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Instrument Panel Lights Failed. '92 940 2.0L

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Old Dec 3rd, 2012, 23:55   #1
Ian21401
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Default Instrument Panel Lights Failed. '92 940 2.0L

Driving home at dusk tonight when instrument panel lights all went out including heater control panel, radio lighting and heated seat switch panel.
All warning lights, indicator tell tale and road lights switch panel lights still working. All external road lights still working.
I had not accidentally brushed against the dimmer slider as that was the first thing I checked. (I think we've all done that at some time)
The dimmer switch is the slider type mounted immediately to the right of the ignition switch, in the same panel as the lighting switch and shares it's position with the clock adjustment buttons.
I don't know whether it is significant but, the clock is still running and the clock adjustment buttons (+ and -) still work normally.

I've had a quick search on this forum but most of the threads seem to refer to the 700 series and are rather old.

Can anyone give me some advice on where to start trouble shooting please?

Last edited by Ian21401; Dec 4th, 2012 at 00:00. Reason: Punctuation & add text
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Old Dec 4th, 2012, 01:07   #2
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Hi,

Check the instrumentation light level/dimmer as that may have gone south. The normal thing to happen when these fail is for the internal instrumentation lights to go full power (safety feature) but that doesn't always happen.

Other thing to check/consider would be the ignition. Check by "twiddling the key once the engine is started to see if this will bring the lights off/on.

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Old Dec 4th, 2012, 08:06   #3
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Hi Ian,
I had this on my 1993 960 2.9, 271000 mls. Also the radio was intermittant and on one occasion, the ignition went off but restored by a wiggle of the ignition switch. I checked the dimmer and all the earth connections on the dash board and side pillars. Following the blogs on this site, I went to Bradons and got a new replacement ignition switch (electric switch part only), not expensive and easily changed over. Its like a new car now and additionally, a number of other niggling electrical issues have been resolved, including a sometimes rough idle. My advice is change the switch for a new one....don't go for a second hand as donor cars are just as likely to have similar problems.
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Old Dec 4th, 2012, 10:26   #4
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Have you got any idea how the rough idle could be connected to the dimmer switch ? Someone else on here has been having problems with rough running and stalling and also a dashboard fogging up issue. I wonder if his dash lights aren't working then when they do it's warming the instrument cluster slightly causing condensation.
Can anyone think of a link that could cause this ?
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Old Dec 4th, 2012, 12:08   #5
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Default Thanks for the input guys.

I'll be checking it later this morning and update eventually.
I'm really annoyed about this as I've just spent four days over the last three weeks locating and fixing an electrical fault on my son's ancient Mk3 Astra heater blower. Finally, after fitting a new resistor, which made no difference, and then dismantling half of the dashboard to acccess the back of the controls, found it to be a poor contact in the plug on the back of the blower switch itself which had melted part of the plug. Visit to breakers yard in the freezing cold to obtain replacement bits and it was working again at 4.00 pm on Sunday. I was just trying to catch up on the other jobs which had been delayed when my panel lights went. So it's another job jumping to the top of the list.
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Old Dec 4th, 2012, 18:41   #6
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Default Update, still not working.

I've tried twice to post this and lost it each time, don't know why, so here goes again.

Dismounted dimmer/clock adjustment switch and removed the plug from the back to find that the plug has a small PCB built into it. This is divided into two halves, one for the dimmer and the other for the clock adjustment. There are two smaller plugs into this plug, one for each half and each of them having three wires into them.
The dimmer rheostat is part of the PCB and the sliding contacts are attached to the operating knob which is part of the switch outer casing.
The three wires into the rheostat mini plug are:
Top one,- Green/brown
Middle one - Pale Blue
Bottom one - Black
Unfortunately not the same colours as in the Haynes manual so I was not sure which wire did what.
The bottom (black) one appears to be earth - checked for continuity to chassis and resistance = 0.3 ohms.
The middle (Pale Blue) one is live when ignition or lights on = 11.2 volts.
The top (Green/brown) one appears to be the feed out from the rheostat to the panel lights and I presume should have continuity to earth/chassis, but it hasn't. It's open circuit.
So how are the panel lights earthed to chassis?
Is it in some way through the ignition switch, hence the comments about jiggling or replacing the switch curing the problem?
If it is the ignition switch, is it possible to remove and replace the relevant part of the switch without dismantling the steering column lock part?
It's dark and raining now so it looks like I'll be driving home from work without any panel lights until the end of the week.
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Old Dec 5th, 2012, 23:50   #7
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Default Correction to my previous.

I removed the dimmer/clock adjustment switch from the panel. I previously stated that I removed a plug from the back of the switch which revealed the PCB. In fact I had unintentionally taken the switch apart. The 2 mini plugs fit into the back of that switch.

I've studied Mr. Haynes wiring diagram again but I'm still completely in the dark (no pun intended) as to where, what appears to be a wiring circuit fault, is.

Help please guys?
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Old Dec 6th, 2012, 23:02   #8
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Have a look at the attached wiring diagram, albeit for a 1994 model car, hopefully there are enough similarities to be useful.

If you short with a small piece of wire pins 1 and 2 on the back of the dimmer switch connector - that's the white (12v from battery) and green-brown (potentiometer out), I guess it is possible that the dimmer switch and the clock controls are sharing power if they are both connected in the back as you say the connection only has three wires on the dimmer side - do your lights come on? if so the problem is within the dimmer switch.
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File Type: pdf 1994_940_instrument_and_control_panel_lights.pdf (1.31 MB, 10 views)
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Last edited by James_EG_940; Dec 6th, 2012 at 23:54. Reason: Added advice back after thinking about it.
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Old Dec 6th, 2012, 23:39   #9
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Default Wiring Diagram

Thanks Xad.

I haven't had time to fully digest that wiring diagram but it looks as if it could be very useful. I've printed it off for study later.

No, I haven't tried your suggestion. (I see that you have just removed it.)
In any case I haven't located a white wire yet as there are only the three I quoted into the mini plug on the back of the switch,although I think one of the three wires going into the clock adjustment mini plug may be white, but I'm not sure about that at the moment. I'll be having another look at it on Saturday, weather and chores permitting.

Thanks again for the diagram.
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Old Dec 7th, 2012, 00:03   #10
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Cos I'm bored I just whipped up a quick circuit in my modelling software; I invented the resistor values. Seems to work! The dimmer switch on the dash basically changes how much power is allowed to get back to earth via the rheostat circuit which, for all intents and purposes and by going from the circuit diagram is probably just a big transistor. Blue will always be live unless all the bulbs have gone/you take all the bulbs out. The Green-Brown wire should have continuity with earth (and live!) whilst connected to the switch but probably has quite a big resistor between it. Try testing with a DMM set to something like 2M Ohms.
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