A/C Behavior

[ATL_Alan]

First, I want to thank Blinkling and Stevehose for all the assistance as I spent the summer getting my car's cooling system and A/C under control (almost). There was a lot of "one step forward and two steps backward" but it's at a pretty good place now.

That being said.... I have some inconsistent behavior with the AC and Stevehose suggested I put it to the forum; I think I already know the answer but I'll leave it to you to verify.

Yesterday, I ran the AC for awhile to make sure the engine wouldn't overheat (it didn't) and the vent temp went from 80 to around 49 degrees very quickly (less than a minute). A few minutes later the vent temp mysteriously started rising and got up to 60 degrees, so I turned the car off.

Today, I tested it again and this time the AC again went from 80 to 49 degrees very quickly and stayed there, moving between 49 and 50 degrees for a pretty long time before shutting it down.

I was curious to see what the pressures were to see if I could get the temp even lower so I hooked up the manifold gauges and started it up; again the vent temp dropped very quickly and the gauges read 35 psi on the low side and 150 psi on the high side to start. Then the low side dropped to 25 psi and the high side went to around 200 psi. Then both the low side and high side started increasing so that the low side was a little over 40 psi and the high side was headed to 400 psi; the vent temp at that point was 60 degrees, as it was yesterday. So I shut everything down.

The AC setup is that I replaced everything original with a Sanden compressor, 16"X 22" condenser, hoses, fittings and a SPAL electric fan. The only original component is the evaporator and expansion valve, which I had cleaned and tested at a local radiator shop.

Like I said, I think I know the problem (the expansion valve) but before I pull the evaporator (ugggghhhhhhh) again, Stevehose thought I should ask the group for opinions.

 

[m_thompson]

The right low and high side temperatures depend on the ambient temperature. The hotter the ambient the higher the pressures. The pressures are also different for R12 and R134a refrigerant. Since you have a new compressor and condenser I assume that you converted the system to R134a.

Your 35 PSI / 175 PSI pressures are OK for a 75F ambient temperature, but just a little low if the ambient temperature was much higher. If both pressures are lower than normal then it could use a little more refrigerant.

With the 40 PSI / 400 PSI pressures it looks like the refrigerant isn't circulating because something is clogged. It could be the receiver-dryer or the expansion valve is clogged. It could be moisture in the system that froze in the expansion valve. If the evaporator ices up on the outside it can also cause unusual pressures.

 

[ATL_Alan]

Thanks for the input, I appreciate it. I just heard from a friend who does refrigeration and used to be a mechanic and he also thinks it's the expansion valve malfunctioning. Even though it passed a static test at the radiator shop it's apparently possible for them to fail under operational stress (and it is closing in on 50 years old so....).

It looks like I'll be pulling the original evaporator and valve and replacing them. Since the system will be open again I'll replace the receiver/dryer when everything comes back together. At that point everything except the blower fans and control switches will be new and we'll see what happens.

I've asked Blinkling for an evaporator/expansion valve recommendation since he appears to be the resident expert.

 

[sfdon]

Flush your receiver-
new expansion valve-
new dryer-
do NOT buy bmw thermocouple- it does not fit. Buy Napa brand.
Both early and late fan switches are unobtanium.
nows the time to install a modern pusher fan for your condenser and get rid of the 60 year old design unit that the car came with?

 

[sfdon]

My guess is you should let 4 ounces of refrigerant out before you get near the dash.
clean the condenser
make sure the auxiliary fan is working.

 

[ATL_Alan]

Thanks, I have a new SPAL electric fan and fabricated an aluminum shroud between the new condenser and the new radiator so that is working very well. Initially the air was all over the place but now is focused. Earlier in the process, you actually helped me identify the radiator temperature switch that had broken and that's installed and working.

Mercifully, my fan switches and the blower appear to be fine.

 

[sfdon]

Let some gas out?

if your pressures take off and the temp rises, it can be too much refrigerant.