[ro80rob]

Does anyone know how the rear screen heater switches on. The handbook merely mentions that it comes on automatically. What does that mean? What conditions? Are there any specifics? Is there any way by some setting of the heater controls that would 'make' it come on? Got to say that I could have done with it being on a couple of mornings this week, but it patently didn't do that.

Any Suggetstions?

Cheers... Rob.

 

[Lew]

The rear window demister is automatic. It is controlled by the climate control electronics. It turns on whenever the upper system is in heating mode, the upper air quantity flap is more than about 1/4 open, and the system mode flap is in the screen position. Later cars have the mirror demisters on the same circuit.

 

[ro80rob]

Thanks for that.
I'm not sure how to check if all these criterion are met, but I'm convinced I haven't observed rear screen heating yet. It would be useful to know where in the car the relay that switches it on is located. The electronic manual I have for the car omits the appropriate page showing where to find the relay. Can anyone help me?

Cheers... Rob.

 

[Lew]

Pull out the fuse (centre panel B4 on post-20,000 series cars like yours) and connect a multimeter set to be a 10 amp DC ammeter in-circuit instead. With the motor on idle, switch the climate control from off to the manually selected demist/defrost position (fully clockwise). You should see the current jump immediately to almost 3 amps. The fuse supplies the side mirror heaters too, so if you only see a bit less than 1A, then check the relay, located in the trunk on the right hand side behind the trim.

 

[ro80rob]

OK Lew

Thanks for the full test procedure. I'll give it a go. 3 Amps doesn't sound like much to clear the screen, I'd have thought more like the full 10 Amps would clear it much more quickly. Do you reckon the screen heater is a little under powered?

Anyway, I'll go looking for 3 to start with.

Cheers... Rob.

 

[Lew]

The demister wires are so small, you can't hit them too hard with current. They warm up pretty quickly as they are.

 

[ro80rob]

Ok Lew

I have a 20 Amp fuse for this item. The key charts on the drop down door agree with your location.

I've done the test you suggest. The current reading is 2.7 Amps. After a short period, the door mirrors are noticeably warm (one would say hot even), but I feel no warmth at all from the rear screen. I reckon I should do, or what effect will it have? Note that after a few mins operation, the current is 2.3 Amps.


Taking the 2.7 Amps figure, that makes only 32 Watts (Watts = Volts x Amps). Not even as much as one headlamp bulb?.

So

I pulled back the right hand panel in the trunk a little, and spied a couple of relays towards the front. One is a plastic bodied item with its connectors facing sideways towards the rear wing (fender?).
I note also from the parts manual I have (which does not seem to quite cover my car since the chasis number is outside the range), there should also be some sort of timer device mounted there with the relay. Maybe this box is it?
The other is a more conventional metal bodied relay (like in the engine bay) that is mounted in line with the car (front to back). Do you know which to address?. I'll pull the panel back/out fully and get a better look.

I had a Ford with this sort of 'in glass' heating for the windscreen (not the rear screen). It was brilliant. It could clear a misted screen in 30 seconds. It drew considerable current (~12 Amps).

This should work better in my opinion.

Cheers... Rob.

 

[ro80rob]

Okay so I checked it out.

The plastic bodied item is the brake lamp failure device. Looks like a simple reed switch device to me. Hey keep it simple.

The other item is 'the' relay. Without it, there is no current flow through fuse B4 and so no heated mirrors either. It seems to be the only item in the circuit, and it switches. That means that if there is a timer, it is definitely switching on the relay.
One point I would like to see is the connection of the heated door mirrors to the relay. The only diags I have show two wires coming off this relay's output. I only see one, which means there must be another bridge somewhere. I don't know where that is. I can't see it in the boot (and I've pulled back the boot liner around the battery switch too.

Any ideas?

... Rob.

 

[WLH]

Rob,

I have a wiring diagram for a 90 Mulsanne S. This is a later, 30,000 series car with Active Ride and ECUs everywhere.

If your car wiring is the same, there should be a 2mm blue-silver wire from the relay to a three-pin connector behind the O/s cantrail mirror (pry the mirror frame outwards from the front; hook retainers at bottom and rear).

My diagram shows a second, 1mm blue-silver wire from this plug to a 24-way plug at the lower O/s A post (by your right foot). From there, it feeds back into the main loom and on to each front door plug (either the 12 or 24-pin, a dashed line to the 12).

The rear window feed connects to a second 3-pin behind the mirror and on to a barrel connector in the headlining above the window. All blue-silver except the final connection which is blue.

There is a black, 2mm earth shown, also above the window.

From the aircon ECU, the relay on my car is fed from pin 18 on the R/H side, which is the main aircon feed from fuse A1. The other side of the coil (ground?) connects to pin one R/H

Happy hunting,

Bill

 

[ro80rob]

Cheers for that info Bill

I did some good looking round and down where the wiring loom disappears, but it just went off into the distance so I'm not sure where it went.
Now you have given me hope of finding the lost connector and maybe a chance to get the window working.
Logically there must be another connector/joint. If I wanted to replace the rear screen, it must disconnect somewhere to get the window away from the car.
My old Mercedes w123 had screw on connections behind the side trims in the saloon. They joined the window element to the wires on nice plastic insulators set into holes in the internal bodywork. Rather overkill, but nice and solid.
I'll have a go at removing the O/S mirrors and see what I can find.

