any auto a/c experts on here?
 

finally got the LS conversion on my '60 Cadillac up and running. charged the a/c and am running into an issue. it will cool down in the vents to 20 degrees with in 20 or 30 seconds and wants to freeze up after a short time. the low side will pull down to 15 lbs after a short time and the high side will not go over 150 or so unless I disconnect the fans at which time it will go to 250 pretty easy and back to 150 within a few seconds when I hook the fans back in. system is a hybrid of things, 1960 evap converted to remove the suction bypass valve and now just has an expansion valve, condenser is 1987 corvette parallel flow, sanden sd7 compressor. right now the cycling switch is bypassed as it cools so fast that it cycles non stop. thinking the expansion valve is the problem? any one ever deal with this?

You're under-charged. Low side should not be getting to 15 psi and freezing up. I believe you need more refrigerant.

it is a big system but it has 60 oz in it now and the sight glass is perfectly clear. dont think I can shove any more in it. it will freeze the outside of the line right after the expansion valve and the suction line coming out of the evap. this is what makes me question the expansion valve not regulating correctly. the low side starts out at around 25 but as it gets colder and colder it will start to drop. according to the static pressure chart it shows I should have around 80 psi at the 75 degree ambient temp and that is almost dead on.

Mis-matched parts may produce odd readings, but assuming you're running 134A, 15 psi is 15 degs F. Way too cold. A low-side press of abt 40 psi would get evaporator temp a little above freezing (35 degs) which is abt where you want to be, maybe a little higher. I'd add some more refrigerant and keep a close eye on the high-side press while doing so..BTW, I've seen high-side pressures of over 300 psi on a properly working 134A system (fan ON).

While charging system, you need the eng RPM's up to what you will see cruising down the freeway (1500-2000 RPMs) Also need a large fan blowing into the grill to simulate doing down the road.

Converting from R-12 to R134A (or using R12 components w/134A) can be done but the efficiency of 134A isn't as good as R-12 so cooling performance may be a bit disappointing. I did the R12/134A conversion on my old Camaro many years ago. I wasn't very impressed with it's cooling performance when temps are 100+ degs outside.

You said it is a hybrid of components. Is there a fixed orifice tube in a line you don’t know about and conflicting with the expansion valve?

no orifice tube, I put it all together so sure on that. unfortunately I am working by myself and it is a fly by wire so hard to get the rpm up and watch everything at the same time. dont need the fan in front as the cooling fans are too good. if I disconnect them I can get the high side up to 250-300 in a few seconds but it will come back to 150 in under 5 seconds when the fans are back on. I have dealt with hybrid systems before and lots of vintage air units but none have ever worked like this one where is does too much too quickly. I think I am going to switch to a trinary switch so the fans at least wont run till the pressure is up but wondering if it is just flooding the evap due to a stuck open exp valve so everything is just going around and not build any pressures? or a stuck closed one and it cant move enough and that is causing the low low side pressure but the over cooling I have on the condenser is masking the high side pressure. these fans cool alot. sitting at idle they never run more than 30 seconds at the most and the engine temp has never gone over 190 which is the stat in it.

Freezing after the txv would indicate a restriction but freezing after the evaporator woud be low air flow... hmmm

that is what is making me think the txv is stuck or restricted. the freezing after the evap may be after it started freezing the core and restricting air flow thru it. hard to tell when it does what when you are by yourself.

did you pull a good vacume

ran vacuum for about 6 hours. got busy and then went to lunch and forgot it was running. think I am going to change the exp valve as it was put on by the shop that changed the fittings on the evap so who knows what brand of chinesium it is.