A/C Guage Readings?
 

I know the answer to this question has to dael with alot of factors,but what would be a good reading on the low side to see if the system is fully charged? Symptoms: A/C will cool after 10 min. of driving in 85 temp. Getting 60 out of vents,but when outside temp goes higher it takes longer to cool. The system is all original with R12 .

You're exactly right. There are too many variables to determine correct charge by reading low side only. You need to get a manifold set and monitor both sides.

Since you are wise enough to stay with R12 on an R12 system, you can use the sight glass to determine that the system is fully charged.

After the system is stabilized which will be about ten minutes at 1500 to 2000RPM with system set for max cooling, the sight glass should be clear.

Good luck,

Also, I think a 40 degree temp drop from the ambient air temp is considered good. Check the archives on that though, it might be a 50 degree drop.

Thanks Larry,Mike i will check that out today. Thanks to you guys on this forum,the decision to stay with R12 was easy.

I use a criteria of my own choosing. The conditions are easy to set up and are very quantitative if done exactly.

I believe airconditioning to start at a 40deg drop from ambient at the center duct in high blow and at at least 50mph and at equilibrium (meaning it isn't getting any cooler if you drive farther).

If this is what you have then it will drop to at least a 35deg drop when not moving and much less at idle engine speed. If you are at equilibrium and you drop the blower speed the temp drop will increase (the temp will go lower).

Really good AC will drop 50degs under these conditions. of course if the day isn't hot enough the system will regulate (cycle or otherwise) before freezing so one probably can't see a 50 deg drop off ambient on a 85 deg day as it will cycle at 40deg duct temp or higher (most of the time).

Wednesday in San Diego, the . . .
 

temperature was 96F! This is the first time that we have seen such high temps in a very long time. But it was a good time to test my A/C cooling after the compressor was replaced.

Travelling along at 70mph and A/C in 'equilbrium', the temp gauge built into the car's A/C diagnostic system (my page, MENU#15) showed the evap temp cycling from 36F to 43F.

The fan speed is set to apx halfway between MIN and AUTO and is on RECIRC so internal air is being recirculated. A 60F deg difference!

"I use a criteria of my own choosing. The conditions are easy to set up and are very quantitative if done exactly."

To do comparisons exact means exact, center duct and an acurate digital thermometer and we can talk round the world. The evap temp is probably 2-8 degrees colder measured at the evap temp sensor than at the center duct (depending how long the AC has run)

All in all in ain't going to get any colder though as you were into cycling.

We had a hot day (33C/92F) and I tried this on the C230. Evap temp was a constant 4C/40F and the air out of the center vent (Dad's super accurate digital thermometer) was 6C/43F.

So, on the C230, it seems that a close to 50F drop is possible. I think that all of the late model MB's would have that preformance, and possibly less on some older cars. I would think that a 40F drop would be a minimum standard. BTW, our Subaru managed a 50F drop, too.