DIY overhaul AC?

I am about to replace fans and do an AC overhaul. System is currently empty. I did an engine swap in December and I am hoping the compressor on the replacement engine is good. The old one had been neutered at some point in it's life and had the clutch removed(I think). I was going to do it all at the same time as the engine swap, but I could not get the lines off the AC drier for the life of me, and it was cold, so I gave up in order to finish the swap in a timely manner.

Can anyone point me to a writeup for all the bits and parts I should clean/replace? I have done ac system work in the past, so have a pretty good working understanding of it. Just want specifics to our mk4's. I have a vacuum pump I will put on it to test and prep with.

The parts I have so far are a new AC drier, new expansion valve, set of o-rings, and both fans. What else should I do while I am in there? Probably going to pull the front clip to get the best access.

Thanks.

If it's been open I would replace the condenser. At minimum you want to flush it but they're parallel flow and impossible to completely clean, so you might consider replacing it. There may be plugged tubes in it and there's no possibly way to make sure the flushing material gets through all of them. There is a proper solvent made for this but it's HazMat disposal and you need a circulating machine with filter for it; there are multiple "back yard" options but make sure whatever you use is non-flammable AND will completely evaporate under vacuum without leaving a residue.

Replace the TXV, ALL of the O-rings (they're a fluorocarbon compound -- DO NOT use regular O-rings or they WILL fail!) and flush the evaporator (which you can do in the car with the TXV out if you pay attention. Use Nitrogen or Scuba Air (very dry and clean) in COPIOUS quantities to purge EVERYTHING you flush before you put it back together, then make sure you draw a hard vacuum on the entire system for a good long while before you charge it.

Note that the flushing will remove ALL the oil from those components, so you need the proper amount of the correct oil as well. If you're going to this much trouble I STRONGLY recommend a new compressor; they're not that expensive (IDParts has them) and a new compressor will come with the correct charge of oil for the entire system already in it, so if you've flushed everything else you're all set on that account.

On the dryer DO NOT uncap and install it until JUST BEFORE you hook up the vacuum pump. It has a one-way chemical in it and cannot be renewed once consumed, so once you take the plugs out of the ports you need to install it and pull a hard vacuum on the entire system immediately or you have installed a cylinder that does nothing to actually remove residual moisture.

Note that when you charge the system YOU HAVE TO DO IT BY WEIGHT. This means you need a scale plus the can(s) of refrigerant so you can measure the amount consumed from the last can. To charge it start the car, put the AC on FULL COLD, open all the windows with the fan on FULL. The system will NOT engage until the pressure starts to come up, and will cycle a few times until a decent amount of charge is in it. **ALWAYS** charge a system with vapor; trying to charge with the can upside down (liquid) is potentially dangerous in that the compressor is designed to compress vapor and you can hydrolock it, which will destroy it. Warming the can gently helps (WARM water, NOT hot; never over 100F!)

Finally NEVER work on ANY refrigeration system without GOOD gloves and a good set of full-coverage eye protection (goggles); if a hose bursts or a seal fails liquid refrigerant can be expelled under pressure and will freeze anything it touches on contact. If it gets in your eyes it WILL blind you and the frostbite it will cause can be quite severe.

Excellent information. Really great, but new compressor is $250, and new condenser is $140, and you have just upped this job by $390! I like the idea of a new condenser, agreed that it is a finicky thing, but new compressor is just a big chunk of added change. Current compressor is an unknown, but only 110K miles on it... Will have to think on it.

Any way of assessing the existing compressor for its condition?

@300D -- Not really. What you could do is tip it and pour a LITTLE of the oil out of it, and see what it looks like. It should also rotate with some drag (resistance) but not a lot, and the front pulley should have ZERO play (from the perspective of your fingers) in the bearing.

Be REALLY careful not to get any dirt into the compressor's hose ports; it takes very little contamination to screw it up badly because the only "filter" is the dryer and it isn't much of a filter. Anything that gets in there is going to circulate and "sandpaper" the insides.

The other thing that's impossible to know with a used compressor is how much oil is in it. The best bet is to take the amount for the dryer, evaporator, and condenser (assuming you either flush or replace those) and add that much, but no more. MOST of the oil is and will remain in the compressor; it circulates when the system is running. Being under-oiled and over-oiled are both bad and there's no realistic way to drain (or flush) a used compressor without completely disassembling it, at which point you may as well just buy one since you'll have to change all the seals and such anyway, plus you have to work in a scrupulously clean environment or you're risking destroying it when you put it back in service.

