A Class rear window just shattered

[222mark]

Hi all, first post so go easy on me, i've just had an issue with my 2017 A200D AMG Line Premium. I was driving home when suddenly there was a loud pop from the rear, i looked in the mirror and the rear window had shattered, i pulled over and immediately noticed the sound of the window making a tinkling sound, i felt the glass and it was hot and you could almost smell the heat coming from it. its currently around 7 degrees C outside and i had the heater at about 23 C inside the car, the heated rear screen was switched off. I can only think that the heated rear screen has a fault and it's heated the glass to the point where it has shattered, has anybody else had this fault or are aware of it happening before?

Luckily the insurance will cover it and have said they'll have it repaired within 24hrs but i need to know what has caused it so it doesn't happen again.

Thanks

 

[Grumpydriver]

Had it happen on my wife’s 1 year old BMW 1. Was parked in a large brick garage at the time which had been locked up for hours with no one anywhere near. Had it replaced within hours. Never happened before or since and never determined why it shattered. The only stress that I could think of that the screen had been subjected to was air pressure due to the garage roller door being vibrated by a fairly strong wind that night. My Merc e class estate which was much closer to the door was not attested.
If I were you, I would get the screen replaced and put it down to an isolated event and get on with life whilst preparing to vacuum the odd piece of screen that rattles its way to a place that you can catch it!

 

[Flyinspanner]

May have been a stone thrown up by a passing car/lorry (describing an arc over your car and landing on your window.)

gravel falling off a lorry etc etc

you‘ll never know why, just be happy it’s covered

 

[222mark]

Its an odd one alright, the only thing that makes me think its an electrical fault was the temperature of the glass, it was hot and you could smell the heat from it, the tinkling sound was like the glass cooling down but i haven't used the rear screen heater for about 3 weeks since the last frost, looking at the car now it looks almost perfect from the outside apart from the bottom left/right corners of the glass have broken off.

 

[zipdip]

Well you have to get the new rear screen fitted,when it is fitted feel the screen,if it is getting hot with the rear screen off,then pull the fuse and get the garage to sort it,my bet is all will be well,but better to look at the manual and know where the fuse is just in case.

 

[222mark]

Well you have to get the new rear screen fitted,when it is fitted feel the screen,if it is getting hot with the rear screen off,then pull the fuse and get the garage to sort it,my bet is all will be well,but better to look at the manual and know where the fuse is just in case.

Cheers, i've found the fuse and relay ready for tomorrow when the new screen is in, fuse 35 and its 40 amp, i'll give that a go.

 

[ToeKnee]

I'd your car a diesel? I ask because some cars auto turn on the rear screen whilst doing a dpf regen.

 

[222mark]

I'd your car a diesel? I ask because some cars auto turn on the rear screen whilst doing a dpf regen.

HI, yes its a diesel.

 

[mioba]

You would need a hell of a lot of heat to break glass.

The smell I guess was from a short in the circuit for the rear heating element.

Sounds like it was under stress.

 

[Jobsworth]

Sticking relay probably

 

[222mark]

I'm hoping the window fitter will have seen something like this before, i'll ask him when he comes to fit the new window. I might change the relay as well just to be sure, its right beside the fuse in the passenger footwell fuse box, i'll report back when he's been, thanks for all the replies.

 

[222mark]

Well no glass fitted today, as suspected they don't carry it in their stocks so its coming from Mercedes, guess it'll be off the road a bit longer now.

 

[222mark]

Quick update, Had the glass fitted about a week ago once it finally arrived in stock, i explained to the windscreen fitter what had happened and how the glass felt hot, he said he's seen it before and not just on Mercedes vehicles, he said its where the heated rear screen terminals fix onto the glass, they become so hot it just blows the entire screen. He had no idea how they become so hot without blowing a fuse or why it would do it without the rear heated window activated. I asked him to leave it unplugged when he'd fitted the new glass, at least i know it wont do it again.

 

[Flyinspanner]

I think you have been way too cautious, you’ll have the aggravation of having to properly clear your rear screen manually for a really tiny risk of reoccurrence!

I bet there are more instances of wheels suffering from cracks than windows suffering from failure due to a HRW fault. (Are you rushing out to buy new wheels?)

(and you have no definitive proof that the failure was due to the HRW, it may have been a stone! The fact that it was warm just meant it had been on….)

connect it back up and stop worrying!

 

[Fastcar155]

Hi 222mark , if you have concerns that the HRW failure was caused by an electrical fault have an auto electrician give the car a quick once over.

I have only experienced a rear screen failure in 50 years of driving and that was a change of underpants experience.

 

[MrGreedy]

The rear screen, as stated, is activated during a DPF regen. It’s unlikely, but possible that without rear screen activation (because it’s unplugged) that your car won’t enter a DPF regen and all the pain that would entail.

Unless someone can confirm 100% this won’t be an issue?

 

[Ted]

Fwiw, a bad connection to the rear screen could cause the local area to be very hot without blowing a fuse.
Just because a small area gets hot, it doesn’t mean an excessive current is being drawn - think headlight bulb which will only be drawing five amps.

 

[BTB 500]

Fwiw, a bad connection to the rear screen could cause the local area to be very hot without blowing a fuse.
Just because a small area gets hot, it doesn’t mean an excessive current is being drawn - think headlight bulb which will only be drawing five amps.

Hmm ... not sure I get that?

Heat is caused by current flowing through electrical resistance. The HRW already has a high resistance, which is why the element gets warm. A bad connection might further increase the resistance of the circuit, in which case even less current would flow. If the connection failed there'd be no circuit, not current, and no heat. Can't see any way you could get a localised short on a HRW?

In general terms I don't think any normal HRW would get hot enough to crack the glass, no matter how long it was left on.

 

[Ted]

a low resistance circuit will generate most heat - not a high resistance - ohms law
So say the circuit resistance is 6 ohms and voltage 12 volts.
2 amps will flow, and the power consumed will be 24 watts.

If the resistance is doubled to 12 ohms then 1 amp will flow and the power consumed will be 12 watts.

I don’t know the exact resistance of a hrw but lets say it is about 2 ohms. (That’s just a guess)
Thats 6 amps flowing in the whole circuit assuming no losses - 72 watts across the whole screen area so it gets nice and warm.
Now one of the connectors gets a dodgy connection. It’s not THAT bad - say 1 ohm.
You now have a circuit that is passing 4 amps or 48 watts in total.
So now you have 4 amps flowing through the 2 ohm screen with 8 volts across it - 32 watts across the whole screen and 4 amps through the 1 ohm dodgy connection with 4 volts across it - 16 watts on a very tiny area (the connection)

It’s a common misconception that the higher the resistance the more power (and therefore heat) is produced when (for the same voltage) the lower the resistance the greater heat is produced.

Hope that makes some sort of sense - I ‘ve made up the values so it is easier to work out.

Edit: just to add for clarity, if the connector is perfect and has zero resistance there will therefore be no voltage drop across it and zero power loss

 

[Ted]

Just to add, while sort of on the subject…
To test hrw elements where they are printed on the glass, connect one side of a testlight to a good known ground.
Now draw the probe of the testlight (you can use a voltmeter for this) down the middle of the screen across each element in turn.
The testlight should be around half brightness (or 6 volts) as it connects to each element. Full brightness or 12 volts means a break between that point and the earth side. No light means a break between the suppy connector and that point.
You can then use the half split method to locate the exact point in any element where the break is.

I’m a mine of useless information