AC Troubleshooting

Not on a TDI model, but I don't see why this is not applicable to the MkV TDI's. I am currently trying to narrow down what I think may be a failed compressor. I still have the wife's 2007 2.5L Jetta that started blowing hot air last summer. I have gauges on it now, and it has been sporadic while trying to figure out what's wrong. Here's what I know:

• Driver's side fan runs when up to temperature. This is on low speed.
• Driver's side fan ramps up when you move click the AC switch. It seems to kick on every time when I move the selector to defrost.
• Passenger side fan does not turn at any temperature or AC setting.
• It is around 95F outside, and both gauges normalize to ~125 psi with the compressor off.
• Now, with the AC switch on, the low side fluctuates 115-120 psi, with the high side running around 130 psi.
• Earlier today, with the system running, the low side was running around 75 psi, and fluctuating, while the high side was around 150 psi.
• I added maybe an ounce of refrigerant (enough to turn a half-used small can cold on the outside, but no big change in weight), and the vents actually blew cold for about 15 minutes.
• The car has been sitting outside where rats have been in out of the engine compartment. It's an extra vehicle that we have periodically used as a spare. Although maybe not likely, it is possible a rat has chewed something.
• I looked in the under hood fuse panel, and none of the fuses appear burnt. I tested continuity on all the smaller fuses, and the two big green 30A fuses are good. The cables going up the side of the box are not discolored and look fine.

I don't know if it's over or under charged, but I could drain it all out and start over. Can the passenger side fan issue be the same reason my AC is not working. In the past, I have had to change the RCV on my 2009 TDI Sanden compressor. However, this in no way seems like the same issue. That one would blow hot randomly at red lights and go right back cold. This feels like a different issue. I will keep this thread updated with any knowns and answered questions.
Thanks

No matter what, the pressure differential is way to low. You should see 45 or so on low, and high 200's on high on a hot day . . . your 10 to 75 psi delta is showing minimal compression - either RCV or compressor itself. Lack of fan would cause things to go even higher . . . No/low charge would show far less pressure when not running . . . and low side pulling down to low pressure cutout if all else was OK.

- Tim

From my reading on the RCV, I think that it can have different symptoms. as part of troubleshooting, if you have the system evacuated and the amount removed measured, you will know if there is an issue with the amount of charge.

You should be able to call around to some shops and find someone that will evacuate it for you, then you can change the RCV and have it recharged with the proper amount. This is the easiest and best way to do it, though it can be done in other ways as well.

I can have it quickly evacuated at a local shop next week. Anything else I should do to troubleshoot anything before then? I can change the RCV, but this is really doing nothing like the last time I had an RCV fail. My previous one would take about 5-10 minutes for the AC to work, but when it did, it would get fully cold. Then, it would go in and out randomly, or after decelerating. I would also think the gauges would go normal when it worked normal. I can't get a normal reading from the gauges right now. Unless there is some kind of mechanical switch, relay, or something else I'm missing, I'm thinking the compressor is out.

Electrically, it looks like things are working since it's trying to work. Do yo have VCDS? There are some things to check with it that will help diagnose it. If you can get them to evacuate it cheap, I would certainly do that and pull the RCV, inspecting the oil in the pump for contaminates.

whitedog said:

Electrically, it looks like things are working since it's trying to work. Do yo have VCDS? There are some things to check with it that will help diagnose it. If you can get them to evacuate it cheap, I would certainly do that and pull the RCV, inspecting the oil in the pump for contaminates.

Click to expand...

I do. Anything particularly to look at? Thanks for the help.

So, I checked it this morning. With the car in an insulated garage, at 86F, the high and low sides both read about 103 psi. It's a little high, but I don't see that being my problem. I then started the engine and let it run for 10 minutes with the AC switched on (engine temp is reading 190F in the middle). Neither fan will turn. The driver's side was turning yesterday, and it would switch to high when the AC was on. The passenger side would not turn. Now, this morning, neither fan is turning.

