Strange Dash Lighting Issue - wiring diagrams?
 

First off, if anyone has any wiring diagrams for the dashboard/instrument area that would be helpful.

Here's the problem. Many months ago some of the main dash lights started to either go dim or not work at all. Soon after that all the main dial lights stopped working. I still have all the warning lights and the indicator "arrows", just the main dial lights are dead.

I re-soldered (probably not very well) many of the connections on the dash. No change.

I then bought another dash. A single bulb in this dash worked for a moment, then back to nothing again. The bulbs are definitely not blown. The rheostat (the dimmer switch) still controls the remaining instrument lights as normal.

Is it possible that the main dial lights are powered separately from the other dash lights, and therefore maybe this separate wiring system is where my problem lies? Could it just be a bad ground? Where should I look? What troubleshooting can I do?

I'm getting good voltage at the battery.

All suggestions appreciated.

You will find all the wiring diagrams including the 940 instrument panel, in the technical pages/wiring diagrams...but you will need the members login and password. Also on the tech/FAQ pages there is a lot of detail on panel problems. I've had these symptoms on my 1993 960 instrument panel and I traced it down to the earth points down by either the pax or drivers footwell panel on the outboard side; and the earth points on the instrument panel. Clean them up and re-make them.

Cheers. Are the footwell earth points those 4 spade connectors positioned in a row?

And for the instrument earth points I'm assuming that's most if not all of the screws on the back?

I've fiddled with those and the aforementioned spade connectors. No joy so far. Also looked at the relays to make sure the relevant ones were seated correctly (read about someone else having that problem).

I'm sure there must be a very limited amount of things that could be causing this considering I've tried two dashboards and the problem was consistent.

Yes, the earths are the spade connectors in a row. Take the spades off and remove the connecting block in order to re-make the connection.
Check also the panel light power supply from the slide dimmer as that can cause the symptom you have. If you by-pass the slide dimmer then the panel lights should come on bright. There is also a rheostat on the back of the combined instrument which are known to give problems. Those earth screws on the back may need quite a bit of cleaning to get rid of any corrosion if its present. All the panel lights come off the slide dimmer so if the other panel lights are also giving the same symptoms, then that is your likely suspect.

Thanks for the info. I didn't try removing that connecting block so I'll try that next. Getting the ECU off and back on again is such a pain.
The dimmer functions correctly for the other panel lights, but I might have a look at it anyway so that I can begin to work out how it's all wired up.

Probably also worth mentioning that the lights were flickering badly before completely dying, as if a loose connection was present.

I use these 940 wiring diagrams to find problems and things. Says MY1995 but they're all the same really. File size is 50mb. There's a dash lighting diagram on page 43.

http://www.volvowiringdiagrams.com/v...20Diagrams.pdf

Cheers, I came across it earlier. I've found Combined Instrument on page 126 (pdf says number 53) and Instrumentation and Controls Lighting on page 98 (pdf says number 38). Maybe my document has different page numbers for some reason.

Tested the connections going to the dash. Everything seems to be in order. There appears to be just over 9v supplied after the rheostat when it's at its maximum setting. Pretty sure that's normal.

The ground connection shows some resistance but I've no idea how much I should be concerned about. Anyone know how much resistance I should check for? Or if I should even be checking resistance?
(absolutely no idea what I'm doing here)

The resistance to ground along an earth wire and through the bonding point on the chassis should be fractions of an ohm.

(It is common with most meters that even if you just touch the leads together you will read about 0.5 to 1.5 ohms - this is the resistance of the meter leads themselves and should be subtracted from the displayed resistance when you are taking a measurement.)

If you have a good earth, the meter should read almost exactly the same as it does with its leads shorted to each other.

Anything over about 3 ohms and I would suspect one of the connections to be defective.

Boots.

Resistance testing is a waste of time and will give misleading results, all it will prove is that you do not have an open circuit but it will not indicate how the circuit will operate under electrical load.

The best way forward when testing any electrical circuit is to use "voltage drop" testing method on both the supply and ground side of the circuit. This will show what amount of voltage is being dropped whilst the circuit is in operation.