I was removing my 52mm clock the other day (with the negative battery cable removed) when I got distracted by the fact that the new clock I was going to swap in had a different bulb base. I went inside to look up the difference when my girlfriend came home. I was blocking the garage, and not wanting to make the lady wait I hooked up the negative cable and moved the car. In my rush I forgot to insulate the two wires that run to the small clock. Not sure if they touched at any point when I was moving the car, but, I thought it may be pertinent.

I ended up not installing the new clock and thus removed the connector wires from the large tach to the small clock, but removed no other wires. When I went to start up my car again I had no tach and no speedo movement –– they were both dead. Additionally, both the lights to the turn signals were lit permanently. The fuel gauge and temp gauge both worked fine.

If I turn off the headlights the tach and speedo spring to life. And, if I have the headlights switched on but the rheostat/dimmer switched turned all the way off I have full instrument functionality as well.

I've been struggling to understand how to attack this. I've checked/swapped fuses, and cleaned grounds, and all appear to be okay. I've checked the boards, but all I can find on the subject pertain specifically to the temp/fuel gauges.

Would touching the two small clock wires together cause this issue? Or was it just coincidence?

And, how is the rheostat involved? I've gone over the wiring diagrams in both the Bentley and here –– Volvo Wiring Diagrams –– and I understand that the rheostat connects via the brown wire to the circular connector on the back of the cluster (connection 10) and that goes to the tach/speedo/turn signals, which are all the components I'm having issues with. What I suppose I'm lost on is why turning it off causes function to be restored?

I have a working backup cluster that I could try, but, I don't want to install it incase the existing issue is persistent and will fry it as well.

Any thoughts/directions on how to approach this would be appreciated! Thanks!