Hi,

Define carefully everything that has failed on your instrument cluster.
What lights fail to work , and what happens to your speedo?
What schematic are you working with?
What you want is the Volvo wiring diagram TP3904202
Find a PDF file on line if you're lucky with uncooperative search engines,
and check at Ebay and used book dealers such as Abe book.

That metal box sounds like the Rheostat.
Confirm that the dimmer switch for the lamps is behaving or not.

Failures of the instrument cluster are often bad solder joints that require
careful reflow and/or jumpers to resolve intermittent through hole connections
where the circuit track goes from one side of the flex card to the other.
Take care that you don't melt the flex card.

Another failure can be loose connector pins.

When troubleshooting an intermittent fuel gauge I finally caught the +12v to the gauge quitting and then probed back to a ratty through hole connection.
A jumper solved that, and there was also a loose pin for for the tank sensor.


For your speedo, start with the 4 pin connector "D" in the center of the board.
1 Signal +
2 Signal -
3 -
4 + 12v from ignition switch

The speedo is grounded via Connector A pin 2
You'll see this in the Volvo wiring info

Yes there is an in-line fuse 11/41 to the speedo D-4
This fuse is common for all of the gauges

Check the series component from D2 to Speedo S -
it is likely a capacitor.

What's your email ? I'll send you Volvo instrument cluster schematics.

Bill