Coolant Thermostat and A/C cycling

[Baumeister]

For the past few months, our 2003 New Beetle TDI's A/C has been cycling off. I WAS thinking that the compressor clutch is failing after 140k miles.

2 weeks ago, the red "temp" warning started flashing after 20 minutes or so of driving, turning on the heat full blast kills the light.

SO...one new thermostat later...not only did the overheating issue resolve, but the A/C no longer cycles off and has been working great.

What's the connection?

Only other observed difference now is that the external temp gauge reads a constant 75 degrees F.

 

[gquenstedt]

The a/c adds load to the engine and the cooling system, so if it's overheating, the fan control module shuts off the a/c to reduce load to not make the overheating worse.

 

[Corsair]

tangentially related- recommend also, if not done already, check the operation of both radiator cooling fans. There's a post on here for how to do that. I think the process is (engine off)... turn the key to the "on" position, turn the inside heater /AC fan on, push the AC button. Then look under the hood at the radiator fans- they should BOTH be turning. Any fan(s) found not to be operating... should really be fixed. "things" under the hood, in general, have to work harder when one or both fans isn't operating as it should.

 

[Baumeister]

Thanks, guys
I did check the fans as described and both function well. Confirmed that the old thermostat was bad in a pan of water on the stove...wouldn't open at ALL, even at rolling boil.
I really need to make an effort to do the same check on our Passat 1.8t.