If it's like the guage in my 740, it only ever goes about halfway through the yellow area, older cars had a red area which was later removed but normal boost is only about half through the boost area on the guage. If it went all the way to the end the overboost switch would cut out, that's bad. I'm really not sure why they didn't put some numbers on the guage, but Volvo never did use calibrated boost guages so they're not really terribly useful, they only show you relative boost so you have to have paid attention to the guage when things were working right to know if something is wrong.
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