At 100k miles it might be fine but if the donor vehicle had fan trouble then it might have coked oil in it, and if it does then it'll fail eventually and will contaminate the rest of the system (meaning you get to do the job over again.) If you KNOW the donor car's fans were working properly or don't mind tearing it apart again (and buying another dryer) then I might take the risk.

If you aren't super concerned about quality I think most auto parts stores have ac kits that include compressor, condenser, and dryer. Do NOT just recharge from cans. Either buy a cheap gauge set or possibly you can rent a set from an auto parts store. You can't be very accurate just going by low side pressure. Also you have to pull the system under complete vacuum for at least 30 min and then verify it can hold vacuum before adding freon. You can usually rent vacuum pumps from auto parts stores. I'm not sure what there is to fear about getting liquid in the system. When you charge from a bulk bottle you turn the bottle upside down so that you are getting the liquid. You could always install your new parts and take it to a shop to have it recharged.

--Greg

The fear is that the compressor is designed to compress gas, hence the name. If it gets liquid in it things can bend. You know, like getting coolant into your cylinders.

The book has the amount of oil to add for each component changed. One needs to dump the oil out of a new pump and add back the correct amount for each component replaced (or flushed).

You aren't supposed to charge by pressure. The book gives the weight of Freon to be added. I dunno if that's fear and superstition or has some sound technical reason but until I know the rational I wouldn't recommend a newb do it any other way.

You could attempt to calculate the superheat but systems with a TXV confound the attempt because they will adjust the flow rate to maintain the proper superheat over a fairly wide range of charge. The RCV combines with this and then the cleanliness of the evaporator coil comes into it as does airflow through the condenser. In addition you need the dry and wet bulb temperatures at the evaporator inlet (good luck on a car getting anything accurate there) and the ambient temperature at the condenser to calculate it anyway.

In short forget it on a modern car system; you have to charge them by weight if you want to have an accurate amount of refrigerant in them. Modern car A/C systems will cool reasonably well down to about half-charged but don't think this means you can get cavalier about it; refrigerant circulation is responsible for cooling the compressor and there is typically no thermal sensor or thermal cutout inside the case, so what you get when it overheats is coked oil which rapidly destroys it. Over OR undercharging reduces cooling capacity on a TXV-based system. You can get a decent idea of charge state by checking subcooling but again that makes assumptions that can be wrong and as such trying to charge that way is a bad idea.

If you don't live in a place where you get really hot temperatures you may get away with playing fast and loose but if not charge the system correctly if you don't like buying and replacing compressors. Oh, and make sure BOTH fans work properly too.

^^Listen to all these guys.

I had non-working A/C when I bought my car. It took a few years until I got to it just because I didn't need it at the time, but then it started getting hot.

I used OEM parts and replaced the entire system...except...my compressor was OEM and I wanted to keep it that way, so I re-built it. It's not very complicated, but in a nutshell I had a bad RCV (only thing left after all the troubleshooting I did, but I'll leave that out).

I cleaned up all surfaces I could in the compressor and replaced all the seals and gaskets that I could.

Oil is added per component either during recharge or prior to part installation. I chose to add it to each component EXCEPT what was required for the dryer (if I recall) because I didn't want it open to atmosphere for very long prior to installation in the system. The remainder I just added to the manifold hose before charging.

After that it was easy going. Draw a vac for enough time to verify that the system is holding. Then you can re-charge. By weight. I had a mini scale and zero'd the can. The harbor freight manifold kit is a really good price and it has been flawless. I've done the A/C on all of my vehicles and they all blow cold.

KLXD said:

You aren't supposed to charge by pressure. The book gives the weight of Freon to be added. I dunno if that's fear and superstition or has some sound technical reason but until I know the rational I wouldn't recommend a newb do it any other way.

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Yes, I use a scales too, but I like to be able to check the system pressure according to ambient temp. I would use a combination of scales & gauge set.

Maybe someone in the thread know (I'm a total ignorant on AC matters).
Can the N280, the valve in the compressor, be replaced without evacuating refrigerant or losing compressor oil?

pedroYUL said:

Maybe someone in the thread know (I'm a total ignorant on AC matters).
Can the N280, the valve in the compressor, be replaced without evacuating refrigerant or losing compressor oil?

Click to expand...

You need to evacuate the system. You won't lose any meaningful amount of oil, though.

Thank you Oilhammer!

Last question, and sorry for the highjack 300D, to make absolutely sure the N280 valve is bad.

Can I disconnect the N280, put the AC on max and check for voltage at the N280 connector, should I see 12V?

The weird thing is that the fans are not coming on at all at the moment.

