The cluster has 3 sections. First work fine. Last one for gas and temp dead. Mechanic says voltage regulator $350 installed. Not sure that sounds good. Any solid help appreciated. Has 206k so just broken in.

 

this is the actual regulator, https://www.ipdusa.com/products/9407/120792-instrument-cluster-voltage-stabilizer-240-1986-1993-genuine-volvo-1362674

here's someone's video on removing the instrument panel

oh heck, this picture looks like the regulator (on middle of the left side) is in a socket, so even easier (I thought you'd have to solder it before i saw this pic), you just need some heatsink paste for under the metal mounting tab...)

(Picture cribbed from Dave Barton's Volvo pages)

 

I must have fixed hundreds of 240 gauges - and never saw a bad voltage stabilizer. In the early cars yes, the stabilizers in the little metal can went bad - but none in the 86+ cars. Your "diagnostician" may be confused with the 1363117 temperature compensation board that is inside the cluster and is commonly replaced because the temp gauge stops working correctly. That part lists for about $150. But - it's usually not bad, just has a poor connection, and replacing it cleans the connection. So - sure 150 part, 200 labor - maybe???

That board has nothing to do with the gas gauge. So - you see how simple it is to remove the cluster - and the circuitry is simple compared to a newer car - everything is testable with a volt-ohm meter.
The gauges are simple ohmeters, They report (move the needle on the gauge) the variable resistance in the fuel sending unit or the temp sensing unit. And use a voltage stabilizer to keep the gauges from fluctuating as the voltage changes in the car. The temp gauge signal is modified by the temperature compensation board - sometimes just banging on the dash above the cluster or turning the lights on and off will make the temp gauge move when there is a poor connection.

 

the temp compensation board that hoonk refers to can be replaced with a couple jumpers, such that the temp gauge reads linearly like it should, instead of always being forced to dead center so the soccer moms didn't freak out