Need A/C Expert...2001 Beetle TDI

Hey Guys...
Long time reader, first time poster here on tdiclub. Hoping one of the sage TDI folk here can help me troubleshoot my A/C issue . I've been trying to diagnose my tricky A/C problem for the past few months. I had a new compressor and receiver dryer installed from a competent shop two years ago and everything worked fine at the time. A few months back, the A/C went out abruptly (not gradually) and stopped blowing cold altogether. I've read quite a few threads here (and on other fora) and have been trying to diagnose the issue before taking the car in to a shop for another obligatory A/C recharge (whether that's the problem or not): "We have to make sure your system is charged with the correct amount of refrigerant first before we can tell anything else...", as they always say. Okay, here's some background:

Basic info:
*Both fans come on when they are supposed to
*A/C light on console switch comes on
*Radiator temperature sensor has been replaced
*FCM replaced
*A/C pressure sensor replaced
*All fuses are good--including fuse #16
*System is fully charged--just took it to a shop that added 1/2 pound R134 but said it had enough to run even before they added any.

Now I've established that the compressor does not come on. So to me that means either the compressor/clutch is bad or else it's not getting the proper voltage. I tested voltage coming in to the compressor and I was getting around 5.6V whether or not the A/C switch was on.
At the shop where the A/C was recharged, we had the car on the lift with the engine running and the A/C switch on. The tech got 3V max on one of the compressor wires with some sort of inductive probe that measure voltage using only one lead.

So, looks to me that I'm not getting enough voltage to the compressor to make it kick in. But why?

My only guess would be the console A/C switch in the cabin or possibly bad wiring somewhere? But I don't know how much voltage is supposed to go to the compressor anyway. Can someone tell me how much voltage I should be looking for? I've heard talk of an ambient air temperature sensor somewhere under the dash or behind the firewall which I know nothing about. Not sure my 2001 Beetle even has one of those, but I heard they can go bad.

Any suggestions of how to diagnose the problem from here would be greatly appreciated and could potentially save me a lot of shop time/money!

Thanks!

There is an ambient temperature sensor under the wipers and rain tray that is supposed to protect the system from running if it is too cold outside. My old 2000 Beetle (now belongs to a good friend's daughter) had a weird "a/c cutout issue" and that was it

Cheap part, slight pain in the ass to get to....

P.S. new part does not look like old part, but fit in the electrical connector (but not the mount bracket)

Check your radiator/condenser fan operation NOW
I'd say Dan is expert. Lots more in that thread than just the fan bit.

Rxfire said:

There is an ambient temperature sensor under the wipers and rain tray that is supposed to protect the system from running if it is too cold outside. My old 2000 Beetle (now belongs to a good friend's daughter) had a weird "a/c cutout issue" and that was it

Cheap part, slight pain in the ass to get to....

P.S. new part does not look like old part, but fit in the electrical connector (but not the mount bracket)

Click to expand...

You can also jump that part, which renders its function gone. You just have to know not to turn the A/C on when it is 20 F outside, which would *seem* like common sense, but....

Does selecting defrost override the button on these cars? If so then jumping that sensor is dangerous to your compressor...... (it DOES on two of my other vehicles; even if the AC button is NOT pushed the compressor will run if its warm enough to be safe to do so when the defroster is selected)

No, thankfully the defrost does not automatically engage the compressor... something I hate like poison, as I have cars that do that.

Thanks for the suggestions, guys...
Unfortunately, I think I'm a little more confused than before, now...
So I do have an ambient temperature sensor on my vehicle...thanks for that. However, when I google it, it looks to me that there are supposed to be two ATS's? I know for a fact that I have one down by the bumper behind the grill, I saw that when I pulled the front clip (didn't know what it was at the time):

https://photos.app.goo.gl/kaLa18ml5pC9mgYx1

Okay so you guys are telling me that there's also an ATS somewhere around the firewall behind the rain tray? All I see in the rain tray is my horn and a metal capacitor looking thing (see pic). Is this it?

https://photos.app.goo.gl/fsQSxPf5APG8FgEw1

All the images I see online show a plastic ATC, so I don't think it's that metal thing. So if it's not the metal capacitor looking thing, then I have no idea where my second ATS is. The diagram makes it look like the second ATS is somewhere inside the cabin near the fan shroud (under the dash?):

https://photos.app.goo.gl/LMg4LT7vMOoW7Wt83

For what it's worth, the temperature display in my cabin seems to show the correct temperature. Also, I do remember my compressor kicking in whenever I used to turn on the defrost in the past--whether or not the A/C switch was on. But that was before. Now the compressor does turn on in defrost mode with or w/o the A/C switch on.
Also still curious how much voltage is supposed to be going to the compressor to make it kick in.

Thanks!

The one down in the bumper is not for the HVAC system, it is just for the overhead display by the clock.

The one for the HVAC is in the cowl, near the passenger side strut tower, under the plastic cowl cover.

Thanks, Oilhammer...
So there are two ATS's then...

Is the sensor in the cowl area that you are referring to the same as what I have in my second pic from my last post? That's the only electrical device I saw in that cowl area besides the horn...but it's metal, not plastic.

I live very close to a pick n pull, fortunately, so I can grab one of these over the weekend and swap it out. Just have to be sure I am grabbing the right part.

I have a sensor in the air tube from the air filter to the turbo.
I always thought it was for air intake temp.
Never looked for any others.

wonneber said:

I have a sensor in the air tube from the air filter to the turbo.
I always thought it was for air intake temp.
Never looked for any others.

Click to expand...

That is something entirely different, and has nothing to do with what we are talking about (the HVAC system). That is the Mass Air FLow (MAF) sensor, and while it does have an intake air temp sensor in it, it is not used the same way on all engines. The TDI also has another intake temp sensor, on the charge side, which is integral with the Manifold Absolute Pressure (MAP) sensor, down on the intercooler.

But again, nothing at all to do with the car's HVAC.

oilhammer said:

That is something entirely different, and has nothing to do with what we are talking about (the HVAC system). That is the Mass Air FLow (MAF) sensor, and while it does have an intake air temp sensor in it, it is not used the same way on all engines. The TDI also has another intake temp sensor, on the charge side, which is integral with the Manifold Absolute Pressure (MAP) sensor, down on the intercooler.

But again, nothing at all to do with the car's HVAC.

Click to expand...

I might have been thinking about one on the Patriot I had.
It was in the intake tube not part of the MAF in the TDI.