Strange dash lights

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discogaz81

discogaz81

Member
Posts
59
Location
Cumbria
Evening good people, I seem to have a strange fault with the digital displays on the trip display and the air con unit, they have a back light on until you turn the headlights on when they both turn off no light at all, turn the lights off and hay presto back lights.
Car is a td5 discovery any help greatly appreciated
 
discogaz81 said:
Evening good people, I seem to have a strange fault with the digital displays on the trip display and the air con unit, they have a back light on until you turn the headlights on when they both turn off no light at all, turn the lights off and hay presto back lights.
Car is a td5 discovery any help greatly appreciated
Click to expand...

I imagine it is to prevent the display interfering with your view ahead at night. A lot of modern cars do this, and older landrovers had a switch to switch off the displays.
 
I was thinking that but the lights on the entire air con unit turn off buttons and display, seemed a bit strange but if that's the way it works then all is good I think
 
discogaz81 said:
Evening good people, I seem to have a strange fault with the digital displays on the trip display and the air con unit, they have a back light on until you turn the headlights on when they both turn off no light at all, turn the lights off and hay presto back lights.
Car is a td5 discovery any help greatly appreciated
Click to expand...
It might sound a bit daft, but is your Disco a Jap import by any chance?
The reason I ask is because on Jap Discos there was a resister pack with a relay to dim the interior lights. If that has developed a fault such as the resister going open circuit then that could lead to the lights going off.
 
discogaz81 said:
I was thinking that but the lights on the entire air con unit turn off buttons and display, seemed a bit strange but if that's the way it works then all is good I think
Click to expand...
what about the other buttons near the aircon unit and the clock? are these still OK? ... observe well everything with care cos any small detail is relevant, this seems a bad contact issue somewhere quite likely within the fusebox or bad earth issue but for that a 100% accurate description of the symptom is needed
 
It is a UK car, the clock doesn't light up at all lights on or off, all the push buttons to the right of the air con panel hill decent, hazard lights ect all work the right way round light up when head lights on, its just the complete air con panel and the digital display in the speedo cluster, speedo and rev counter work correctly.
 
The LCDs are made to dim when the headlamps are on but not to go out completely so just confirm that they are completely off when it's dark cos in daylight they might look like off but just dimmed, the clock has most probably a blown bulb
 
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In this case i'm afraid that it's a problem within the interior fusebox on that particular dimmer circuit cos it's managed by the IDM
 
That sounds like a rather complicated fault finding mission, looks like it could be one of those problems I can live with.
Thanks for your help
 
Yes, this LCD's dimmer thing is completely uncovered in the documents, i've been trying to figure out how it works for a good while now without a clear result, in the odometer LCD description appears that there is a photocell which controlls it's brightness and it dims on headlamps, for the aircon unit the only thing which proves this circuit's existance is the connector's pins description in the workshop manual where at C0792 pin 1 appears "Night lighting/dimming = Input" in the connector view appears RB wire but this pin/circuit is not shown at all in the diagrams, also the clock's LCD illumination works the same, it dims on headlights but only the main lighting feed is shown from fuse F16... a real mess, from all my research i suspect the IDM so if other problems will appear which can be due to the interior fusebox/IDM(cos it used to suffer from water ingress) and you'll change it this issue might be fixed as well... though i'm not 100% certain in this lack of info about it

Aircon LCD dimmer.jpg
 
sierrafery said:
Yes, this LCD's dimmer thing is completely uncovered in the documents, i've been trying to figure out how it works for a good while now without a clear result, in the odometer LCD description appears that there is a photocell which controlls it's brightness and it dims on headlamps, for the aircon unit the only thing which proves this circuit's existance is the connector's pins description in the workshop manual where at C0792 pin 1 appears "Night lighting/dimming = Input" in the connector view appears RB wire but this pin/circuit is not shown at all in the diagrams, also the clock's LCD illumination works the same, it dims on headlights but only the main lighting feed is shown from fuse F16... a real mess, from all my research i suspect the IDM so if other problems will appear which can be due to the interior fusebox/IDM(cos it used to suffer from water ingress) and you'll change it this issue might be fixed as well... though i'm not 100% certain in this lack of info about it

View attachment 131081
Click to expand...

In your description of the dimming on the ATC display, I would venture that the actual dimming circuit is in this case inside the ATC circuitry.
During daylight running, the backlight for the display is powered solely by the +12 volts fed to that whole unit.
The RB (Red/Brown) wire which you've marked is the interior lighting circuit for all the switch illumination. The voltage applied to pin 1 of C0792 switches an internal circuit to dim the backlight.
A similar arrangement occurs in the radio head unit and probably the clock too.
 
I can't contradict the theory that the dimming thing is within each unit, seems logical but as both of them have problems i suspect that dimming input from the IDM to be the cause which is not mentioned at the instrument pack cos i presume it's through the serial data link from IDM and at the clock it's not shown at all
brian47 said:
The RB (Red/Brown) wire which you've marked is the interior lighting circuit for all the switch illumination. The voltage applied to pin 1 of C0792 switches an internal circuit to dim the backlight.
Click to expand...
AAMOF the one i mentioned RB which is Red/Black to C0792-1 is not shown in any diagram the one you say RN(Red/browN) for overall dash switches illumination is to C0791 - 5...you can see in that table i posted and that's present in all the diagrams.
 
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