a/c compresser

wondering if an a/c compressor will turn without resistance if it is in good shape. I am not talking the pulley but the compressor itself, it is still on truck[a frieghtliner fl 70] not sure if it has pressure in system,

There's always gonna be at least some resistance (albeit small) because bearings have friction which develops heat even in a new compressor. Of course, with the electric clutch disengaged (which is the case if shes too low on freon) there's far less resistance as its only a couple bearings involved versus if it had Freon and was working in which case youre having to turn a compressor against wayyyyyyyyyy more resistance.

The only way to have no resistance in an AC compressor would be to re route the drive belt (if possible) so its not turning at all and theres no friction or heat or resistance.

A mechanical device such as a compressor or a shaft rotating on bearings is NOT 100" perfect or 100% efficient. The energy you put in is the energy out (that's conservation of energy, you cant create or destroy, only change its from). HOWEVER the say HP (basically Torque X RPM) you put into a gear reduction drive is more then the HP you get out, because some of the energy you put in was wasted as friction heat loss in the bearings. Energy in HP in = energy in HP out PLUS heat energy.

John T Electrical NOT a Mechanical Engineer but still picked up a few fun mechanical tidbits in physics classes, albeit nearly 50 years ago grrrr

A friend said if it turns easy it is no good, but if it is doesn't have Freon should it. Don't want to waste money putting gas in if compressor is junk

On allllllll the ones Ive owned if Freon was low the electric clutch never engages so they indeed turn pretty easy as its only a small bit of bearing friction if alls well in the clutch and bearing assembly. If Freon is present and at the correct pressures and the thermostat calls for cooling, its only then the clutch engages and she indeed turns much harder then. If freon is low there must be a leak and unfortunately its often the compressor grrrrrrrrr. Ive had some that leaked slowly so the compressor clutch never engages but as soon as I hang a can on her she will eventually start to engage and short cycle a bit until theres enough freon and she works fine until she leaks out which may be almost an entire summer if its only a slow leak but otherwise its compressor time............

John T

Sounds like you need to invest in a set of AC test gauges to check if the system does have pressure. If there is no pressure, then you may need to charge the system enough to do a check to locate the leak. (A cheap set from Harbor Freight will suffice for a non-professional.

If the compressor is leaking around the main shaft, the compressor can be re-sealed by a good AC shop or AC parts supplier. Sometimes it is the hoses that leak. Interestingly,I had to replace the hoses on my Ford car just after the warranty expired!

Typically if the refrigerant has leaked out, the compressor will turn with minimal effort. Since it has likely leaked out, look for oily residue around the clutch, slung on the bottom of the hood, fender well, etc. That will indicate the shaft seal is leaking.

The same dirty oil residue on any other component will indicate a leak.

The good thing about the compressor turning easily and smoothly is there hasn't likely been a mechanical failure in the system. Once that happens, it is near impossible to get all the shrapnel out of the system.

Normally after ran some they will turn by hand EZ charged are not a few turns... A new one may need assistance... I would worry more if it did not turn...