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Discussion Starter #1
I have problems with Climatronic HVAC (A/C) system in B5 Passat. I was able to diagnose the problem using the Climatronic interface and channel 40 ("Evaporator Temperature - Actual value") says that the evaporator temperature is -51 °C (-60 °F) which is obviously not the correct reading. (Press Fan down, Temp down and Econ simultaneously, switch channels with temp up/down, exit menu: short press Econ.) All the other channels output sensible values except channel 39 does not display degress celcius but some value without unit (I've seen values between 80-110.) and channel 43 always outputs 0 even though the compressor activates which means that low pressure switch has detected pressure.

The A/C compressor clutch engages when toggling AUTO / ECON and both fans do work in the engine compartment but the system barely emits any colder air than the outside temperature.

Where is the evaporator sensor located and how to test it? I know how to use multimeter and I can tear down parts if needed but does anybody know an easy way to access the sensor?
 

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This is the only thing I found about it:
 

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Discussion Starter #4
Here's an explosion diagram of b5 Passat evaporator housing:
View attachment 100019

... and here is the evaporator temperature sensor if I guess correctly (this picture also shows the incoming air temperature sensor with number 23, also known as G89 in some cars, AFAIK):
View attachment 100020

The question is, is this supposed to be accessed from the driver footwell with left hand drive cars or by some other means?
 

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I don't know about left hand drive cars, but it should be able to be accessed from under the cabin filter with a little prying. I've reached the electrical connector from behind the dash on a LHD, but it was not easy.
 

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To access the evaporator sensor, Bentley says to remove the glove box than reach behind heating and A/C unit from below. Turn sensor 90° and pull out. (Bentley p V 87-26, item 14)

But I would first test the resistance of the sensor at the Climatronic connectors.

The sensor common, brown/white wire, is on pins 9 & 12 of the black (12-pin) connector.
The evaporator sensor, brown/blue wire, is on pin 7 of the red (20-pin) connector.
You can compare the resistance to:
Floor outlet sensor, brown/yellow wire, pin 5 of red connector.
Center outlet sensor, brown/red wire, pin 6 of red connector.
(Ref Bentley schematic 59/4 & /5)
 

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Discussion Starter #7
An another picture of the system. The evaporator sensor is #14 in this image and I think it's called G263. The sensor in the previous picture is #28 here and it's actually footwell vent temperature sensor G192.
100025


test the resistance of the sensor at the Climatronic connectors
Do you mean pulling out the climatronic unit from the dash and measuring the resistance from the wires behind it? Is that really easier than just pulling out the glovebox and twisting the sensor away?
 

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My guess is it's probably a wash--I wrote that before consulting Bentley for how to access the sensor. (Some Climatronic components, like damper motors, are very difficult to get to.)

I was fortunate with my Climatronic and never had occasion to remove it, so no personal experience.
 

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Discussion Starter #9
It seems that the Climatronic unit is attached with 4 bolts to the dash and bolts are under a fragile plastic trim cover. The glove box is attached with 5 visible screws and the only electronic part is glove box light. I'm pretty sure the issue is with the sensor so I'll be taking the glove box off in any case with very high probability.

Thanks for the info about the wire colors. Do all the temperature sensors use common ground?
 

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Yes, they all use a common ground. Very likely it's electrically the same as chassis ground, but a separate connection to the Climatronic head unit to avoid electrical noise.

I have had the glove box out a couple of times, no cursing involved, so hopefully the sensor is easy to reach. Good luck!
 

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Discussion Starter #11
Okay, removed the glove box and searched for the evaporator sensor and found nothing. I remove Climatronic unit, too and checked the pin 7 in red connector and it wasn't even wired in the connector! Searching around for more information I found VW Passat (3B) Heating/Air Conditioning - Ross-Tech Wiki and it turns out that our car has software coding 05000 which means that the compressor is "Internally regulated" (whatever that means) and this car does not have separate evaporator sensor. The coding should be 15000 if the sensor is after the evaporator in the A/C housing. In addition, the Climatronic unit self test reported no errors (hold down ECON + RECIRC while turning the key on but do not start the engine, keep holding the buttons until the Climatronic unit display turns on all cells and then wait for the display to show results around 30 s later. Value "444" on the left side of the display means no errors, any other value means some kind of issue, clear codes by holding ECON or exit the display with short press of ECON).