Can I ask where you got your manual from. I got one of those IETIS ones (as well as the quite poorly 'scanned in' items for earlier Spirits etc) from Ebay. The scanned in one claims to have a supplement for the 87 on cars, but has no wiring diagrams. The IETIS manual seems to have chunks missing from it. Wiring diagrams for portions of my car included. Most frustrating. The Australian club's online manual is also lacking in the wiring area.

Of the two items I wanted to work on so far, information has been sadly lacking.

Cheers... Rob.

 

[ro80rob]

OK Bill

What you told me got me thinking so I took a look in the manual at removing the rear window. It tells you in there where to disconnect the rear screen heater. I've copied the picture here below to show that.

It seems on my car its a little bit harder to get at than behind the mirror unit. Unfortunately to get this trim off, I need to remove all the back seat and side rear trims. I was going to pop the rear window to sort out some very minor corrosion just appearing from under the glass at one side, but it looks like a bigger job than I want just now. I'll see how my enthusiasm takes me.

Cheers... Rob.

 

[WLH]

Hi Rob,

it might still be worth your while pulling the mirror and checking for power/continuity at the plugs behind, before you take out all the trim

My manual came from Flying Spares and I was told that it is time limited, but it hasn't expired yet and I can set up another PC if it does and turn the clock back.


The diagrams I have are not from the electrical section! Look in TSD500 under aircon, section C4 electrical circuits and test procedures, then scroll right down to C4-7, fan control and demister and mirror heaters, 4 door cars prior to vin 5400.

Larry Halpert also offers manuals. Look on the RRBEW website

Bill

 

[ro80rob]

Hello Bill

Thanks for that info. I managed to pilot my way to the drawing you speak of. Its the first time I've seen that one. Somehow the way I've been accessing it from a different end of the programme, I couldn't get that wiring diagram to view.
You are right. I realise that this is a drawing for an active suspension control car, it should be broadly similar. The drawing shows 3 connectors on the way to the screen element. The last one being in the roof trim, but two others being possibly available as you say behind the rear inside mirror. I agree that it would be worthwhle to take a peak.

As to your manual being 'date limited', so is mine. All I do is wind my computer calendar (double click on the clock in the system tray at the bottom right of the screen) back to make the programme work, then change it back again later when I'm done viewing.
No probs.

Cheers... Rob.

 

[Lew]

Been on an RROC Fall tour for the past 3 weeks(4700 miles). You are now passing my expertise on locating the problem. Let me know what you find.

 

[Adamski]

I thought I'd ride shotgun on this old post. I was out list night in the Conti R and it was raining and the inside was well misted up. I noticed the heated rear window only demisted in random stripes of about 4 inches. Rubbish really. Does this mean that certain elements are burned out?
This would be the same for any of the SZ cars. Any advice?

 

[guyslp]

I do not know if the setup is precisely the same as in the SY cars, but on the SYs the rear window defogger is only in the center area of the window and there is about 8-12 inches on the outside where no elements run.

The setup on the elements, which are sandwiched between the layers of safety glass, make it unlikely that they will burn out and they cannot be "scratched off" like most defoggers that use electroconductive paint.

I would take a look at the electrical connection to the defogger itself in the boot. Things could have gotten knocked about or you could have corrosion on some of the connections to individual elements and not others. You should, at a minimum, get a nice, solid clear patch in the center of the rear window. I believe this will only activate if you turn on the defrost/defog setting on the climate control.

 

[Adamski]

I'm afraid the demisting was like a candy/zebra stripe whereas I can see elements throughout. I wonder if they die?

 

[jeyjey]

My wife's Ford SMax started to lose "lines" in its front window about 18 months ago. It's now down about 1/3 of them. Candy/zebra striping is irregular. Not sure if it's the same design as your Turbo R rear window or not, but I at least it's a data-point....

Cheers,
Jeff.

 

[mikebentleyturbo2]

Hi All
I have had my screen out and the connections to the elements are crude beyond belief.
There is an aluminium strip running along the top and bottom unsealed ! so water gets in the slot in the rubber corrodes thr strip and thats why you see the white strip along the bottom of a lot of rear windows as water permeats between the layers of glass.
At the top of the screen is where the power is connected ,the wires appear just wrapped aruond the aluminium strip and corrodes away.
If yours works at all your lucky!
Hope that helps
Mike

 

[guyslp]

Hi All
I have had my screen out and the connections to the elements are crude beyond belief.

Which is one of the reasons I always suggest that people check the connections. The setup of the heating elements, which are actually very thin wires, is relatively robust.

Another suggestion, and particularly applicable here, is to treat any electrical connection you ever have reason to visit with electrically conductive grease after you've cleaned it up and before you put it back together. I know I've posted about Sanchem No-Ox-Id A-Special and Gardner Bender Ox Gard in the past. There are others. This stuff is really a bit of a miracle worker in preventing any oxidation when applied in the very thinnest of coatings. The fact that it repels water should there be water incursion is a big advantage.