Rear heated window not working, found breaks in elements...

[celoader]

Maybe this is one of many threads about this, but I can't seem to find the answer I need...

As the cold weather has really started to set in, it's time to start using the rear heated window. Now the switch on the dashboard lights up, the fuse is fine and the heated wing mirrors de-mist, but the heated rear window won't demist at all none of the elements work).

A couple of weeks ago I bought some of this granville conductive paint/window demister repair kits as I found a couple of breaks in the lines/elements. I applied the paint into the gaps, left it to dry but it hasn't helped. Today I found another 3 breaks, right at the ends of some of the lines/elements, all on the drivers side of the window.

So I thought I'd better try a bit of fault testing with a multi meter... In resistance mode, I tried touching the probes on the two small areas, that I repaired with the conductive paint (bear in mind that these two points are on opposite ends of two different elements), and I got a resistance of about 0.9 ohms (I'm no electrician, but I think that's pretty good?). I then unplugged the two metal lugs from the rear window from the sockets on the tailgate, tested the resistance across these 2 and got a reading of about 80 ohms. I also tried probing the parts where I found the 3 new breaks (2 at time of course in different combos), and the resistance was again about 0.9 ohms...

So, is it likely that these 3 breaks can cause a high resistance across the whole grid? I thought if an element was broken, just that element wouldn't work? I'm just curious if any one knows if having a few elements broken means the whole rear demister won't work?

Sorry about the long post, but any help would be most appreciated!

 

[RUM4MO]

I've not examined the window heater, but I would think that there are two "bus bars", probably one at each side, one should be "+12V" and the other "0V" - so yes, as you are thinking, only the broken side to side strips should be "missing". Maybe you will need to look at all of the rear heater with a magnifying glass to prove that there are no more damaged areas (good luck).

One thing that you should do is, check "across" the two power connections to make sure that you are getting the 12 volts supply and return, next connect them back up to the heater and check again on the heater close to these connections. If the supply and return are okay with it disconnected but not good when its connected, then there is a high resistance somewhere on the supply or return side - supply lead could be partially broken round about the hinge area with flexing when the hatch is opened, or the return - ie earth connection could need cleaning up.

 

[Russ 1.8t]

i have breaks in two of the elements, top and bottom of the window but havent got round two fixing them yet as the rest of the elements do still work!!

 

[celoader]

I've not examined the window heater, but I would think that there are two "bus bars", probably one at each side, one should be "+12V" and the other "0V" - so yes, as you are thinking, only the broken side to side strips should be "missing". Maybe you will need to look at all of the rear heater with a magnifying glass to prove that there are no more damaged areas (good luck).

One thing that you should do is, check "across" the two power connections to make sure that you are getting the 12 volts supply and return, next connect them back up to the heater and check again on the heater close to these connections. If the supply and return are okay with it disconnected but not good when its connected, then there is a high resistance somewhere on the supply or return side - supply lead could be partially broken round about the hinge area with flexing when the hatch is opened, or the return - ie earth connection could need cleaning up.

OK super, must try a voltage test when I can next borrow a multimeter

So if I have high resistance across the whole grid as mentioned (with the plugs disconnected from the supply and return), is this probably a good sign that there is many breaks in the grid i.e enough for the whole grid not to work?

Or as you mentioned about a break in say the supply wire, or earth wire (where ever that is!), is this the more likely reason why the whole demister has stopped working?

Cheers for your help!!!

 

[RUM4MO]

OK super, must try a voltage test when I can next borrow a multimeter

So if I have high resistance across the whole grid as mentioned (with the plugs disconnected from the supply and return), is this probably a good sign that there is many breaks in the grid i.e enough for the whole grid not to work?

Or as you mentioned about a break in say the supply wire, or earth wire (where ever that is!), is this the more likely reason why the whole demister has stopped working?

Cheers for your help!!!

I don't know what the resistance of a "good" heater is meant to be, the return wire will probably only be a short heavy lead back down to the metal frame of the rear hatch, which relies on good contact through the hinges to the metal of the car body - though there might be a dedicated earth (return) wire. I've had broken wires in a 1990 Ford Fiesta but, so far have not heard of this happening to VAG cars with rear hatches.

