AC low is high, high is low

Hello,

On my '00 Jetta TDI, the AC does not work properly, the low pressure is too high (over 70 psi) and the high pressure is too low (90~100 psi). Because of the high pressure on low side, it is difficult to charge the system, sometimes the low pressure gets lower and I used that to manage to get some gaz into it.

I played with it for few days and checked the pressures every evening. The fans are working properly, as the compressor clutch. Sometimes the clutch does not disengage when I turn the AC off, but it does disengage if I turn the motor off then restart it.
On each evening, the pressure is about the same before I start playing with it, so it does not seem to leak.
Tonight I noticed that if I shut the AC off, then on after few seconds, low pressure gets low for a short period of time (~50 psi) then slowly climbs up to 70~80. High side is steady at ~95psi. I tried to add more gaz into it, since I guess the previous one did not completely discharge into the system and it probably is undercharged. I'm using RedTek 12a, so my leakage to atmosphere is human friendly .

Any idea what is going on? Any suggestions as to what to check to confirm what is defective?

It takes much less RedTek than R134a. I don't have the numbers in front of me, but it is less. Seems the pressures are lower too. But, your low-side is definitely too HIGH. (EDIT: your low side pressure is too high. sorry about that.)

I suspect that the RCV may be the culprit. The Refrigerant Control Valve is internal on the Sanden compressor in your 2000 model.

We assume you vacuumed the system before adding the RedTek. And, do you add any oil (PAG)? If so, how did you know how much to add?

I did not vacuumed the system, I do not have the pump, it is my buying list for this summer
I add a can of RedTek Dry to deal with the atmosphere in the system, so far I did not damage any of the AC in my cars with it.

I wanted to check the oil level with the small tool from RedTek also, but I could not get the low side valve to release pressure enough, the valve seems to be sticky, and the tool is not pushing it low enough. The gauge valve pushes it properly, since I have readings. I tried yesterday to add a can of RedTek oil, but since the low pressure is high, it did not suck a lot.

On the RedTek cans, it is written how much ounces it contains, so I use that to calculate how much gaz is getting in, when the can gets fully sucked in...

There are some very good Threads in the Samba.com about RedTek. The gurus there that swear by it, have put together some good info and shared their experiences.

So, maybe you should go there and search RedTek as I've been a member here for over 15 years and have not seen any Threads on the topic.

I will say this, it would seem logical to evacuate the system before introducing any other refrigerant. I really don't think RedTek is compatible with R-134a. I suppose a Google or Yahoo search would provide some good info on the use of RedTek.

Let’s keep the discussion about my issue. I’ll gladly discuss RedTek, I’ve been using it for years, it is not the issue on my car today. When the system is empty, as soon as gas enter it, the low pressure should stay low and this way suck the gas in. I just did that on my MDX and it is working fine today.
Any suggestions about the pressures issue?

The RCV is most likely the culprit as I stated previously.

The AC Compressor clutch not disengaging when you turn off the AC is not related to the RCV, in my opinion, as the RCV is totally mechanical.

I will replace the compressor, as I also think it is the most culprit presently.
For the clutch, my hunch is a sticky relay, I'll see with the new compressor if this behavior continues.

It's almost-certainly the RCV, which can be changed for the cost of the valve, gaskets and (I'd change the) reed valve set too.

Before I bought a new (really reman'd) compressor I'd take the old one off and see what you got in there. I bet the RCV is bad and the rest not, and reman's are a crap-shoot as to what you're going to get. Either way you need a dryer and the means to pull a hard vacuum on the system, plus the correct amount of make-up oil if you keep the old compressor to cover whatever you lose when you pull it.

Or if you replace the pump and drier you need to drain the pump and add back in the specified amount for those two components. If you just install it you'll have too much oil.

I've read of people whacking the back of the pump with a hammer to get the RCV working. When I tried it something shook loose. It sounded like it was full of rocks. Some day I'll open it up to see wuhappened.

