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Phill

Rear window demister

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Discovered rear window demister is not working. It was the last time I used it (possibly a couple of years ago) but the other day, nothing.

Started with the basics. Green light comes on at switch at both positions. 

Now it's a bit strange. Picture for reference.

IMG_20200105_103556.thumb.jpg.847757f7b8851fc0076c3d22ad3170b7.jpg

There is power at both fuses 2 and 3 whether the switch is on or off (ignition on and assuming switch runs directly to the relay, black wire?). I always thought fuse 3 was the demister fuse but the wire appears to be the same as that at terminal 31 which is from fuse 2. However, at terminal 31, which appears to be the same wire as at fuse 2 there is no power. I suppose the wires at the fuses could be inserted into the wrong holes. Only terminal 30 has power (ignition on). 

Is this correct so far, if not why not?

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Update. The black at 30 (could I have my terminals opposed) does indeed come live when the switch is operated. Should a second terminal come live when the switch is pulled to the second position?.

Fuse 2 is live at all times, fuse 3 is live with ignition on.

Where are the connections at the screen itself and are they accessible?

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44 minutes ago, Phill said:

Update. The black at 30 (could I have my terminals opposed) does indeed come live when the switch is operated. Should a second terminal come live when the switch is pulled to the second position?.

Fuse 2 is live at all times, fuse 3 is live with ignition on.

Where are the connections at the screen itself and are they accessible?

The screen needs to be removed to get to the spade connections. They're tucked away inside the seal but it's worth checking that they haven't been chopped and re joined with bullet connectors in the location shown in the image. Body shops typically do this to avoid messing with the seal or disconnecting them properly.

P1040750.thumb.JPG.50551bf6de1c2aa22774a3b622c78ac9.JPG

Cheers

FF

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Thanks FF. My screen is original (with dealer sticker!) So I would rather have no demist than take it out. I will check the wires though. Presume I need to remove parcel shelf to see them?

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18 minutes ago, Phill said:

Thanks FF. My screen is original (with dealer sticker!) So I would rather have no demist than take it out. I will check the wires though. Presume I need to remove parcel shelf to see them?

Yep that's right Phill

Cheers

Andrew

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Update. It seems my fuses are upside down and are from an 81-83 model year. Top fuse (1) has a thick red wire which when tested for continuity is the one at the relay. I further tested this by jumping fuse 2 to 1 and then there is power at the relay terminal 30. Also established that terminal 86 is indeed live from switch position 1 and terminal 86a live from switch position 2. So new diagram is as below.

IMG_20200105_144315.thumb.jpg.09a766392b3588528b30ddbace2994ba.jpg

Now, for some bizarre reason I have no life feed to fuse 1 which his the permanent live feed to the relay (when ignition on). I don't know where the wire has gone but I'm hoping that if I connect it up all will be well. However, I need to trace where it came from originally.........

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So, Bentley says it comes from the generator regulator (Bentley has been useless for this so far). There are two red wires shown from point B+ on the back of the generator, one to the starter and one to "power distribution system". Alternative diagram shows this wire going to fuse 24 which his one of the engine bay fuses. Where has that wire gone?

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Unfortunately the red wire from the generator leads to the big multi plug......

IMG_20200105_151036.thumb.jpg.7c1702b26e5c0d790a531b5250ce78a7.jpg

What happens from there will need much more invasive investigation which I'll have to wait until another day.

If you know where it goes to from there please let me know.

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I wonder if each row/fuse drives a different section of the heated rear screen?

 I ask as the top and bottom on my rear demister works but the middle bit not and I can't see any breakage lines on the screen itself?

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13 minutes ago, Beaky said:

I wonder if each row/fuse drives a different section of the heated rear screen?

 I ask as the top and bottom on my rear demister works but the middle bit not and I can't see any breakage lines on the screen itself?

The screen has two sections for the two stage operation. The centre section is operated by the pulling the switch out (so called "defog" position) the whole screen is operated when the switch is turned (so called "defrost" position). The relay that controls all this is not a simple relay, it detects alternator output and if there is not enough power (low revs, idling, lights on etc) it switches to just the centre section until it detects enough power. 