 

[krussel]

Can I just say good luck (and good on you) for trying to repair the heating elements.

I ve had 2 fail on different cars and tried to repair both without success. Although the repair paint makes a connection when the current flow the repaired section tends to heat up the most and steal all the amps from the grid. I guess the electricity follows the least path of resistance.

Anyway on one car I put up with a couple of bars not working and on the other a hit it with a hammer and got Autoglass to put a shiny new window in. Cost £40 excess at the time. Guess that’s more like £70 in the UK now.

 

[celoader]

Can I just say good luck (and good on you) for trying to repair the heating elements.

I ve had 2 fail on different cars and tried to repair both without success. Although the repair paint makes a connection when the current flow the repaired section tends to heat up the most and steal all the amps from the grid. I guess the electricity follows the least path of resistance.

Anyway on one car I put up with a couple of bars not working and on the other a hit it with a hammer and got Autoglass to put a shiny new window in. Cost £40 excess at the time. Guess that’s more like £70 in the UK now.

Yeah I think you are kinda right after doing some proper 'fault finding' tonight, as I have a feeling a may need a new window. So here's a little the electrical nerds, who can confirm if my heated rear screen is probably dead

Hope you like the awful 'paint' drawing of my rear window! I borrowed a decent multimeter from someone at work, and I had spent a good 15 mins or so checking, and double checking a variety of different points on the window for resistance/voltage (I'm no electrician, so thats why a need a definite answer on this!). Lug 1 and lug 2 is the small wire straps, which plug into the sockets in the tailgate for the heated window wiring loom.

With the window still plugged in to the wiring loom, and car/heated window not turned on, resistance across lug 1 and lug 2 was 250 kΩ.

With the window unplugged from wiring loom, (so obviously car/heated window not turned on), resistance across lug 1 and lug 2 was 580 kΩ.

With the window still plugged in to the wiring loom, and car/heated window turned on, voltage across lug 1 and lug 2 was 14.1 V. With car/heated window turned off, voltage across lug 1 and lug 2 was 0.15 V.

I did a few tests at the points marked numbers 3 - 7. 3 & 4 are two breaks I found in the elements, which I have tried to repair with conductive paint (so as can be seen, these are on different elements to each other). 5 - 7 were new breaks I found yesterday. Trying various combinations across the lugs and the points marked 3 - 7, the lowest resistance I got was 225 kΩ. Some were as high as 1 MΩ

So, does the above make it sound like the rear window is to blame? Is the anything else I can test, or have tested wrong?

Cheers!

 

[krussel]

I think you have done all the testing you can.

One small break in the grid is borderline to repair with prob 50/50 chance of success without drawing to many amps from the circuit. Two breaks is even tougher to fix but if your looking at four i doubt if you can do it at home.

I amnot sure if there are any mobile firms which could help you but i doubt they could charge very much as you can simply have a new screen fitted and pay the excess.

I would check your insurance and see how much your glass cover will cost you in excess. It should not effect your no claims bonus. You could simply say that it was frosty and you turned on your rear screen and POP it blew. They WILL believe you as its a common Ibiza fault and at one time Autoglass run out of rear screens here in the UK.

 

[icezar]

When you have a circuit with several parallel lines and one of those lines fails, it's intended tension/amplitude is equally distributed to the remaining lines.

Now, if you have a circuit with 6 elements and 3 don't work, the load will basically be 2 times higher that it's suppose to be on the remaining 3.

Depending on the design of the defogger (meaning what is the maximum load a line/element can carry) you might have a total failure sooner or later. Or, in the best case scenario, the remaining elements will keep on working forever.

What I'm trying to say is that you may succeed in fixing the circuit but if you don't restore the original parameters (meaning the resistance is the same across all elements) the work might be in vain.

I would try to find out the exact resistance of the lines/elements of the heater and try to fill the cracks with a similar resistance metal. Conductive paint ain't gonna do it.

 

[RUM4MO]

I think that I repaired a rear window heater once, using conductive "paint", though I have heard some bad stories.

If there is only 1 or 2 or 3 cuts in a "cross piece", then repairing these breaks with a conductive "paint" that will provide a low resistance "link" between the cuts, then the overall resistance of that "cross piece" will still be almost the same as the other "cross pieces" - so job done.