I think I need to get the fans right before I dive into the compressor.

The gauges are also erratic. The gauge pressure dropped down to about 40 psi on both high and low. Now, it's up to about 75 on the low, and 110 on the high with the engine running. I'm hoping this is all somehow related to the fans not turning.

You have it backwards. The fan control logic is driven by A/C high side system pressure. No pressure buildup, the fans won't turn on.

By leaps and bounds, the RCVs on these cars are the most likely A/C component to fail. Incidentally, it is also rather easy to replace.

If you want to soothe your mind about the fans you can test them by running the output tests in VCDS under Engine control module.

I pulled this from VCDS.

01-Engine -- Status: Malfunction 0010
2 Faults Found:
006481 - Radiator Fan 2 (V177)
P1951 - 002 - Movement Restricted / Jammed - Intermittent
Freeze Frame:
Fault Status: 00100010
Fault Priority: 0
Fault Frequency: 1
Reset counter: 255
Mileage: 198414 km
Time Indication: 0
Date: 2000.00.00
Time: 12:05:54
Freeze Frame:
RPM: 826 /min
Load: 20.8 %
Speed: 32.0 km/h
Temperature: 91.0°C
Temperature: 61.0°C
Absolute Pres.: 1010.0 mbar
Voltage: 12.827 V
001152 - Coolant Fan Control Circuit 1
P0480 - 004 - Electrical Malfunction - Intermittent
Freeze Frame:
Fault Status: 00100100
Fault Priority: 0
Fault Frequency: 1
Reset counter: 255
Mileage: 200119 km
Time Indication: 0
Date: 2000.00.00
Time: 12:46:38
Freeze Frame:
RPM: 668 /min
Load: 39.2 %
Speed: 1.0 km/h
Temperature: 87.0°C
Temperature: 51.0°C
Absolute Pres.: 1010.0 mbar
Voltage: 11.684 V
Readiness: 0000 1000

I ran the AC for another 10 minutes, and the main fan didn't run at all. However, when I turned it off, it ramped up with the engine off. So, it seems to have a couple problems with the fan circuit.

When I run the output test on the fan in the engine controller, it ramps up to high. I didn't see a test for the cooling passenger side fan.

Newgene said:

When I run the output test on the fan in the engine controller, it ramps up to high. I didn't see a test for the cooling passenger side fan.

Click to expand...

There isn't. These fans are designed to run together at all times. Inside the primary (larger) fan is the controller for the smaller fan.

The smaller cooling fan can fail in two ways; the common issues are that the brushes stick in the small fan or the controller in the primary fan fails. That maaay cause the issue you have also with the fan ramping up after you shut the car off. The fans are fed constant 12V, so conceivably a controller failure could cause the symptoms you are seeing.

Glad that you ran that output test. I apologize if I seemed dismissive of your concerns about the fans.

No problem. I think my next step would be to see if I'm getting 12v to the fan, and/or see if the fan will run with straight 12v to it.

So, you think the gauge pressure and the rest of the AC could be related? I'm thinking I may have a couple problems at the same time.

Conceivably, yes. The HVAC controller may disable the A/C if it doesn't see the cooling fans spool up in response to increasing high side pressure.

Using VCDS, in the HVAC module -> measuring blocks, you can see what the HVAC controller is commanding the A/C compressor to do via the RCV. If I remember right, 0.840A is 100% duty cycle for the RCV. You can also see if there are any compressor shut off codes, I want to say its under fault codes.

Last year, I had to troubleshoot what I thought was a controller issue that turned out to be another bad RCV. I took a pretty deep dive into the HVAC measuring blocks as part of the process. This also happened to happen soon after my primary fan failed and I had to put the repair off for a week....fan issues and a/c issues can definitely be intertwined.