In case the car had climatoric coding 15xxx the sensor should be as displayed in the middle of the image https://a.d-cd.net/jYAAAgEiJOA-960.jpg (white sensor about 10-15 cm up from bottom of the A/C housing and 5 cm towards blower fan from the A/C drain. With the car with coding 05000 the plastic housing doesn't even have an opening for the sensor so VAG has been manufacturing totally different plastic housing for this car. Reaching this sensor (G263) is would have been very easy after removing the glove box. Reaching the G89 sensor that's above blower fan towards to front of the car is pretty hard and even though I was able to touch it, it would have been very very hard to replace it without moving the whole dash due very narrow space available.

The glove box was pretty easy to remove. Two things are worth mentioning: the T20 screws above the hinges are hard to reach without a really long screwdriver. In addition, the top of the glove box has plastic taps next to the 3 screws that were hard to detach. As far as I know, the best way would be to remove the screws (2 at the bottom, 1 per each hinge and 3 at the top of the glove box) and then just carefully pull the box towards back of the car and hope that the top edge of the box does not crack.

The climatronic unit is easy to pull after you've successfully removed the trim plastic which may be hard to do. After that you just unscrew the T20 screws in each corner of the unit and simply pull the unit from the dash. The wiring was really short so be careful not to break anything while pulling the unit. Unfortunately, the wiring sockects have no measurement points so wiring cannot be measured while the unit is working.

In the end, the A/C still doesn't work or has such a low performance that I consider it broken. I guess it's time to take it to A/C specialist because I don't have equipment to check the A/C system pressure with external gauge nor ability to vacuum the system to check for leaks. Hopefully the amount of refrigrant is simply low enough to not cool effectively but still high enough to allow the system to engage the compressor clutch and pass self-test.

Do you know what "internally regulated" A/C compressor means in practice? Is the sensor for internal regulation replaceable or adjustable?

Additional info here: Климат-контроль с CAN от VW Passat B5+ — Volkswagen Golf, 1.6 л., 2001 года на DRIVE2
 

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I know the B6 Audi used a "variable displacement" compressor, but I don't know it that is what they meant by "internally regulated.
 

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Discussion Starter #13
Yeah, here's a very well done video about variable displacement A/C compressor:
(it's about Ford compressor but as far as I know, all work with the same idea). I previously thought that Climatronic units monitored the temperature of the evaporator using a temperature in the housing and using PWM pulses to drive the solenoid in compressor to adjust the variable displacement as required.

However, as it turns out, our car with Climatronic does not have temperature sensor with the evaporator so the Climatronic unit obviously cannot adjust the compressor to acquire any specific temperature for the evaporator.

"Internally regulated" sounds like the A/C compressor would be adjusting the variable displacement to target some specific pressure because it obviously cannot monitor the temperature at the evaporator without a sensor. However, I haven't found any information how to check or adjust this.
 

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Discussion Starter #14
I found more information about mechanical A/C regulation here: Variable Displacement A/C Compressor

I guess our car has similar setup. Basically the compressor has been tuned to use the pressure differential between high-side pressure and low-side pressure as the input for the displacement control. This works because the low-side pressure should depend mostly on evaporator temperature if overall system pressure is correct. I think this kind of setup is very sensitive to the low-side pressure and if any gas has leaked or the system has been overfilled even slightly this kind of setup will work very poorly.

I would have preferred electronic control with an evaporator temperature sensor but it doesn't make sense to retrofit such a system in this old car. I hope A/C specialist can get this to work again.
 

