately I do not make a living dealing with electrical circuits.
 

coolasice

Member
Oct 27, 2013
1,019
Northern Maine
Testing from ground tab across the grid, from component test instructions, "If the test lamp remains off at the middle of the grid line, test for an open between the test point and the voltage supply side of the grid."

This is the condition I have, in fact, the test light does not come on anywhere on the grid when testing from the ground tab with power on. So this means that no power is getting to the grid if I am interpreting this correctly.

But I do have power at the left tab, 12.7 volts tested at several grounds including the ground tab. So this would mean that there is a break between the power tab and the grid, correct? But if I test from the power tab to the grid, I get continuity and approximately 12v everywhere on the grid, which would mean that there are no breaks between the power tab and the grid, right?

None of this makes any sense to me, fortunately I do not make a living dealing with electrical circuits.
this video explains the testing very well...
 

Mooseman

Moderator
Dec 4, 2011
25,262
Ottawa, ON
As I said earlier, your grids are all likely broken under the bakelite rubbed by the weatherstripping. That's why you're not getting any ground or power in the grid.

147298-a9d69dd0ff6e8bcb1accf0fe25b962ce.jpg
 
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budwich

Member
Jun 16, 2013
2,027
kanata
Testing from ground tab across the grid, from component test instructions, "If the test lamp remains off at the middle of the grid line, test for an open between the test point and the voltage supply side of the grid."

This is the condition I have, in fact, the test light does not come on anywhere on the grid when testing from the ground tab with power on. So this means that no power is getting to the grid if I am interpreting this correctly.

But I do have power at the left tab, 12.7 volts tested at several grounds including the ground tab. So this would mean that there is a break between the power tab and the grid, correct? But if I test from the power tab to the grid, I get continuity and approximately 12v everywhere on the grid, which would mean that there are no breaks between the power tab and the grid, right?

None of this makes any sense to me, fortunately I do not make a living dealing with electrical circuits.
You need to do the impedance testing that I suggested at the fuse box to confirm your "grid readings" otherwise, your "sectioning testing" is somewhat "unjust".

Actually, can you explain what "not working" means. How are you deciding about this? thermal reader or eyes?

Lastly, get / borrow a clamp meter. Monitor the current on the wire (either one).... 10-12A. IF you have a regular "good meter" that can handle 20-30 amps, you could use that instead. Lots are limited to about 10 amp so be careful with your meter.

As suggested by mooseman, you likely have a "crack" which when the power hits, it heats up and breaks the connection shortly there after.
 
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Lckent48

Original poster
Member
Dec 4, 2011
43
I'll get a clamp meter & run further tests. Out of for a few days, I'll check when back. Thanks for all the help.
 

Lckent48

Original poster
Member
Dec 4, 2011
43
Finally figured it out. A very small break between power tab and grid. It would work sometimes & not others. I repaired by taking some 400 grit sandpaper and carefully sanding off some of the bakelite left of the power tab until I got down to the conductive material. Tested & sanded off more until tests were OK bridging the cleared area and tab with copper tape.

After cleaning off about a sq inch of bakelite I cleaned up everything, including the exposed part of the tab. Then using copper tape, I covered the tab and the area cleared of bakelite so I ended up with about a 1x2" area covered with copper tape which completely covered the area cleared of bakelite and the tab. Ran all the tests again & everything checked out. Today it snowed & I got confirmation that it's working properly. I covered the copper tape with electrical tape so the repair is barely noticeable.

I probably could have used that defogger repair paint stuff, but the copper tape is cheaper, $8.50 for 33 feet, and much easier to work with if you don't have to repair the grid lines. We'll see how it holds up, but so far, so good.

copper tape
 

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