Author Topic: Rear Window defogger Rocky F75  (Read 464 times)

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Offline Rusty

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Rear Window defogger Rocky F75
« on: April 28, 2019, 03:30:42 AM »
My rear window defogger hasn’t worked since I got the car about a year ago.  The switch tests ok, there is 11.8v on the positive side of the window grid and earth side of the grid tests ok to ground too.  The grid appears in perfect condition and the solder lugs have continuity to the vertical bars of the grid on each side.  The grid seems to be the type where you can have some broken wires, and any good wires should still work. When I check resistance from one side to the other, there is an open circuit (full resistance) on all horizontal grid wires.  Going along the wires, the resistance is zero to about half way (testing from each side).  So the meter is telling me there are breaks in every wire about halfway across...  If there are breaks, they are all invisible even with a magnifying glass, so I’m reluctant to scratch what looks like a good wire away to repair it.
When I check the manual, the circuit is really simple, no relay and no timer which would be standard on most vehicles.
I’m hoping someone else has had a similar problem and found an answer. Im sure there must be a relay somewhere and would like to check that before I start any scratching...

Offline Gonzo_Descente

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Re: Rear Window defogger Rocky F75
« Reply #1 on: April 28, 2019, 06:22:19 AM »
It appears to me from your description that the whole grid is defective.  If full voltage is measured across the grid, and the continuity is open circuit as measured, it has to be bad.

Offline Rusty

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Re: Rear Window defogger Rocky F75
« Reply #2 on: April 29, 2019, 06:12:03 AM »
Thanks Gonzo, I’m sure the meter is telling the truth but if there are any breaks, I certainly can’t see them.  i did measure the voltage at the negative end of the grid (multimeter in series between negative grid terminal and battery negative terminal) and it appears to be about 1 volt coming out there with the engine running. I’ve been thinking that even though there are almost 12volts going in, that maybe the power supply is not sufficient to run it. I’m tempted to connect a battery directly to the grid and see what happens, hopefully I don’t fry it lol.
Probably the only other option to get it working is finding a new rear window with a grid built it, not so easy from what I’ve seen so far.

Offline Gonzo_Descente

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Re: Rear Window defogger Rocky F75
« Reply #3 on: April 29, 2019, 07:30:20 AM »
Ok, now I'm a little confused.  If there is 1 volt at the negative terminal side of the grid, that indicates there is some current flow.  Seeing that says there is a small resistance between the grid negative and the battery negative.

I read many moons ago that someone glued a piece of aluminum foil over a break in one of the grid lines and restored one grid line, but I am not sure that works, considering the grid lines should have some insulating cover.

Can you put your meter on a current scale between the negative side of the grid and the wire from the door?  What current is read?  Start with high scale on the meter, then go to lower scales until you get a good read.

I'm sure if you connected directly to the grid with the battery it won't hurt anything.  However, let me suggest connecting only one side to the battery at a time, as a test whether your switch may be bad or some resistance such as corrosion is present somewhere in the circuit.

I'm sue I saw a window for sale on Craig's list some months ago, but it probably was expen$ive.
« Last Edit: April 29, 2019, 07:48:48 AM by Gonzo_Descente »

Offline Gonzo_Descente

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Re: Rear Window defogger Rocky F75
« Reply #4 on: April 30, 2019, 07:09:29 PM »
Yes, there is a window on Craigslist for a mere $450.00.

Offline Rusty

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Re: Rear Window defogger Rocky F75
« Reply #5 on: May 01, 2019, 04:03:15 AM »
Thanks Gonzo, I’ll give that a try over the weekend probably. I’m heading away but might get time on Sunday.  That sounds very expensive for the rear window, but I guess they are like hens teeth haha

Offline Gonzo_Descente

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Re: Rear Window defogger Rocky F75
« Reply #6 on: May 01, 2019, 09:30:46 AM »
This morning I used my handy-dandy multimeter and measured the resistance of the heating element.  I seldom use the rear defroster, but I seem to remember that 2 elements didn't work.

So the calculation is simple:  I measured 2.2 ohms.  There are 9 elements, and assuming 2 are indeed bad, I made the following calculations:

9 good = 1.71 ohms
8 good = 1.925 ohms
7 good = 2.2 ohms
6 good = 2.56 ohms
5 good = 3.08 ohms
4 good = 3.85 ohms
3 good = 5.13 ohms
2 good = 7.7 ohms
1 good = 15.4 ohms.

From this you can evaluate your window and any window you could purchase.

Offline Rusty

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Re: Rear Window defogger Rocky F75
« Reply #7 on: May 07, 2019, 02:11:06 PM »
Thanjs Gonzo, great info. I tried measuring the current and got zero, but after checking the fuse in my multi meter I found it was blown. I’ll probably buy a better meter...It does seem strange to me that none of the wires heat and theres no sign of any damage.
I also tried direct connection to another battery and no luck, but still has just under one volt at the negative end. I guess that rules out relay or timer problems.
The main reason I want to get it working is so I can get the windows tinted.

Offline Rusty

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Re: Rear Window defogger Rocky F75
« Reply #8 on: May 30, 2019, 07:16:00 PM »
ok I found a window for AU$85, very cheap so Ill use Gonzos resistance figures to check it out