My airconditioning system (R-12) does not cool at all. With gauges hooked up, at idle with clutch engaged, I have 5# on the LOW side and 75# on the HI side. With an increase in rpm, LOW side goes to MINUS 5 and the High side to 100#. The system appears to be full of Freon and will not take any additional when opening the valve. Are these indications of a bad expansion valve, or bad valve AND compressor??

Thanks, Bill

My book does not give numbers for a 1985 300SDL. It does say that 1984 through 1986 should be running 24 to 34 on the low side and 230 to 275 on the high side. I would recommend getting the correct charge in there. Use caution running at the low pressure you have indicated - you could possibly ruin the compressor. My suggestion would be to evacuate it, replace the dryer and lube/charge to the correct capacities of both.

__________________
Jim

What is the system pressure when the compressor is off?

Since no "SDLs" were made in 85, it is reasonable to assume that the car is an "SD." Use the 84 thtu 86 numbers. The first SDL was in 86.

__________________
N. Hodges
1994 S350
1987 300 SDL (for sale)

I know I goofed; a senior moment let me add the "L" to my "short" body 126. My static A/C pressures, all systems OFF, are about 50/50 #.

Thanks for the inputs so far.

Bill

300 SD
B250 Van
356C
BRD
Caracal

Static pressure is too low.... should be at least 80-85 psi or >, depending on ambient temperaure. Your refrigerant charge is too low. Are you seeing any bubbles in the R/D sight glass? Or just oil slicks?

As to the inability to charge... is your charging technique proper? What engine RPM are you charging at?

Compressor is likely worn, also. Is this the factory original compressor? At your current refrigerant charge level, you should see a greater dip into the negative on the suction side as the RPM increases above 1500.

1. The fact that your low side will pull into vacuum while the high goes to 100 psi would be a good indication that your compressor is OK, old R-4, maybe not great, but I wouldn't be looking there as a first choice.

2.The high side doesn't go bonkers while the low side is pulling down, so the numbers you give don't point to a bad TXV. Again, might be bad, but I wouldn't be under my dash as a first move.

3. The static pressure means little and even less if we don't know ambient. R12 curve will show psi ~equal to ambient in degrees fahrenheit, even if one drop of liquid refrigerant is present. Static pressure won't fall off the chart until ALL liquid refrigerant has been vaporized.

4. YOU AIN"T GOT ENOUGH FREON.

5.Like they said: have it evacuated (legal stuff), I'd flush (I'm assuming this system is new to you and no evidence of being recently serviced, so current oil charge is a complete unknown), clean and re-o ring, new R_D so I could get a measured amount of oil back in and recharge with R12.

__________________
The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)

Did one more series of checks today; at 58F, static=50/50#, at idle 5 Lo and 75 Hi; increase in RPM yielded minus 10/75. When attempting to add R-12 today, system took one can with resultat pressures of 22/150 at idle and +3/125 at 2000RPM. It appears I have 2 problems; an intermittant restriction AND a leak. Even after adding 12oz, sight glass is full of bubbles but system is cooling (for now) and will not take any more R-12.. My guess now is that the restriction would not let me add R-12 (2) days ago(with appearant clear sight glass)....intermitant expansion valve?? and now that I can add some R-12.....system also has a mystery leak. Off for a dye check and hopefully an inexpensive(relative term with a MB) leak. Will definitely replace valve and dryer at a minimum, plus leaking item.

Thanks for all the help. Bill

I'd say you are still low on gas. A/C systems can behave kinda strange when low on gas. System should take more R12 without any trouble. You are charging with the engine running and the A/C running at max speed, right? Should be able to gas charge (car upright ) into the low presure side. Set the can in a pan of warm water to help encourage the freon out of the can and into your car. I usually shoot for 35 degrees F. on the low presure side. Forget exactly what presure that is with R12, think maybe around 35psi. Do this at around 2000 rpm. Then stuff a thermometer in the air vent and go for a ride. See what outlet temp your get and how much of a split between cabin temp and outlet temp. On a moderate day, I like to have a 20 degree split.

Thanks Bobk; I already did the hot water/increased RPM procedure with the one can I was able to get in the system. Am going to try to add some dye today and see what happens; hopefully the system will take some more R-12.

These may help...


www.motor.com/MAGAZINE/Pdf/041999_03.pdf


www.motor.com/MAGAZINE/Pdf/041999_07.pdf