A friend put his gauges on the low side and it showed only 40psi ( compressor running). Shouldn't this be a lot higher, up around 140 psi with compressor running?
Interesting, no, it should be a bit lower, depending on which refrigerant gas is being used, ( R12/vs/R134 etc.) since we don't know what refrigerant it's using, it should be between 18 to 25 PSI on the low side near the evaporator on average, and less than approximately <150PSI on the high side . in a 70-degree room air temperature.
what is the running vs idle/off pressure on the low-side? ( the "low-side" is the large diameter copper line leading away from the evaporator directly back down to the compressor)