Just sharing a problem I ran into recently with my FC's converted A/C system. Back in 2017, the compressor seized up, and I restored the system with a rebuilt Denso compressor, flushed out all the reused components, new expansion valve, dryer and O-rings throughout, and charged the system with R152A (aka - computer duster gas) as described on another thread on here. It worked flawlessly and cooled as good as the old R12 system did until earlier this summer, when the rebuilt compressor started leaking from the shaft seal.

So I went and purchased an new aftermarket Denso compressor to replace it, put in another new dryer, evacuated and since there were no leaks I decided to charge the system with R134A this time around. Performance was good until the last few weeks, when I noticed it just wasn't cooling that well. So I tested with the gauges, and found it was low on charge. Figured I had a leak, but after evacuating it held a vacuum overnight, so no leaks - WTF? Anyway, since no leaks I charged it back up again with R134A, and cooling was back to normal. Until one very HOT & humid day (90*F+) when I ran the A/C, the cooling performance suddenly fell flat on its face, and when I shut the car off, I can hear a weird hissing coming from the A/C compressor area. Knowing it's got to be a leak, I grabbed the electronic sniff detector to isolate the source of the leak, and found it quickly - it was the compressor's pressure relief valve that blew open. That part is circled in red below in the picture:




So you may be wondering, how the hell does that happen? Answer is simple - the FC A/C wiring only has a switch that cuts off the compressor clutch under LOW pressure conditions, but if your running R134A on a very hot day, the "normal" high pressures the system can generate with R134A (i.e., > 350psi) are higher than what that pressure relief valve can handle (designed for R12), and since there's no electrical cut-off for high pressure conditions, the valve goes POP!

The fix for this is to replace the existing refrigerant pressure switch with one that opens at a LOW and HIGH limits; so a switch that opens at pressures < 20 psi and > 350 psi and is closed between those pressures will prevent that from occurring.

Also, for the Denso compressor pictured, the relief valve is actually part of the manifold, not the compressor itself. So now my repair options are limited to (a) Add in the Low/High pressure switch, AND (b) Either find another good, used manifold & relief valve, OR plug the relief valve opening with a suitable sized bolt & thread sealant

 

When converting from R12 to R134A you need to do the correct charge amount as it will take less R134A.

 

Roger that, I used a similar table for my conversions and charged by weight. But even when accounting for the correct charge by weight when converting to R134A, the pressures the A/C system will normally generate with R134A are significantly higher than what you would see with R12 for any given ambient temperature/humidity condition. My point being that the FC's A/C electrical system doesn't cycle the compressor off if/when pressures go too high, and on a very hot day, you may end up popping the relief valve which is the only protection mechanism for high pressures at that point. So if you're going to convert, you should swap out the pressure switch to an R134A type that cuts out the compressor at low and high pressure conditions.

Odd thing about my relief valve is once it popped open, it vented all the refrigerant out, but if I evacuate/vacuum the system, the vacuum will hold for hours, but if I try to charge it back up with refrigerant, it will start leaking out again from the relief valve. So it's apparently a one shot & done valve.

 

Interesting... Looks like the FSM states that switch is tested at ~32 PSII or above, which is odd since it specs nominal suction at 14-24 PSI. . Never ran into this problem personally, even having done this conversion in the Tucson heat, but I do see that the upper limit is ~324 PSI. Have you spec'd out a replacement switch? Did you charge with the sight glass or manifold gauge?

 

Have never had an issue with 1st-gens when converting to R-134a. High side pressures should not get high enough to blow off a safety, I have never seen over 250psi on the high side....

WITH A WORKING FAN.

It might be a good idea to get an electric helper fan, with a relay that turns it on when the A/C clutch is engaged. Actually, it WOULD be a good idea.

 

Charged it by weight, with a manifold gauge set. I used a table similar to what IRP posted above to figure out how much R134A it needed. Basically the procedure is to weigh each R134A can (after bleeding the yellow manifold hose of trapped air/moisture), and do the math with before/after weighing numbers to figure out how much refrigerant went in when you charged.

The OEM switch is installed on the high pressure side piping, between the condenser and dryer. In the old R12 system it's only purpose in life was to ensure that there was at least enough refrigerant in the system to allow the lubricant to circulate with it, so a spec of being closed at pressures > 32psi on the high side piping is fine. The only way the pressure would drop below that on the high side would be if almost all of the freon leaked out.

With manifold gauges hooked up, and the system fully charged by weight, I've seen high side pressures approach 300psi with the R134A. Being over 30 years old and used on 3 different compressors now (it's part of the manifold piece that gets reused for the FC Denso compressor), I'm guessing my relief valve may have just been a bit weak.

I do have a working auxiliary pusher e-fan that comes on whenever the A/C is switched on - that's a factory feature on all S5 FC's. I also have a big 16" e-fan for the radiator, which is managed by my ECU (AEM Infinity), and it too will come on regardless of coolant temp when I run the A/C, so airflow thru the condenser is not an issue here.

 

Good info thanks ! I’ve been considering adding an electronic fan to my condenser. Running 152A and can get a nice 46-48f from the vents on the highway, however in traffic for long periods it begins to creep to 55-60F until I get moving again. Running oem cooling fan and shroud and engine temps are solid around 192-195 in traffic.

Tampa Florida heat is a burner man.

 

Derp, yeah that would make sense on the high side. Did you happen to select a suitable replacement yet?

 

Not yet, I still need to remove the old OEM switch to figure out what size thread/pitch is used to select one. Basically any low/high pressure switch should work though - I recently restored A/C on my FD, and used a mix of OEM and aftermarket parts. In that case, I got an aftermarket dryer that fits the FD dryer bracket, and a switch that fits on the dryer's switch port. It cuts off the compressor at pressures < 30psi and > 390psi or something close to that.

 

Interesting, my 89 vert doesn’t seem to have that Efan, and it’s stock.
might be a good addition.

 

Yah, my '81 has factory A/C and it has a high pressure cutoff switch as well as a evaporator temperature switch.

I REALLY like the idea of an evaporator temp switch, since the only reason you care about low side pressure is if the evaporator is close to freezing. Don't want it to freeze over. So this means that the compressor cycling is independent of pressure, so they take much MUCH more easily to refrigerant conversions. My '81 blows 40F out the vents with an 80% charge of R-134a.

My '80 and '85 also had temp switches, and the neat thing was that it was actually adjustable on the '85. (Never had the other two apart)

 

All FC's equipped with A/C (factory or port/dealer installed) had a thermal switch on the evaporator. IIRC, it cuts off the compressor at temps just above freezing (~34F) to prevent condensation from freezing & blocking up the evaporator core.

 

the fan for an S5 is Turbo or Automatic only. pretty sure the wiring is there. the stock setup works from the fan switch, but its better if you set it up to have it run with the compressor on

 

Interesting, didn't know the fan was an S5 T2 & auto only thing. In reviewing the FSM wiring diagrams, that e-fan will activate whenever the A/C compressor is running OR whenever the fan thermal switch goes above its set temperature (207*F, IIRC).