I got one here in my garage with that EXACT symptom. But the kid can't be without the car for several days, which IS a risk -- if I take it apart and the compressor is trashed, it's several days to get a reasonable (not "autozone Chineesium") replacement. I can order the RCV, seals and dryer, and IF all goes as expected it's done in ~4 hours, which is fine.

But if I get a "surprise" then the car is down since without the compressor the serpentine belt won't work, so.... yeah.

So it is what it is for now -- when I can get a solid five day window I'm ripping it apart.

The RCV would explain the drop then raise of pressure on low side? Please explain the concept. A link to a RCV to buy on eBay can be provided?

If the RCV is "open" (failed open) then the low side will read high and the high side read low, almost identical. This is the common failure mode of the RCV.

I wouldn't buy one of these from eBAY. You need the valve and a seal kit, as the seals have to be replaced when you open the compressor to replace the RCV.

Try here: https://www.polarbearinc.com/index.cfm/product/405/ex-10067.htm

Note that you also need a new dryer (NOT optional!), some of the correct oil and the seals. You also need a vacuum pump to pull hard vacuum on the system before it is recharged (you HAVE TO get the air out of it, including the moisture in said air, or it WILL fail down the road), and of course refrigerant, and in addition a way to MEASURE the charge (it's by weight, NOT by pressures since this is a dynamic valve-controlled system!) which means you need a means to WEIGH the charge you put in.

So the valve works when the system activates then return to its off position in my case? Why would it move then not? How does it move, by an electrical motor?

It controls the position of the swash plate which determines the stroke of the pistons. Not electrical, never really thought about how it funcfuncfunctions. I imagine it acts like a relief valve maybe?

If you get a momentary drop I figger it's just that the compressor is pumping a little but not enough to maintain a pressure drop across the expansion valve so the pressures equalize or nearly so.

For what it is worth, for the use in my Vanagon "AC System" I removed the RCV. Below are some pics of what you'll see inside the AC compressor. And, for the record, I re-used the gasket. In fact, I opened the compressor two times and re-used the OE gasket. These photos were in 2012, thus, six years later the compressor does not leak and appears to function properly (do at your own risk)







Below is a close-up of the RCV with nut on it.



Below, the RCV is replaced with a 7/16" bolt and nut to function as the OE compressor on a Vanagon.



Below, re-use of the original gasket



Since the RCV in this compressor (from an 02 Jetta TDI) is mechanical and by design and location only controls the flow of refrigerant, I cannot imagine how it might vary the swash plate angle. Edit/Correction: This compressor is a SD7V16 and does vary the swash plate angle.

How does it control refrigerant flow? I don't recall if I've read these are variable displacement or made the assumption.

I just looked at the one I pulled out of the '98. It's an SD7V16, "V" indicating variable displacement from what I read on Sanden's site.

From the little browsing I just did it looks like a fixed displacement pump will have a nut holding the the valves place rather than the RCV.

Okay, after doing a bit of research, it appears the "mechanical" RCV is operated by pressure with a small internal bellows (diaphragm) that senses pressure change. At X pressure it releases some of the pressure from the housing into the intake side of the compressor head which applies downward pressure on the piston(s) causing a change in the angle of swash plate. Evaporator temperature and pressure affects the RCV function. The intent is to keep the AC compressor rotating at continually (no kicking in and out) with constant flow of refrigerant, thus, not annoying the driver/passengers (Duh). Also, it is suppose to be more efficient.

Apparently the mechanical RCV has become obsolete as the electrically controlled RCV seems to be the trendy thing of late. As I understand, the eRCV's operation is sort of in reverse of how the mRCV functions.

(Or, just maybe I have misunderstood it's function entirely, mechanical or electrical.)

Read for yourself: https://macsworldwide.wordpress.com/2013/05/29/variable-displacement-ac-compressor/

Yep. It's basically a pressure-regulating valve. Since it has a spring in it the wear pattern is that the spring fatigues and eventually fails open and then of course the compressor doesn't compress.