I an of the opinion that high temps perhaps caused by fan issues contribute to the RCV problems.
My first problem was the loss of one of the little runner flappers that seals the fan shroud (one of four).
This lets the air bypass the radiators at a standstill, but allow for flow thorough when moving.
I replaced the flap and the A/C worked for a while longer until the RCV stuck and I replaced it.
If you haven't check those flaps They have to be there for the fans to do any good.
Otherwise they just pull air through the opening and not the radiators.

Thanks for the responses guys. I'll get to checking as soon as I can get it in the air this weekend (at least I'm lucky to have a lift in the garage). The part that has me the most baffled is that the fans and compressor do nothing when the engine is first started. It used to fire right up with the first start of the car. Now, it really does nothing. It's like there is a lockout on the mechanical side from the controller. When it worked fine, it would cool immediately, long before any lack of fan cooling would start to increase air temps in the car.

Newgene said:

Thanks for the responses guys. I'll get to checking as soon as I can get it in the air this weekend (at least I'm lucky to have a lift in the garage). The part that has me the most baffled is that the fans and compressor do nothing when the engine is first started. It used to fire right up with the first start of the car. Now, it really does nothing. It's like there is a lockout on the mechanical side from the controller. When it worked fine, it would cool immediately, long before any lack of fan cooling would start to increase air temps in the car.

Click to expand...

It sounds like multiple failures here, fan and something within the HVAC system. If you do a VCDS scan of the fault codes within the HVAC controller (they won't set a CEL) you'll see whether or not the controller is disabling the compressor. As for the fan operation you describe, it is "normal" when you consider how the fans are commanded on by the ECM in response to high-side A/C system pressure. No pressure buildup = no signal to turn the fans on.

threeshot223 said:

It sounds like multiple failures here, fan and something within the HVAC system. If you do a VCDS scan of the fault codes within the HVAC controller (they won't set a CEL) you'll see whether or not the controller is disabling the compressor. As for the fan operation you describe, it is "normal" when you consider how the fans are commanded on by the ECM in response to high-side A/C system pressure. No pressure buildup = no signal to turn the fans on.

Click to expand...

Thanks. I'll probably get a chance to dig into this further tomorrow. To clarify, is that a regular VCDS auto-scan, or a specific scan run within the HVAC controller? Without the laptop on the car, it won't let me see the options within the 08-Auto HVAC control module. I ran the auto-scan last week, and it only showed the faults within the engine controller for Radiator Fan 2 and Coolant Fan Control Circuit 1.

It's specific within the controller. I tried to play with it too on my laptop but you are right, has to be connected to car to open the module.

Open the controller, then you can play with fault codes, measuring blocks, and all that fun stuff.

Ok, so here is what is going on in VCDS today.

• Ambient temperature is 33C
• No fault codes in HVAC controller
• Small fan never turns
• Evaporator temperature ranges from 24C to 32C, so there is some cooling going on.
• Compressor load varies from 1.2Nm (or lower) to 5.0Nm
• Large fan signal goes from 0% to 34% when the compressor load peaks
• When fan gets up to speed, the compressor load then falls off.
• Lowest evaporator temps logically correspond to highest compressor load

I copied the screen before I loaded the evap temp.

Here's what I'm thinking. The 2nd fan is dead, and unless that is causing a temp shutoff problem, it's unrelated, but needs to be fixed. There is a mechanical issue going on, that is maybe the RCV. I can replace the valve and fan (or wiring), but I'm still trying to get my head around what is actually going on.

Thoughts?

It sounds like you're on the right track. You know the small fan is dead, so replacing it is a no-brainer. With no compressor shut off codes and indications of at least some cooling, you know that the system "works" in that there are no errors or faults preventing the compressor from doing it's magic. In terms of A/C repair items, the most likely problem is the RCV at this point.

They do fail in weird ways. When I had my troubleshooting fun last year, the symptom was that the A/C would work normally if I powered the RCV using a 7VDC power source, but when I had the RCV under computer control it would ramp up to max. amperage and then I'd get a compressor lockout code of implausible RCV activation. New RCV, back in business.