As I understand it, fuse 6 at the front of the car supplies power to the dash switch to operate the relay and fuse 3 in the engine compartment supplies power to both sections of the rear screen, controlled by the relay.

Mark

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1 hour ago, Beaky said:

I wonder if each row/fuse drives a different section of the heated rear screen?

I am 100% the screen is powered by one fuse only.

32 minutes ago, SilverWT said:

The screen has two sections for the two stage operation. The centre section is operated by the pulling the switch out (so called "defog" position) the whole screen is operated when the switch is turned (so called "defrost" position). 

There is no twist on mine. I know for a fact that one pull is centre screen, two pulls is whole screen. I remember this from when it was working and thinking how pointless it was to have half the screen demisting.

The relay that controls all this is not a simple relay, it detects alternator output and if there is not enough power (low revs, idling, lights on etc) it switches to just the centre section until it detects enough power

I did not know this but it makes sense and explains the "pointless" half screen demisting.

As I understand it, fuse 6 at the front of the car supplies power to the dash switch to operate the relay and fuse 3 in the engine compartment supplies power to both sections of the rear screen, controlled by the relay.

That's what I believe too.

I still need to know where to "find" the missing wire though.

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Posted (edited)
14 minutes ago, Phill said:

There is no twist on mine. I know for a fact that one pull is centre screen, two pulls is whole screen. I remember this from when it was working and thinking how pointless it was to have half the screen demisting.

Mine is just a button, don't think there is two stages movement in it but I could be wrong

 

image.jpeg.a1d307004602caff833a7cba7c33f3c9.jpeg

Edited by Beaky

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Ah, yours is completely different to mine. Mine pulls out and lights up green.

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I think there are significant differences over the years but in post 15 of the above the poster gets the wiring connections right but I think he is mistaken that there are three sections. I think the extra electronic gubbins is the part Mark speaks about above in relation to the lack of current failsafe.

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Hi Phil

You probably know this already, in which case sorry, but a number of fuses are linked, so beware when belling out cables that are not disconnected.  For example, at the front, Fuses 11, 12, 13, 14 & 15 are all linked at their inputs (switched live).  At the rear, Fuses 23 & 24 are linked at their inputs (permanent live).

 

On my '79SC the red fuses are (now) wired

At the front, Fuse 12 supplies power (via black wire) to the HRW [I believe HRW switch (pin 1)] and (via the red/black wire) to the rear wiper.  [I believe that pin 3 (position 1) of the HRW switch supplies pin 86 of the HRW relay, and pin 4 (position 2) of the switch supplies pin 86a of the relay]

Fuse 22 - has red wire coming from input side of Fuse 11 at the front (just provides an un-fused, switched live supply).  This also supplies the CDI box (no fuse) and the output of the fuse (red/black) goes to the Heater vent relay (pin 86)

 

Fuse 23 & 24 are linked, and both have a permanent live supply.

Fuse 23 outputs to the Htr Vent relay (pin 30) and the rear wiper

Fuse 24 (red/black) outputs to the HRW relay (pin 30)

 

Relay - pin 87 (red/white) goes to [I believe, middle bit of] the HRW element

Relay pin 87a - [I believe goes to the top & bottom bits of the HRW element]

 

 

 

 

 

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Thanks for this Peter. Will have a better read when I get home. Might need to phone you.

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I just want to be clear here before I progress. The three fuses in the engine bay. Top one, 22, middle, 23 and bottom 24, yes?

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Mine are the other way - 24 at top.

 

23 & 24 are linked at the input side, so remove fuses and check continuity to check which is which ?

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57 minutes ago, PeterK said:

Mine are the other way - 24 at top.

 

23 & 24 are linked at the input side, so remove fuses and check continuity to check which is which ?

That makes much more sense, thank you.

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To close this off the solution was simple after Peter's clarification above. Connected fuse 23 and 24 and hey presto, working demister :D Where or why the wires got changed will have to remain a mystery, lost deep in the mists of time......

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