If this is a MK4 I'm pretty sure that if the fan control module is out it won't engage the compressor. Also if you have a bad fan (one that is drawling too much amperage) it will take out a new fan control module.

Last year I started to see a code for N280, P2612, but back then the AC worked, and the fans also worked....not this year.

EDIT: You guys were right, I don't have an N280, didn't have P2613. The issue was me badly translating a Hex error code from a tunning box. The code I do have is for the smart actuator position. My AC does not work however.

We seem to have a thread hijack in process here. The A4's A/C system is completely, entirely, different than the A5's. So what applies to one won't necessarily apply to the other. Everyone stay in their correct lane.

pedroYUL, your car (an the questions about the RCV) has nothing at all to do with this thread. The answer is you check it with a scan tool and watch the refrigerant pressure request and change in the Climate control scan data. Again, this does not apply to the A4 cars. They have no such device, no such control module, no such scan data, and no such problem.

Anymore, given the commonality of the RCV failures, I don't even bother. I just evacuate the system, do NOT pull a vacuum on it (as that makes it harder to get the RCV out), replace the RCV, then pull it back to vacuum and recharge the system with the correct amount of refrigerant. Then I make sure that BOTH fans come on shortly after the A/C request signal is sent out and the compressor starts working. Usually within five seconds of the low side line starting to feel cold (the quick underhood test that requires nothing more than my hand).

TDI Greg said:

If you aren't super concerned about quality I think most auto parts stores have ac kits that include compressor, condenser, and dryer. Do NOT just recharge from cans. Either buy a cheap gauge set or possibly you can rent a set from an auto parts store. You can't be very accurate just going by low side pressure. Also you have to pull the system under complete vacuum for at least 30 min and then verify it can hold vacuum before adding freon. You can usually rent vacuum pumps from auto parts stores. I'm not sure what there is to fear about getting liquid in the system. When you charge from a bulk bottle you turn the bottle upside down so that you are getting the liquid. You could always install your new parts and take it to a shop to have it recharged.

--Greg

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Thanks everyone for the helpful info. I am not a complete newb with AC. I have rebuilt two systems previously. My 1995 Toyota T100, which just had rotted lines so was pretty easy, and my 1985 300D which I converted to a modern Sanden system and did a complete flush with. Measured in the new freon with both gauge and scale. Worked fantastic until the 'automatic' selector at the dash stopped behaving itself. Was going to replace and get working again, but then a neighborhood kid backed into the beautiful old girl at high speed and that was the end of that.

I have the Harbor Freight gauge set, which does work pretty well, except the valves are a bit dodgy and require fiddle diddling to make sure everything is working well. I also have a right sized vacuum pump for the 30 minute check.

IDParts is having a nice sale right now. Might do the new condenser route, but will see. Feels wasteful to toss a maybe perfectly good working OEM unit. Although I witnessed the donor car running, I don't know it's history that well. It's engine is working great in my car now though.

My car is an mk4 too. 2004 Jetta wagon BEW with 410k km.

I am a total noob when it comes to AC. I want to make sure I need a new N280 and it is not just a wiring problem.

EDIT: again oilhammer is correct, my mk4 does not have RCV

pedroYUL said:

My car is an mk4 too. 2004 Jetta wagon BEW with 410k km.

I am a total noob when it comes to AC. I want to make sure I need a new N280 and it is not just a wiring problem.

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Then that car has no RCV either, sorry I was under the impression you were talking about something with one of the newer cars.

300D said:

T
I have the Harbor Freight gauge set, which does work pretty well, except the valves are a bit dodgy and require fiddle diddling to make sure everything is working well.

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Which version do you have? They started with the kit that had black o-rings (rubber) in at the connections...then they switched to this plastic washer setup instead of o-rings...this is when I purchased my first set

After taking those plastic washer versions back 2x, they ditched those and brought the original o-ring sealing version back and it's been flawless for me.

Not sure what they were thinking with that fubar...

remember ac clutch is most of the time problem on this cars - you can test it by disconnecting connector at compressor and checking for resistance. It should be around 4.3 ohms

another most frequent problem is FCM - fan control module and I found good solution for that by ordering special fan controller from autocoolguy.com it is little more expensive but will take care of the problem with fans for good. It has soft start for fans and will gradually increase speed of fans as the temperature increases. So no hard on and off fan starts every min or two in hot weather which is what kills fans and fan control modules all the time. There is one writep that I wrote for replacing clutch without removing ac compressor and disconnecting lines. It is on our sister forum for mk4 TDIs.

