MatraSport Forum

Now that autumn is upon us, I'm trying to get my rear screen heater to work.  I can measure 12 volts between the two connection tags and continuity between them, but nothing much seems to happen.  Am I expecting too much from those thin little elements on such a large thick screen?  Does anyone know what the current consumption should be, or the DC resistance?

If you open the engine cover a little, you allow engine heat to enter the luggage compartment. This will demist your rear window for free!

Just put the engine cover on top of the clamps. As easy as that.

Just put on a ferrari tail.
No window, no demist needed.

Thanks, guys, all really useful stuff  ;)  Seriously, do I take it then that the heated screen is something of a lost cause?

yep.
Because the thrunk gets very hot, the slightest humidity in the back make's youre windows misty.
Control for leaks at the backlightrubbers if it is wet in there.

Hi
On my Murena the electric heater is removing the mist from rearscreem with out any problems excep for on "wire" that has been damaged. So it is not a lost case.
Try to messure the resistance through the wires on the windows. If all "wires" have been damaged there is not much you can do.

Hans

First there must be battery/charging voltage on the right hand connection and the left hand connection must be a good connection to earth.  The volt drop across the lines on the window should mean that if you check the voltage between the left hand terminal and earth it should be zero.  If you have any voltage on the left connection there is a fault.

Second, if you have the ignition and heated rear window switched on, then breathe on the area around the lines, you should see the mist clear quicker around the lines, showing that they are passing current.  Any line that does not give that effect, is probably broken.  Sometimes you can repair these breaks with a kit that 'paints' a new connection across the break.

Third, if you are driving along on a cold morning with just a light misting inside, you should see it starting to clear but it tends to be slower than a normal car with more upright rear screen for one simple reason.  On any car where the angle of the rear screen is 23 degrees from horizontal (or less) the airflow over the car stays 'attached' to the glass.  Most hatches and saloons have a sharper angle and so the airflow breaks away at the rear of the roof and the glass has a 'pocket' of less cold air that is not moving along the glass creating a cooling effect, which the Murena has.  This cold air flowing down the glass on the Murena makes it harder for the heating elements to heat the glass and thus slows down the effect.  If the car has been stood for some time and there is a great deal of moisture in the rear compartment, then it is best to at least wipe the glass off first otherwise the slowness means it will take a long, long time to dry it off and clear.  Regular use means the heat from the engine compartment will keep the rear compartment much dryer (unless you have a water leak into it!) and easier for the H.R.W. to have some effect.

You can tell the airflow stays attached on a Murena (without rear spoiler) since if it is wet the airflow dries the glass off.  On the Murena with a factory rear spoiler (Prep 142 or 'S') the airflow travels from the rear of the roof to the top of the rear spoiler, and does not flow over the glass (so it stays wet in rain) and has less cooling effect to counteract the H.R.W.

Finally, the H.R.W. relay is only a 20 Amp one if I remember correctly.  Replace it with a 30 Amp relay to pass more current, but make sure all the connections can handle it.  The relay takes it's feed from the front fuse board via connection CC2 and it has a habit of corroding badly and breaking there.  The most obvious way it should have been wired is from the back of the alternator which is much, much closer, so will have less chance of any voltage drop.  If you do change it, however, make sure you put a fuse with the 30 Amp relay.

Roy

One funny thing I noticed on my car was that whenever the rear screen heater was engaged, my fuel gauge showed 1/4 less!!  ;D

Done that, got the volts but it fails the breath test.  I think every element must be broken.  I've done this in the past with some success, but the Murena elements are so thin I have difficulty in seeing the breaks.  That or old age is taking its toll on my eyesight  :(
Sorry, I must take issue with you there Roy.  If you are using a relay of the correct current rating then the voltage drop will be negligible.  If it is not, then something must be getting very hot at that sort of current (a drop of one volt at 20 amps equals 20 watts of heat  appearing somewhere - that's more than my soldering iron!)  Changing to a 30 amp relay will not pass more current because the resistance of the switching path of any relay will be as near zero as is practicable, although it may improve reliability because relay current ratings take into account the current that can be interrupted without the resultant arc destroying the contacts. 
The screen supply already comes from the back of the alternator on my car, I had assumed it was designed like that, although the absence of a fuse seemed odd.  A quick check of your wiring diagram shows that it is a modification though.  I need to add a fuse.

