Evap freezing at low fan speeds

[JohnHere]

In re-reading the thread, I'm trying to identify anything we might have overlooked.

You mentioned in your first post that you may have accidentally slugged the compressor by momentarily opening the high-side valve on the manifold gauge set. I don't think that's a concern, though, because at that stage of the cycle, the refrigerant is a gas, not a liquid. It's liquid on exiting the condenser on its way to the TXV, and your high-side gauge tap is normally before the condenser. So I think you can rest easy about that.

I noticed that you didn't mention pulling a good vacuum on the system before recharging it. You probably did, but if not, we might have some non-condensables (air and moisture) in the system that are causing problems. Just a thought.

Lastly, I still wonder about the anti-icing device. If it was working correctly, it seems to me that it would open and shut off the compressor before the low side hit 19 psi or lower, pressures that point to evaporator temperatures well below freezing.

Waddya think?

[bohica2xo]

The refrigeration system is working just fine. It produces predictable pressures & temperatures, and the freeze switch works as advertised.

The only time you saw a 19 psi low side was with the freeze switch disabled with a jumper wire. A useless thing to do.

The clutch is supposed to cycle off to prevent ice formation on the evaporator. It does that.

If there is anything to look at on a 26 year old VW it is the airflow across the heat exchangers, and by extension the electrical system. Corrosion in unsealed high current connectors for all of the fans. Tired fan motors. Problems with the cabin blower speed control / resistor. Low charging voltage from the alternator. Check for voltage drops at the fan motors with the system running.

Your high side pressures are slightly elevated. Reduced condenser airflow will do that. Clean the condenser air side carefully. Make sure the fans are getting full voltage.

[Cusser]

Ambient Temp: 80F

System cools great. ....Problem doesn't occur when rear AC is also running.

Did you add the refrigerant by weight from cans, and assume 12 oz from each? Because there is some loss, and some refrigerant should be used to purge air from the service gauge lines.

80F is pretty low to NOT have evaporator sensor shut off the compressor periodically on most vehicles, at least judging from my older Frontiers.

Many mechanics caution AGAINST having the rear AC shut off when the AC is running on a dual AC vehicle, say that rear AC must be blowing at least on low fan, to prevent liquid refrigerant from getting back to the compressor. See FAQ 35 here http://salemboysauto.com/marksalem/faq.php

[thp]

Thanks for the responses guys.

@JohnHere: Yeah, pulled plenty good vacuum--below 300 micron for an hour. Holds there for ages, so no leaks.

@Cusser: No, I added by weight from a 30lb cylinder of genuine DuPont Suva R134a. I pulled a vacuum in the manifold gage and lines before opening the cylinder into the lines. I did my best to account for the weight of refrigerant in the lines, too. Rear AC has a solenoid valve that closes off the high side when turned off.

@bohica2xo: Blower and condenser fan volumes seem OK, and evaporator coil is clean. The practical problem is that the front freeze switch cycles the compressor pretty rapidly. Maybe 5 or 10 seconds off, 10 or 15 seconds on. Are these systems meant to cycle so rapidly? Maybe I need a switch with more hysteresis?

[bohica2xo]

The short answer is "Yes". Systems can cycle repeatedly without issue.

Do you have Item #16 as shown on page 87-50?

[thp]

Item 16 is the solenoid valve. Yes, it's present, but I suppose I haven't checked to see if it's actually closing or not.

I can look at the differential temperature across the valve to see if it's operating. This is something I was thinking about the other day.

About cycling: Cycling on and off every 6-15 seconds really isn't an issue for the compressor or the clutch? Is that typical for systems with freeze switches?

[bohica2xo]

Rapid cycling can happen with any CCOT system in the right climate conditions. It does not hurt anything, and the compressor / clutch is designed for cycling.

The solenoid valve in your system shunts all of the refrigerant flow to the front TXV when closed. I would expect cycling on front operation with low air flow across the evaporator.

The euro-weasels were probably looking for the least energy consumption with that complicated system design. Adding the solenoid valve allows for a driver only cooling situation while the back of the van is unoccupied. So the compressor, which is large enough to cool the whole van runs only in tiny spurts.

[thp]

Rapid cycling can happen with any CCOT system

It's not an orifice tube system--it's got TXVs.

The rapid cycling is still expected? I checked again and it's cycling every six seconds or so. Seems like an awful lot.

[bohica2xo]

Same compressor, same clutch - either TXV or CCOT. I used the CCOT as a common example of a system that will cycle the clutch for it's entire life.

You seem to be obsessed with the cycling. It has probably done this since new. If it bothers you, turn on the rear system.

.

[thp]

Yes, in fact I was concerned about the cycling. There's a lot of material out there suggesting that rapid cycling is problematic in itself, and symptomatic of larger issues.

But if your experience tells you it's normal, that's good news for me. Thanks for the help.