Jetta2005TDI said:

remember ac clutch is most of the time problem on this cars - you can test it by disconnecting connector at compressor and checking for resistance. It should be around 4.3 ohms

another most frequent problem is FCM - fan control module and I found good solution for that by ordering special fan controller from autocoolguy.com it is little more expensive but will take care of the problem with fans for good. It has soft start for fans and will gradually increase speed of fans as the temperature increases. So no hard on and off fan starts every min or two in hot weather which is what kills fans and fan control modules all the time. There is one writep that I wrote for replacing clutch without removing ac compressor and disconnecting lines. It is on our sister forum for mk4 TDIs.

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Interesting tip on the Fan Control.

The compressor that was attached to the previous engine had been butchered and hacked. So, it may have had the clutch issue in the past. But now that one is out and the AC attached to the 110K engine is in. Would love to save a few bones and try out this compressor to see if it works, but like previous posters have suggested, all the work to clean and flush and perform may be wasted if the thing has issues. Probably going to go the replace everything I can get my hands on route...

I once spent $1200 to have AC fixed in a sweet volvo V70 that I had while living in Philly. And it was totally worth it. This won't be that much, and will be totally worth it. Take today for example. May in Maine. 87 degrees. ***?

Ok then. I took off the front clip, dissembled ac system, flushed everything, and then reinstalled with new compressor, new condenser, new drier, new expansion valve, new seals, and new fans. Drew a vacuum, waited, and it held. Turned vacuum back on and ran for two hours. Finally, turned on car, loaded up 22oz of new refrigerant, and...

Drum roll please...

No nothing. Compressor never kicked on and no fans either.

Ambient air temp about 65 degrees and the high and low side pressures were about even at 85psi each.

So, I found the in-depth ac diagnostic and have read through it several times. Going to get started with it on the car today or tomorrow, just wondering what people think. No condenser and no fans together means that something is putting the full stop on? The pressure sensor? The ambient sensor? Could the fan control module prevent everything from even starting?

Would be great to have a little direction before going through the whole troubleshoot process.

Thanks.

Most-likely places to start are ambient or pressure sensor. Note that the pressure sensor is NOT a switch -- it sends a PWM signal.

Both have to signal "ok" for the system to come on.

Genesis said:

Most-likely places to start are ambient or pressure sensor. Note that the pressure sensor is NOT a switch -- it sends a PWM signal.

Both have to signal "ok" for the system to come on.

Click to expand...

Thanks. Will start with this. Turns out one of my multimeters does have the ability to measure duty cycle.

Measured 18.5% duty cycle on the pressure sensor. Ambient temperature was about 68 degrees. Psi inside the system about 70 psi on both high and low. Seems correct, no?

Tested resistance on the ambient temp sensor, and it read 1.6 k ohms, and then this reading went down when placed in the sun.

So. Seems like both of those are working correctly?

Again, I have no fans and no compressor engagement.

Going to start tackling the fan control module troubleshooting. Unfortunately it’s a bear to get at because of the frost heater that is installed.

Can the FCM prevent prevent both fans from turning on and no compressor engagement? Or is there something other than the pressure switch or ambient temp that can prevent both of these? Fans are new, compressor is new.

I just had my AC repaired, and one thing that was found after replacing a dead compressor and a (possibly not dead) FCM was that the system wouldn't engage more than intermittently.

Shop ended up tracing it to a broken wire in one of the harnesses under or behind the battery, think it had to do with getting the "turn on" signal to the compressor clutch/fans...

Although the AC has not been fixed (with no compressor, nor any fans, ever), I have found at least one glaring problem. The T14 connector at the fan control module is seriously corroded and 12v at the wire is not coming through as 12v at the connectors in at least two spots.

Does anyone have a spare one laying about?

The connector's part number is 6X0 973 717, but searches return just the blank connector with no wires. Do people buy new connectors and re-pin each wire? Is that a thing? Soldering 14 wires at that spot won't be no joy either.

Yes you can actually crimp the pins on. Check with findpigtails.com or connectorexperts.com. If you call either one of these places they can get you the pins you need by application.

Last on my hijack...I had the wrong error code, I do not have error P2612. I was been lazy, taking my tunning box error C0A34 and converting to P...

from here https://forums.tdiclub.com/showthread.php?p=4464392

But that translation did not work in my case. Once I connected VCDS it was obvious.

Now, my AC still doesn't engage. Fans check out themselves, and if I connect pins 1-2 from the radiator thermoswitch connector I do get low speed on both, but nothing when I connect pins 2-3 with the key on. I checked all fuses, both on top of the battery, and inside (5,16,25). Also, I have new FCM and blower resistor pack.

Did I get a DOA FCM?