appatula

With the August heat right around the corner I started troubleshooting my plauged A/C system. I have not touched the refridgerant since buying the car 3 yrs ago and has been correctly converted to R134a during the PO's ownership.
I hooked up my manifold guage set and got a solid 50psi on the high and low sides with the engine off. I also measured the same 50 psi upon starting the car and adjusting the climate control to full AC.
My first thought was that the compressor is shot or is not engaging. I bypassed the KILMA relay and bridged pin 5 (Pos. DC) to the compressor clutch via pin 7. During this quick test I watched the guages quickly rise up to 75psi on both the high and low sides.

At this point, I'm stumped...I need a few pointers as to what to do. I dont want to add refridgerant because 75psi on the low side seems quite high. Also, It seems that the compressor is functioning as there is a clear differential in pressures when it is engaged and disengaged. What are your ideas and or suggestions? I hope I can drive cool for the first time in this car this summer .

E3002.8

50 on both sides indicates to me that you have a low charge in the system. unless you have a contaminated mix of freon which will throw you in a loop cause the numbers wont mean anything at that point. I'd evacuate the system and confirm a proper charge before anything.. and if your compressor is operating normally, the low side should drop below 30 or more depending on what compressor you have.. not rise

appatula

I feel quite confident that the system was converted and filled with R134a correctly long ago, by a MB dealership in my area according to mty receipts.

Would it be safe to say that the compressor is not faulty due to the rise in pressure upon engaging?

What is a normal PSI measurment for the low side while the compressor is not engaged?

What would perhaps cause the low side PSI not to drop in the vicinity of 30PSI after the compressor is engaged?

Thank you for your help!

E3002.8

when your compressor engagues your high side pressure should rise and your low side pressure should drop, and that all depends on what was retrofited itnto your vehicle. if your compressor is a variable displacment type then low side pressure's have to be 29 psi +/- 1 psi. or you can have a cycling compressor which kicks on and off between 15-30 psi(txv setup) or 22-42 psi (orfice tube setup). as for your high side pressure it should rise up to 2 to 2.5 times your ambient temperature for example if it is 100 defrees fairenheit out, your high side pressure should be roughly 250 psi or more ( compressor on). As for why i think you have a low charge, with the system equilized you say 50 psi on your high side withe the compressor not engaged, that means it was roughly 54 farienheit when you had your guage set hooked up (which i doubt it was that cold, but i may be wrong). your equilization pressure's should equal ambient temp, when it's lower then ambient temp it usually indicative of a low charge. most ac systems wont work with a low charge. you could have mostly air in the system and when you kicked on your compressor, it turned into a air pump raising pressures. or as i said,, you may not have a pure r134a which can also throw your numbers off...

appatula

Very well then! Thank you very much for explaning all of this to me. I will evac the system tommorow. Should I assume 25" of mercury over a 5 min period of time should be sufficient for a satisfactory evac? How many ounces of oil/refridgerant should I stock up on to replenish the system after? Thanks again!

E3002.8

you'll wanna hook it up to a recovery station that will measure the amout of freon coming out, the station will also drain the oil so you will know how much comes out. then put the system into a vacuum for about 20 min. leave it in a vacuum and make sure it does not rise otherwise you know you have a leak. if it does not, install the same amount of oil that came out. charge the system with the recomended amount, i usualy go 50 grams over to compensate for the lines. whenever I have ac problems i always confirm that i have the proper charge in the vehicle to start, 95% of the times it's improper charge ( i find at least). Hope this all helps fix your problem, otherwise I asume it's electrical diag which is never fun!

E3002.8

I dont know how much r134a your retrofitted system takes, you may wanna contact the dealer for that info..

appatula

The MB Service manual specifies the capacities at: R12 = 1100g and R134a = 1000g.
I also found that they specify new oil capacity at: R12 120cm^3 and R134a 160cm^3

Does that sound about right?


Thanks!!

E3002.8

sounds reasonable on the r134a, but note with the oil that you dont want too much in there, during evacuation you'll never get all the old oil out hence why i mentioned some recovery stations allow you to measure what you pull out. The most i would put in is about 1 ounce.

appatula

After reading more, the MB service literature stated just exactly what you said! I will be pulling the vacuum with my Robinair cooltech (6-7cfm) so I will not have a way to recover the oil but I will be sure to keep below one ounce of compressor oil when I replenish the refrigerant. Is it important to replace the oil with MB specified compressor oil, or will any R134a type can of compressor oil suffice? Thanks again!

E3002.8

Glad i can help, you can use any type of 134a oil, it's all pretty much the sme stuff. As i mentioned, with any a'c diagnosis you have to start off knowing you have the correct charge in the system, you may wanna consider putting in a leak detection r134a oil, then if the freon does leak out again, you can take a black light to find out were it leaked from.

appatula

Great! After my summer course finals I'll post back with the results and I'll take it from there. I sure hope this is a simple problem due to improper charge, we'll see! I really appreciate your leading me through this, I have learned a lot.