Rear defroster not working well

BMW i3 Forum

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Nov 13, 2019

First post for me in the I3 Forum.

I purchased a used 2014 I3 REX a few months ago. I am an experienced EV driver owning 2 Imievs since 2012 and 2015.
I bought the I3 because the design is very similar to the Imiev but everything is up a few notches on the I3.
I specifically bought the I3 REX because I wanted an EV but I also wanted flexibility in fuel source. I want to be able to do a longer trip with or without charging along the way depending on my schedule, availability of quick charger and the wait time for the quick charger.
The I3 give us a lot of flexibility that the Imiev does not have in this regard.

The reason for my post is that I noticed a few weeks ago that the rear defogger seems to not be working.

I first noticed this when it was raining and about 3 Deg C. about 2 weeks ago.
I had a look at the schematic:

To get an idea of how this works. The fuses checked OK and I noted that I could hear a definite click when I would turn on the defroster.
So I proceeded to the rear window and took off all the plastic dressing to look at the voltage directly across the defogger circuit after the rejector circuits.

I found the voltage to be around 14.4 volts so that looked good. Then I looked at the current flowing in the grid and it was .37 amps not too much. So I disconnected the grid on both sides and measured the resistance with a multi meter and found it was 40 ohms.
So 14.4/40 is .37 or close enough so that all makes sense but 14.4*0.37 is only 5 watts which does not make sense.

So I started trying to inspect and make measurement on the grid itself looking for broken lines etc. I could not really find any and the grid itself looks good.

Very puzzling. I was convinced that the grid was the problem.

Fast forward a couple of weeks:

Today was -12 Deg C and On the way to work I turned off the heat and kept the windows closed, as the car started to fog up I turned on the rear defroster and I could see minimal defrosting on all the lines.

Here is a picture:

The entire window does not defrost but it looks like all of the lines are warming up enough to have a small amount of defrosting just where the line is.

So it looks to me like the grid is actually intact but for some reason the grid alone is measuring 40 ohms.

This measurement is done right on the grid connectors excluding the wave rejector boxes.

Does anyone know the value or can they measure the resistance of the rear defrosted grid on thier I3 ?

If it is 40 ohms normally then the only thing I can think is that the BDC that controls the power supply to the rear defroster has to increase the voltage output to about perhaps 50 volts to give a power of about 60 watts which I would consider normal for a rear defroster
If that's not the case then there is some area of high resistance before the grid that I am not aware of.
In any case if someone knows how this works I would appreciate some direction or comments as I am really wondering what is wrong.

I am hesitating to bring the car in because this should be a fairly straight forward thing to trouble shoot but it would seem not.

BTW I did try this in all drive modes comfort eco and eco+ no difference.


Thanks for going to all that work and posting it here, Don. My 2014 Rex rear defroster works just as well as yours, so it makes me wonder if it's a design issue with the car?

Thanks for the reply. I was wondering if would get any. It's good to see I'm not the only one.....(not good that we both have this problem)....

At least there are two of us seeing this issue.

I have not really figured it out yet but I added a small defroster that blows hot air on the rear glass that I had lying around as a work around until I can figure this out.

So for now I'm monitoring the situation as the winter progresses.

Anyone else up north in a similar situation ?

Here in Vancouver , currently I'm getting the same poor results from the rear defroster . At this time of year humidity always seems to be at it's highest levels . That might be a factor .
Thanks for the information.

I think the main reason I posted this is that my experiance on the imiev and other car's I have owned. The Rear defroster just turns on full power from the get go. It actually gets warm to the touch and if you fire it up in the summer it gets very hot. No smarts just power.

It looks like the BMW i3 Rear defroster system is more intelligent either relying on sensors to throttle back the power or something like that. That would not surprise me as the car has a lot of "automatic" systems on it related to the hvac.

Thanks for the data point I hope to hear from others in cold climates on the rear defrosted performance.

I have no complaints about the rear defroster performance in my 2015, other than the button symbols between the front and rear being easily-confused.

I need to add a disclaimer that I have departure times programmed, so the windows are already warm by the time that I climb behind the wheel.....
I have also been wondering about the defrost because as far as I can tell, it doesn't work at all... Will most likely be bringing it in soon to get it checked
My 2014 is the same, you can see marginal clearing right along the lines, suggesting that the system is functional, but judging by the results I’d still say that crap ain’t working. My recollection from some battery education years ago is that rear window defrost typically is several hundred Watts, not 60 & should never be used without the engine running to not deplete your battery. I think BMW might just have the system severely underpowered for range preservation.
Hi Benrk,

Thanks for posting. Maybe you are right they just limit it like crazy but 5 watts ? That's useless.....

I really don't understand this issue. Hopefully one day someone will read this thread and shed some like on this.

Thanks again.

Hello All,

So A little update on this issue.

I had another whack at this over the last few weeks and re-measured the resistance of the window grid. This time is was basically open circuit. THe grid is basically toast. The grid was in bad shape when I looked at this last year and I think I may have finished it off over the summer when I gave the interior of the rear window a cleaning. I used windex and a paper towel. I think I just destroyed the last few grid lines that were actually making contact at all.

So I think I understand this problem its just a worn out grid. When I inspect it closely I can see broken area's and complete lines that are worn down.

The solution I opted for for this winter was to connect a 12 Volt fan driven heater something like this:

I hope this link stays around for a while. I wired it into the existing wiring to the rear defroster so it works from the panel button up front.

Seems to work fine although we have not had any really cold temps this year yet.

I would like to replace the actual grid lines but i think that will take me awhile. I ordered some 3 mm copper tape to try to rebuild it but i think i will practice on an old window first to see if I can make something that looks good. I did try some of that repair liquid but the grid was too far gone and it looked like hell so I just cleaned it all off.

So the long and short of it is the defroster now works.

Also something to note is that since it's a fan you have more awareness of when it's on or off. I noticed that when I initially turn it on it stays on for about 4 minutes then turns off and the button on the dash goes out. Then later on after a little while it will come back on and cycle on and off every few minutes or so but the dash defroster button light stays off. It's actually the BDC Body Domain Controller that controls the power to the defroster and it has it's only little algorithm to cycle the power on that thing. So it can be powered with the button off. I was not aware of that.

Installing a new rear window. It'll have factory fresh defrost tracings.

I'm just thinking that those tracings were probably heavily scrubbed at some point, so it's probably not just a spot correction to regain functionality, but basically retracing the entire path. It may be worth it to part with the cash just to get a fresh start.
Hey eNate,

Yeah I could have brought it into BMW to get it looked at but I was fearful of the bill and I was able to put in an acceptable workaround for winter I'll probably address this some time in the future.

I thought it was worthwhile providing as update considering a few people answered this thread saying they also had poor performance in the rear defroster.

It might be a good idea to NOT clean the interior side of the rear window or to clean it very carefully and not touch these grid lines.