I think the aluminum strips corrode over time - unless they were made very thin and with rough edges? I don't think so. Last time I measured the current flowing through mine, I noticed that it was quite small - only 2 A or so. That's with good connections everywhere. This is just too little to produce much demisting effect as it displaces less than 30W of heat to the screen. I can hardly believe that it was designed to be this weak (if so, why use a relay at all?), so I think corrosion has eaten the strips and thinned them so much, that they are nearly useless by now.

I took this photo last year, showing the roughness of a single strip.

Bart, thanks for your suggestion about the engine cover - it's simple and useful, although it only demists the lowest part of the window.

By the way, Roys point about keeping the trunk dry is very valid - remember that a slight amount of water there will turn to mist as soon as the engine starts warming up. This can make an otherwise clear rear window completely opaque ;)

Well... just discussed this with the old man (my father).

He mentioned something about aftermarket kits that was sold more than 30 years ago  ???

Guess what, they are still around, do a search for PRSH01  ;)

https://www.europaspares.com/HEATERS_and_DEMISTERS/HEATERS/ELECTRIC_REAR_SCREEN_HEATER___PRSH01__2152.html

Not that expensive either... £25 incl. UK delivery  ;)
Could someone please buy and test one, and let the rest of us know if their are any good  :P

Waldo wrote
Could someone please buy and test one, and let the rest of us know if their are any good  :P

Hi
I am old enough (I guess that I am older than "your old man") to remember these stick on rear screen heaters.
I have used one on a Rover SD1 because the original rear screen heater had stopper working. The Rower SD1 is Rovers very large hatch back.
But to be honest the stick on heater did not do much good.

I remember that a few years ago (in my age that might be 10 to 20 years) there was a small Danish workshop, which cut grooves in the rear screen and glued in proper heater wire in the groove.  That solution worked and the solution was long lasting but expensive.

Hans

Impressive piece of macro-photography Anders!  Waldo, I remember the stick-on elements too, they never seemed very successful as I recall.  I think it's back to the multimeter and magnifying glass to try and find all the breaks.

Sorry I did not phrase that well.  What I meant was that many relays are not high on quality and the points not good when they get near their rated capacity, so if you use one that is rated for more, it trends to handle the actual (lower) current better, giving 'more' current.

Roy

Well, I've been struggling to repair my screen over the last three weeks and after about 30 repairs, I hadn't got a single element connected.  It seems that as fast as I mended them, they burnt out somewhere else.I decided to try one of these, so ordered it on ebay.  The kit came within a couple of days.  It consists of 9 adhesive conductive strips (the elements) in strips of three, two double-sided adhesive strips , two metal connection bars and a switch, cable and connectors.  To install it, you first fit the two double sided strips vertically at the side of the screen.  I fitted mine over the existing verticals so they wouldn't show.  Next the elements are stuck to the screen.  I chose to cut mine up into individual lengths in order to stick them over the existing defunct elements, again so they wouldn't show too much.  They are wider than the originals, but not too much, so the effect doesn't look too after-market.  Finally, trim the element ends off and stick the metal connection bars to the verticals, so that they cover the element ends.  Connect the terminals to the existing screen wiring, you don't need the switch and cable supplied.
How does it work?  I tested it this morning on one of the coldest days of the winter so far.  It cleared mist off the inside in about 4 minutes and off the outside in about 6 and it stayed clear.  A bit sluggish, but a lot better than nothing.  Didn't try it with ice, my Murena lives in the garage.  Because the original screen has 13 elements and the kit has 9, I fitted the new elements to the middle nine, which gives a slightly narrow view out of the back.  Current consumption is about 5 amps with the engine running.
My verdict - looks neat, easy to fit but the performance is only just adequate.  Still a good way to revive a basket-case screen though.

Pete, it's great to see such a well researched post here about something which is really useful to others. Rear screen heating isn't the most "sexy" thing on the car - it's a safety feature! - but we've had the discussion a few times now, and it's really good to have a tried, and reasonably good solution to the problem.

If you can, I'd like to see some detail photos of the result, but in any case: congratulations with the repair!

Cheers,
Anders 8)