Hello, I'm 3 miles down the rabbithole of A/C retrofitting on my 63 Cadillac. I've become quite familiar with the different retrofit kits offered for eliminating the STV and converting to 134a. The retrofit kit for my 63 uses a thermostatic probe wrapped around the output tube of the evaporator to tell the compressor when to cycle on and off by measuring the tube temperature. This seemed crude. I set off to explore options to convert this to using a digital pressure switch the way modern systems cycle the clutch. There seems to be two switch types - a clutch cycling switch (what I need) and a safety switch that only cuts off at very low or very high pressures. Question: is it worth it to switch to a digital switch? it seems more robust and precise. if so, where would the switch actually be placed? Seems like I can stick it on a service port on the STV housing, or put it on the output of the condenser. Or am I totally lost?
Been a long time since I messed with trying to understand AC systems...but... there were generally two types of systems. The type with an expansion valve, which opens when the temperature of the evaporator gets too high. This was used on most older cars. and the cycling clutch/orifice tube system, which always has the orifice tube open, but turns on the compressor when the pressure of the accumulator gets too high. This was used on GMs starting in the 1970s. So, do you have an expansion valve, or an orifice tube?
I have an Expansion Valve. Sorry if that was confusing. Here's the retrofit kit: https://autoacsolutions.com/product...nator-kit-w-two-12-5-8-female-oring-fittings/ In the description, it says it converts from using the STV to a cycling clutch. But the expansion valve stays in play.
I'm a big fan of "crude". Why do you want to make it more complicated? If you want to make it like a modern system, then take it all out and get a modern system. Although the modern aftermarket systems work nothing like modern OEM systems, so don't expect much improvement.
Totally valid, and perhaps this is what I needed to be told. It seemed like to me the pressure reading from a digital pressure switch would be better than relying on a thermostatic probe wrapped around the output tube. But I've been sent on many fruitless endeavors by such stomach hunches.
When you attach the t-probe to the output tube, tie it tightly(nylon tie-wraps work well), & encapsulate(wrap) the probe & tube portion w/a/c cork-tape(it's a rubberized-tar-type compound mixed w/cork pieces in tape form). Squeeze it tightly around the tube/probe to seal out any air/water/other-heat-cool/etc from making contact w/the probe. Do *not* get any in between the two, as you want the probe to only sense what the tube is doing. So, this means, in no case ever - *never* use dum-dum. It is exactly the wrong compound for this, since it will get in between the tube n probe, insulating them from each other. = a/c doesn't function correctly. I knew a couple "techs" that did this on the buses, as it was easier than properly wrapping the components w/ac-tape. & I got to fix their F-ups & resulting service calls for "bad" ac. & yes, I'd rather use the older mechanical-style of components, than the new or newest E-lectrickery versions. Less to go wrong, rarely have failures, easy to fix. Cheaper, too. Who the F cares if the cycling-rate is 3.5*F instead of 1.5*F. Near impossible to tell any difference. Marcus...
Copy that. I spent $80 on a replacement TXV with a curved sensor bulb just so I can have plenty of room for that t-probe to wrap around the tube and they can be nice and neat next to each other. I think I should stick to the t-probe - its what I have already and many have had success with that kit on the same model. Thanks for the input!
Spent about a third on my career as a Mech Eng designing mobile HVAC systems and never heard of an SVT system control. Haha! But after reading @nrgwizard post, it did jog my memory, and I do recall using a “probe” from thermostatic valves that were placed on the outlet tube of the evaporator, and then wrapped with this black sticky cork-like tape to insulate it. I think the workers called it monkey $hit. lol! Typically, the main reason to control the cycling of the compressor is to prevent the icing up of the evaporator coil. For most “non-electronically” controlled systems, we used a simple AC thermostatic switch. You set the probe between the fins of the evaporator coil, and it was wired in series with the compressor clutch circuit. The probe had an adjustable switch, and again, was designed to prevent the coil from getting below freezing. Once it sensed the temp getting close to 32F (or whatever the set point was), it would “open”, and power to the clutch coil would be interrupted. When it sensed the coil had reached the upper set point (adjustable by the knob), the switch would close, and power was sent to the compressor. Also in series with this thermostat, you typically had refrigerant pressure switches. Usually a low-pressure switch (typical of a low refrigerant charge condition), and a refrigerant high-pressure switch (would trip due to excessive compressor discharge/head pressure). This could be a result of a refrigerant over-charge condition, no or insufficient condenser airflow, blocked refrigerant piping, etc.). On a lot of vehicles, there was also a Wide-Open-Throttle (WOT) sensor that would disengage the compressor clutch under hard acceleration to provide additional power, and to prevent damage to the compressor. Hope this helps a bit. Pic of a TXV with external sensing bulb. Pic of typical automotive style thermostat with de-icing temp sensing probe.
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