Instrument Panel Lights Failed. '92 940 2.0L

[Ian21401]

Thanks xad for your input. I’m probably about to show you and whoever else who may read this how thick I am. I have a very basic knowledge of electrics and virtually no knowledge of electronics, other than being aware of the existence of magical components such as resistors, capacitors, diodes and transistors and such.

But first, another correction to what I have already posted. I described the back of the dimmer switch as having two mini plugs, one to the clock adjust side and one to the dimmer slider side. That was wrong, It is one plug with 6 pins although it looks like two.
1 = white. 2 = green/brown. 3 = blue/green. 4 = pale blue. 5 = red. 6 = black.

I’ve tried valiantly to understand the wiring diagram and thumbnail you posted but due to my lack of knowledge I’m not entirely sure what I am looking at so please feel free to correct me if I have got this wrong.
It appears to me that all the white wires are feeding power in via fuse 21 and each of the bulbs, which are wired in parallel, affected by the dimmer have a blue wire feeding out.
(Curiously, when I removed fuse 21 to check whether it was good, the rear ashtray/central storage box illumination and the seat heater switch illumination lit up and these appear to be fed from fuse 22, but when I replaced fuse 21 they went off again.)
To continue, the blue feed from the bulbs eventually connects with the transistor (which Haynes names as a rheostat but as you state is shown on the diagram as a transistor ) mounted in the instrument panel and also to pin 4 of the dimmer.
Am I correct in thinking that the power is then taken from the transistor by the green/brown wire (essentially the earth from the transistor) to the dimmer switch pin 2 where, as you suspect, it is controlled to earth (the black wire from pin 6)
I don’t understand how the dimmer switch does this, except that it appears to be using those clever resistor thingys, and I do not understand how the white feed in at pin 1 and the blue at pin 4 fit into this scenario.
I tried your suggestion of jumping between pins 1 and 2 and this made no difference, so I suspect that the fault is not within the dimmer switch.
I then tried a jumper between pin 4 (the blue) and pin 6 (the black) and the bulbs all lit at full brilliance. Is it correct to presume that this is effectively bypassing the transistor and therefore the transistor is defective?
I now have functioning panel illumination again but have somehow managed to mess up the clock adjust forward button which does not work now, but the adjust back button still works.

I hope this makes some sort of sense.

[baggy798]

I think the dimmer switch is a red herring, because if I disconnect mine the instrument lights still work, they're just slightly less bright than when the dimmer switch is connected and on max brightness. This is my switch, it's the same style as yours right?



The actual dimmer 'control module' is located on the back of the instrument cluster here:



I'm going to a scrapyard today, I don't mind getting another switch + dimmer module for you if you like.

[Ian21401]

My dimmer switch is mounted in the lighting switch panel, to the right of the ignition switch between the ignition switch and the foglamp switch. It occupies the same size hole as yours but the slider only occupies the left side of the switch and the right side of the switch has two buttons on it to adjust the clock forward or back.
Is the transistor/rheostat that large rectangular thing with the screw through it? I haven't seen mine as I haven't had the instrument panel out yet.
If you are lucky enough to find any at the yard I'd be grateful, then we could PM or 'phone re cost etc. I haven't seen any 900 series in any of my local yards lately.

[baggy798]

Hello, I got the dimmer, but the switch that both cars at the scrapyard had were like mine:





Does your instrument cluster have the knob next to the clock to 'manually' adjust it?

[Ian21401]

Thanks baggy798. My switch is a dimmer and a clock adjuster so mine has six pins. The clock adjust is by two buttons on the right side of the switch, one for forward and the other for backwards so there isn't a knob to manually adjust the clock. I tried to post some images, well my daughter tried on my behalf as I'm not that clever yet, but we couldn't make it work. We've just received some guidance from The VOC Forum Support Team so will try again, probably tomorrow.
Thanks again for you efforts.

[baggy798]

The actual dimmer/rheostat module (silver box) should be the same I would think. Anyway, if you would like it, just send a PM with your address and I'll pop it in the post to you.

Edit: Hmm... I've just seen this cluster in the FAQ, another style, the rheostat is obviously different:



And yes, I know what you mean now, yours is like this:

[Ian21401]

Yes, my dimmer switch looks like the one in your thumbnail.
I'll try to access the back of my instrument panel at the weekend to see what my transistor looks like and get back to you after that.

Thanks, Ian.

[James_EG_940]

Hi Ian,

My 1994 dash looks like the second one posted (with the large slanted connector in the bottom right of it) I would suspect that yours is the same but nothing can be confirmed until you have yours out and have a look at it.

The other thing I would suspect is that the rheostat module is unlikely (but not impossible) to have died itself. What I would find more likely is that there is a problem with the back of the dash. These dash boards are plagued with various problems including but not limited to: dry solder joints, cracked PCB traces, oxidised and corroded screws.

Your rheostat may be totally fine but one of the above problems suddenly appeared when you went over a bump in the road etc.

Once you have your dash out I would suggest going over it with a fine toothed comb looking for any of the above, there are other articles on here and the maintenance FAQ with regards to what to look for. If everything checks out swap the rheostat, if it then works, swap your old one back in again to check that it wasn't just re-tightening the screws that hold it on (and more importantly form the electrical connections) that did it.

On the flip side, it could indeed just be the rheostat (which probably has a chunky transistor at its heart but will be much more than that, Volvo will have just drawn a transistor symbol on that diagram I posted to show that it is its own discrete block of circuitry rather than a direct illustration of the component) a bulb may have blown in such a way that it caused a short and fried something further up the line in the rheostat...

[Ian21401]

I don't think I'll have time this weekend now. Christmas and all that. My brother is my tame electronics expert and he is down in Wales fitting a bathroom for his daughter so he is not yet available either. Once I am able to show him the back of the instrument panel,the wiring diagrams and this thread he should be able to help out.
I'll update the thread when I know more.

[trebor]

I've just come across this thread and I have what looks to be the same issue as @Ian21401 . What was the eventual resolution?

I'd love to see @James_EG_940 's circuit diagram but the forum seems to have expired the attachments, presumably to save space on the server. Any chance you could re-post it?

I haven't taken out my dashboard cluster yet but jumpering across blue and black restores power to the instrument lamps.