Can i add refridgerant to ac on my own? - 1999 Volvo V70

The charging port is located in the larger of the two hoses/lines that come from the AC compressor, the fill port will accept only the right fitting from the charging source hose fitting. Before you add refrigerant you need to locate the leak, the refrigerant doesn't just disappear, it leaks out somewhere.. My guess would be the AC compressor front seal.

add refridgerant

Look back on the firewall for the two a/c lines. One is large and on is much smaller. The larger line is the low pressure line. Follow that line back to the compressor. The low pressure port will be on that line.

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At or near the accumulator canister.

you could ha e a leak in your ac system. listen while your car is running with the ac turned to full blast and see if you hear the compressor cycling on and off. if you dont, hook up a set of ac pressure gauges and see if there is any pressure on the system. if nothing else try adding a can of refridgerant into the system. if unsure how to do so take it to an auto parts store and they can show you where you need to add the refridgerant and probably do it for you.

The AC not working wil not affect the engine coolant temperature. The engine cooling system is a seperate system fromthe AC system on your vehicle.
The AC could simply have a small leak and the refridgerant has leaked down and needs to be replaced. If that is the case you can have what is called a "pep charge" performed and you will have AC working again until it leaks back down. Before you ask, I can't tell you how long that will take because it depends on the sevarity of the leak(s). Might be a couple of days might be a year. That should generally be about $40 or less depending on how much refridgerant they need to add to your system.
Of course you may have AC components that have failed such as a compressor. This will be an expensive repair. There is no way around this. Usually when a compressor goes out, you need to replace the compressor, receiver/dryer, orifice tube, all of the refridgerant, and have the system flushed. This is costly.
Many shops have proper AC equipment and properly trained techs that can peform an AC diagnostics for $40-$50 (in my area) that takes about an hour or so. This will tell you if the refridgerant is low, indicating a leak, or if your compressor or other component is not functioning properly.
At my shop when a customer pays for the Diagnostic and we find that the system is low on refridgerant. We offer to re-fill the system (customer has to pay additional for the refridgerant of course but no more labor) and add a special UV dye to the system. Then I ask that the customer drive the vehicle for a couple of days with the AC running then return. I can then use a special UV light and glasses to find the leak point on the AC system by locating the point where the dye leaked out. This is all included in the initial diagnostic charge and does not take too much time.
From there we make the decision of whether to fix the leak or not. Often times the leak is repaired for minimal cost as the leak ponts are commonly very inexpensive parts.

You have a leak in the ac system, u will have to have the system leak tested and the leak repaired before adding anymore very expensive R134 refridgerent.

it's not suposed too for several minutes this keeps you from starting it with load of ac compressor on

if it does not come back after a few minutes ck refrigerant pressure at low side port (static no compressor engauged) it's gotta be over 10 psi or press switch thinks it's M/T and won't engauge compressor
add refridgerant

highley doubt it because you serpentine belt is on a torque pulley, and you should have added ac refrigerant to a nozzle usually covered by a blue cal about the width of a dime, add the refrigerant to that one

Look on the threaded caps that cover the connections,on top of the caps one should have a L for lowside pressure and the other a H for high side.Add the refridgerant to the low side.If your doing it yourself and buying a ac recharge kit from a parts store or shopping center, the kit comes with the refridgerant and the hose you will need to recharge it.The hose fits only the low side connection on your ac line and willnot fit the high side,so you shouldnt have a problem.Hope this helps you.