VW Jetta MK4 AC problems

[gregoron]

Looking for help from anyone here familiar with a 2000 MK4 / A4 VW Jetta's manual (not Climatronic) AC system:

In the last week, my AC would intermittently stop working. Sometimes it would work, sometimes not, either from start or while idle. When it works, it is cold and it does not turn off as long as I have the engine on. When I shut down the engine and turn it on again, sometimes it will work, sometimes not.

I found the following while troubleshooting:

1. With ignition on, fan speed to lowest, AC button on, both cooling fans are not turning. With engine on, and fan speed on, and AC button on, the cooling fans and compressor are still not turning.

2. I checked both low & high pressure of my AC with my manifold gauge with engine off results in 100 psi static pressure. I could only check static pressure because the AC compressor wont turn on.

3. There is a voltage of 3.5v from the AC compressor wiring harness - normal voltage is 9 to 9.5 though.

Given these findings, I've narrowed down the faults to three parts:

a. the high pressure sensor which sends a signal to the Fan Control Module to stop both fans and AC compressor if there is too much refrigerant pressure or not enough pressure in the system is not sending a signal,

b. the Fan Control Module which receives signal from the pressure sensor to power or cut off power to both cooling fans and the compressor is receiving the signal but can't send power to those parts,

c. the temperature control dial in the central stack which would basically not turn the AC compressor on if it's stuck in heater mode - though no heat is coming out of the vents.

Any ideas? I just don't want to throw all three parts at this problem if I can eliminate them one by one first.

 

[Trav]

This year VW Jetta and Beetle had these common problems. You need to jump the fans to make sure they are still good (many are not). Check the A/C relay.
There are two sensors you need to check, the thermo switch for the fans in the radiator and the A/C thermo cutoff switch in the upper hose housing on the engine (its the one closest to the hose).

The fan controller usually goes out because of a bad seal on the bottom. Its not uncommon to have to replace this and the fans together.
I have some good used fan controllers, thermo switches and cutoff switches if you need them.
The ones ending with IIRC (i will look) A or a U are 20 pin and are interchangeable the K is a 24 pin and is not. I have a nice K unit with hardly any miles on it.
I grab them from junkyards when i find a clean low miler just to have around, thats how often they fail now the cars are getting on in years.

Rock has Dorman fan assemblies for cheap money, its not a brand i usually recommend but these are good, no problems whatsoever with the ones i have used and its quite a few.
They fit right, the wiring is correct and color coded properly. Don't buy used, lots of dead or near dead ones!

You can jump the pressure switches to eliminate them.
The SD7V16 Compressor is known for some problems, if you get it working and the pressures are high on both low and high side there is a cheap fix.
This place can pride the parts and answer any questions.

http://www.polarbearinc.com/PBPC/Homepage/product.html

Let me know if i can help you with this or you need some used parts (i am not a yard or retailer just a guy with a few parts in my bin) by PM, these can be a real PITA to diagnose and repair with this problem.

This is for a Beetle but they are all very similar in these years.

http://newbeetle.org/forums/1-8-liter-tu...html#post759625

 

[Finz]

Trav... I think it is simply fantastic how generous you are with you time, knowledge & experience

Thanks for all the help: past, present and future

God bless

 

[supton]

IIRC the fans have to run in order for ac to work. However on my car only one fan still works, but the ac runs just fine.

 

[gregoron]

Trav, thank you very much for your input and offer to help. I'll check those parts you mentioned and PM you if I need any.

The fans are still working as verified when I remove fuse 16 in the cabin fuse box. Have you ever seen the temperature knob in the AC controls in the central dash to fail?

 

[Trav]

I haven't but that doesn't mean it cant happen. Is the A/C button lit when you push it?
If you turn it to heat does hot air come out and room temp air when its turned to cold?

 

[gregoron]

Trav

Yes, the AC light in the button turns on when pressed. Warm air comes out when I turn the temp knob clockwise. I also hear a vent closing when I turn the knob to full cold.

Thanks.

 

[Trav]

It seems all is well on the inside of the car.

 

[Thermo1223]

Originally Posted By: supton
IIRC the fans have to run in order for ac to work. However on my car only one fan still works, but the ac runs just fine.


Until it hits 90F+ and high humidity then it's a little meh.

That is how I noticed my second was dead, then the dorman one burnt out within a week.

So I ended on buying an OEM after that and cold A/C no matter the outside temp.

 

[Trav]

Originally Posted By: Thermo1223
then the dorman one burnt out within a week.

Why didn't you return it under warranty, its not hard to change. I haven't had any troubles with them so far.

 

[gregoron]

Originally Posted By: supton
IIRC the fans have to run in order for ac to work. However on my car only one fan still works, but the ac runs just fine.


Both fans need to run at low speed when AC is on at normal pressure. Both will turn faster at higher AC pressure. If it's really hot and only one fan is cooling your condenser, the high pressure could result in your compressor's safety blow off valve to release pressure. When that happens your compressor might be shot - it might release refrigerant at normal pressure.

I'd fix or replace the faulty second fan.

PS. You'll also get cooler air inside the car in traffic.

 

[gregoron]

Originally Posted By: Trav
It seems all is well on the inside of the car.


Thanks for your advice and the OEM parts. I don't trust 3rd party parts especially the fan control module - I read in forums that it's a hit or miss on new ones made in China.

 

[Trav]

Your welcome. Yes the only good semi aftermarket one is from Stribel which is an OE manufacturer. They are a little cheaper than VW branded but still over $110 bucks.
The Chinese ones are really terrible, you might as well throw the money away.

 

[gregoron]

Ok, I think I fixed the problem. I replaced the G65 high pressure sensor with an aftermarket Alstrom part from NAPA. It's made in Germany. It wasn't working before I bought it and when I installed it, the fans and compressor started working. It's been 2 days now and it hasn't stopped yet.

Now, I'm glad I also have the fan control module plus some switches on order from Trav as my backup parts. The thing with VW's AC system - and any electronic system - is that if one component fails sometimes the entire system still works (ie fans). Then that puts a strain on other components and when you replace the failed part, it's only a matter of time when other parts fail. It's an endless cycle. The way to avoid this is to diagnose the entire system to make sure all are working when one part fails.