Capacity should be around 2.9lbs.

so should I go by the capacity spec or the site glass. it is impossible to measure how much actually got charged.

thx Larry. yes I did fine regarding the re-charging procedure. For (14) oz cans, I think you are right. it would take a little less than 3 cans. but for (12) oz cans that would be 4 cans to get 2.9 lbs. However, since it is not possible to measure the actual amount of freon being pumped-in, should I use the site glass as my indication that it is fully charged - when it is "Clear" and compressor is engaged?

I also noticed that even if the can feels empty by shaking it, there is still some pressure inside when releasing the tapper. so I think there will be some loss somewhere in the process.

yes Larry. I am on R12 system. my car is a 91 560SEL and I was using 12 oz cans. With more than 3 cans used, my site glass is still telling me I am undercharged. there is cooling inside the car, but I feel it still can get better.

How come it is taking so long to empty a can? I am guessing while the car is running, the ac pressure goes up therefore the pressure inside the can might not be high enough to push the gas into the system. this is where the hot water can help. but even then, the can does not get totally emptied.

just by memory,

with engine off, the low side = 90; high side = 120; ambient = 65.

with engine at idle; ambient = 80s; low side = 110; high side = 150

with engine at 2000 rpm; ambient = 80s; low side = 115; high side = 190

these are just my recollection.

The label on a 1989 560 SEL calls 2.9 lbs. The pressures listed in your last post are not ok. They could be ok if the system wasn't calling for cooling at the time the gauge readings were observed. A reading of 30 to 40 psi low and 170 to 200 high is wide range depending on other variables.

I can see the clutch turning when the ac is on. I there a way to isolate the cause one way or the other?

what are old fashioned service valves? and how is it used to diagnose the problem?

I noticed also that the AC compressor stays on (engaged) all the time while the AC system is turned on. It disengages when the AC is turned off. There is cool air inside the car. do you think it may be possible that the compressor is faulty (weak) and is unable to get the pressure on the low side to go down low enough to trigger the pressure switch to disengage the compressor?

Sounds like my logical next move then is to replace my exp valve. I will need to find a shop that can reclaim the R12 first, replace the exp valve, evacuate then recharge.

does this make sense? any shop you know in the San Jose area where I can have the R12 reclaimed?

I spoke with a local benz shop and with the symptoms that I have, he said it is either compressor, or exp valve or low on freon that could cause the problem. so since my sight glass is still cloudy, I will try and get some more freon to clear-up the sight glass and then observe if the pressures align to spec.

yes I have - (4) 12oz cans. however, the sight glass is still cloudy and with lots of bubbles. we are dealing with old cans here and we are not really sure if the net weight were indeed 12 oz per can. so should I follow the weight via the number of cans or should I follow the sight glass indicator?

after driving for a couple of days after the re-charge, I decided to hook-up the manifold gauge set again to check, and to my surprise the pressure readings are now within limits - Low side = 40-45; High Side = 150 at idle. I am puzzled as can be but am glad I am back on track. thank you for all the help and guidance.

I need to get me a thermometer to measure the vent temps. what pressures do you consider very good? I got those pressures while on idle for about 2-3 minutes after starting the engine. also the fact that the sight glass is still with bubbles(kinda pulsating spray pattern) might also indicate more freon is needed?

Well then use propane. Its even more efficient than R12 really cheap, and compatalbe with R12 mineral oil. Now, you need to do the charge yourself because its illegal for a mech. to do it. Why? Stupid government rules. R134a doesn't work well in a conversion because you need a larger condenser and evaporator due to the lower heat transfer rate of R134a. Propane is only slighly more flameable than R12 but, it will freeze your nads off!

here are some temp readings:
Car in idle, dahsboard outside temp reading = 87F; ac wheel = high; fan = high;
duct center vent = 55F

Car in motion, dahsboard outside temp reading = 75F; ac wheel = high; fan = high;duct center vent = 40F