Disco 2 heated screens gremlin

i've got front and rear heated screens, the front doesn't seem to work, the back one does but seems slow to kick in. i think one or two of the elements are broken so that may be the issue there. both lights come on on the switches so power is going somewhere. anyway, i've looked into possible causes but not really a priority. however... the past 2 days, i've noticed that the switches come on by themselves when i start the engine (not just ignition on). they switch off ok but why would they come on by themselves??

Bobdog said:

i've noticed that the switches come on by themselves when i start the engine (not just ignition on). they switch off ok but why would they come on by themselves??

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There is a section in the Rave electrical library about it coming on automatically depending on the Electronic Air con settings.

Cheers

i'll check that, i didn't think i had it set to auto but i could have accidentally pressed something

In addition for what neilly poosted is that the screen heaters will be activated by the aircon unit only if the set temperature is higher the ambient temperature...as about the screen not heating first of all check the fuses F7, F8 and relay R8

Bobdog said:

i've got front and rear heated screens, the front doesn't seem to work, the back one does but seems slow to kick in. i think one or two of the elements are broken so that may be the issue there. both lights come on on the switches so power is going somewhere. anyway, i've looked into possible causes but not really a priority. however... the past 2 days, i've noticed that the switches come on by themselves when i start the engine (not just ignition on). they switch off ok but why would they come on by themselves??

Click to expand...

The front and rear screen heaters are a bit of a convoluted circuit. Not only do they involve the BCU for the timing but also the IDM in the case of the rear screen and the climate control system also has an input. The tell-tale lights on the switches don’t tell the whole truth, they only show that the BCU has sent a signal to operate the relays. There are 2 relays, 1 for the front screen heaters which is mounted in the engine compartment fuse box; the front screen is split into the left and right halves and fed via two 40Amp fuses, one for each half. The relay for the rear screen heater is built into the IDM on the back of the fuse board.

To check exactly what’s happening, use some kind of voltage detector, a 12 volt bulb being the easiest and cheapest or a voltmeter if one is available. Connect the detector across each heater in turn and check the indications.

As I said, there are two switching systems for the screen heaters, the fascia switches or the climate control system and each system feeds into the BCU. Check the settings of the climate control before you go to deep into the circuitry.

Edited to correct errors

brian47 said:

...There are 4 relays, 1 for each heater...

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could you explain that cos i dont understand what you mean

sierrafery said:

could you explain that cos i dont understand what you mean

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i'm guessing mirrors, rear screen, then both halves of the front screen. (its in two halves and one can work without the other, though mine doesn't work at all)

The mirrors are on the same circuit with the rear screen = 1 relay and the front screen has only 1 relay but 2 fuses

sierrafery said:

could you explain that cos i dont understand what you mean

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My mistake, I was reading the diagram without enlarging it fully so that I could read the designations. It looked like there were two relays in the fuse board.
Rechecking, there is just the one relay feeding to two 40A fuses, one for each half of the front screen.
The relay is however driven from the BCU.
Please accept my apologies.

no need to apologise, anybody can missread a diagram once in a while... even me

turned out, that when the wife drove it the other day, she faffed about with the heater, making them come on automatically...
i might get round to fixing the problems at some point. i also get radio interference when the rear screen is switched on, apparently a symptom of broken elements.

Bobdog said:

turned out, that when the wife drove it the other day, she faffed about with the heater, making them come on automatically...
i might get round to fixing the problems at some point. i also get radio interference when the rear screen is switched on, apparently a symptom of broken elements.

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You can get a proper paint which is loaded with silver intended to repair the rear screen heater, which unlike the front screen heater is surface mounted rather than built into the glass.
Having looked at the job on previous vehicles, I can see that while it sounds easy, it's not that simple in practice. First you have to find the break in the element, then apply enough paint to effect the repair without the blob of paint spreading and becoming unsightly but with patience, a steady hand, some masking tape, a good magnifying glass and an understanding of how to use your voltmeter there's no reason why it shouldn't be possible.

brian47 said:

You can get a proper paint which is loaded with silver intended to repair the rear screen heater, which unlike the front screen heater is surface mounted rather than built into the glass.
Having looked at the job on previous vehicles, I can see that while it sounds easy, it's not that simple in practice. First you have to find the break in the element, then apply enough paint to effect the repair without the blob of paint spreading and becoming unsightly but with patience, a steady hand, some masking tape, a good magnifying glass and an understanding of how to use your voltmeter there's no reason why it shouldn't be possible.

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yes, i've looked into it briefly, some conductive paints are better than others. apparently you also have to keep the repair as small as possible, otherwise if you leave a big blob, it acts as a resistor.