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Discussion Starter #15 (Edited)
Here's an even better article about internally regulated variable displacement compressor: Variable Displacement Compressor: How It Works

Short list of known acronyms:

FDC: Fixed displacement compressor (needs to be toggled on/off by the ECU and will cause performance loss all the time A/C clutch is engaged), not used in Passat B5 as far as I know. This kind of compressor needs to be low enough maximum performance to not cause evaporator to freeze or some other method for avoiding freezing the incoming air.

VDC: variable displacement compressor (broad class for all compressors that can adjust the displacement and therefore the required power to run)

ICVDC: internally controlled variable displacement compressor, a VDC with mechanical adjustment based on the pressure of the low-side. Used in Passat B5 if the Climatronic software coding starts with zero.

ECVDC: externally controlled variable displacement compressor, a VDC where a solenoid is used to adjust the displacement. Here ECU or Climatronic unit monitors the evaporator temperature and sends (usually PWM) signal to the solenoid to adjust the displacement. Basically the idea is to set a target temperature for the evaporator and ask for higher output from the compressor if the temperature is too high. Or reduce PWM signalling to reduce compressor output if evaporator temperature is too low. In addition, the ECU can ask for reduced compressor output if e.g. accelerator pedal has been pressed so that the A/C system doesn't reduce the engine power output during quick acceleration. In addition, the evaporator temperature can be tuned to any temperature: instead of constantly running the evaporator at 4 °C as ICVDC does, the ECU or Climatronic unit can adjust the compressor output to e.g. reach 14 °C if that's enough for getting the cabin temperature to requested temperature. A car with ICVDC or FDC will always run the compressor with "too much power" and instead re-heat the air to required temperature. Obviously this process (cooling too much and re-heating to acquire required temperature) will waste energy compared to simply cooling to requested temperature. Used in Passat B5 if the Climatronic software coding starts with 1 or 2. I'm not sure about the difference between software coding 15xxx and 25xxx but I guess first one (15xxx) is controller by Climatronic unit software and another (25xxx) is controlled by simple relay setup. I have no idea why coding 25xxx would ever be used because FDC or ICVDC would be more sensible implementation if you don't want to implement full software control.
 

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Discussion Starter #16
Additional notes for debugging:
If you have A/C pressure measurement gauge, check the pressure while the engine is off. It should be between 60-120 psi. Start the engine and turn on A/C with LO setting (maximum cooling performance), verify A/C clutch is engaged and measure the low-side and high-side pressures. The low-side pressure should be between 30-45 psi and high-side pressure should be between 100-160 psi. If the low-side pressure is between 50-70 psi instead, the control valve (internally regulated or ECU/Climatronic controlled) is bad, or the compressor is bad. Replacing either one requires removing all refrigrant and replacing the dryer is strongly recommended if you replace the control valve or compressor.
 

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Discussion Starter #17
I got the A/C system serviced and it turns out that they got it working by replacing the refrigerant. About half of the refrigerant was missing. The low-side pressure was around 30 psi before the service and around the same after. The high-side pressure was 100 psi before the fix and 170 psi after the service. The pressure was around 90 psi before the service when the A/C compressor was not running so there probably was some liquid refrigerant still available but not enough. The worker told that there was some leak marks near compressor axle but his opinion was that the leak was not bad enough to warrant replacing the compressor yet. (I assume he was using UV light to check for leak marks but I'm not sure about that.)

It seems that the internally regulated compressor is really picky with the amount of the refrigerant and the valve will not work correctly if system has too little refrigerant even if the pressure regulating valve is okay. I'm pretty sure that the correct replacement compressor is part 8D0 260 805 RX which doesn't seem to have a replaceable regulating valve. As a result, if either the pressure regulating valve or compressor is bad so you have to replace the whole compressor.

In addition, even if the compressor, pressure regulating valve (or solenoid if your car has evaporator temperature sensor) and evaporator all work correctly, B5 Passat may still fail to produce cold air if the flaps inside the evaporator housing have gone bad (which seems to be pretty common). Note that this is different from flap servo not working. See this video for more details:
 
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