Lupo air conditioning issue

[szrdave]

I've been going round in circles for the past few summers trying to get the AC on my car working, I've replaced the air dryer, most of the hoses, and all of the o-rings up to and including the expansion valve, plus the controller in the LH wheel well.

I took the car back at the end of last week to have the system leak tested and re-gassed, it finally passed the leak test! Jumped in and turned the AC on, lovely cool air. Started driving home, and a couple of minutes later...hmm, no longer feels so cool. By the time I got home, AC on or off made no difference.

Went out today, and exactly the same. Starts off nice and cool, after a minute or so it begins to warm up. 

Things I have noticed:

• When the AC button is pressed the motor labours a bit, engine speed doesn't change
• The clutch on the compressor engages, and stays engaged even when the AC has stopped working
• The high speed fan (resistor is blown, so I don't have low speed) comes on when the AC is first turned on, then stops and cycles for a few seconds every couple of minutes

Any suggestions on what I could check next? When the car is outside in the carpark at work and the sun is shining it's like an oven inside, I've stopped using it when the weather is hot.

[mk2]

If it's a clutch based compressor (not the variable flow type), you can quickly tell if the problem is plumbing or electrical. If the compressor is running, you should get a hot discharge pipe and a cold suction pipe. Which means the compressor is ok, the gas level is ok and the libricant level is ok. The radiator fan should be running and the interior blower should be running.

The engine revs should stay the same with AC on or off because the ECU has been programmed to maintain the same RPMs at idle.

The other symptoms show that the AC control system can see that something isn't right. You need to figure out if the problem is electrical or mechanical first.

[szrdave]

Thanks for the reply.

I had another look today, it's definitely a clutched compressor, and the clutch is engaging when the AC button in the cabin is pressed.

Upon first press of the button the discharge pipe gets very hot to the touch and the suction pipe very cold. Both pipes entering the cabin after the expansion valve get cold and cold air comes out of the vents. The radiator fan does not start until after a minute or so, it then cycles on and off with longer pauses than run time.

After a few minutes the discharge pipe loses a lot of it's heat and the suction pipe is no longer as cold, the temperature of the cabin vents is cool rather than cold.

The compressor clutch remains engaged and the radiator fan continues to cycle as above.

I'll drive in to work tomorrow, and see if turning the AC off for 10 mins after the initial start will allow if to run cold again.

[mk2]

Sounds like it might be the expansion valve? Compressor is obviously fine. But i think the radiator fan should stay on all the time (if it's like other vdubs). I've never worked on a Lupo AC system.

There is usually a radiator fan controller module that can fail. Often near the battery (just below). On other vdubs, there is a common hidden fault, where the big battery mounted fuse block has loose internal connections (inside the plastic!)- caused by corrosion.

[szrdave]

 

On 6/24/2024 at 12:25 AM, mk2 said:

Sounds like it might be the expansion valve? Compressor is obviously fine. But i think the radiator fan should stay on all the time (if it's like other vdubs). I've never worked on a Lupo AC system.

There is usually a radiator fan controller module that can fail. Often near the battery (just below). On other vdubs, there is a common hidden fault, where the big battery mounted fuse block has loose internal connections (inside the plastic!)- caused by corrosion.

Well, it seems to have fixed itself!

I had noticed before it was making some whooshing, swooshing and squeaking sounds every now and again. Driving home yesterday I turned the AC on, expecting to get a few minutes of cool, and it just kept going all the way home. No noises now so I wonder if like you say it was the expansion valve, and it has freed itself up.

The radiator fan is still doing the same thing which is strange, need to replace the low speed resistor at some point so will have a look at it then.