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Old March 8th, 2018, 18:08   #10
Join Date: Dec 2013
Location: Deville, LA

OP, I had a similar situation with my old '98 Jetta with the AHU. If your diagnostics confirm that you have enough freon in the system with no leaks, here are a few things you can check easily yourself.

The power for the AC compressor comes from the supply, through the blower fan/AC switch system in your dash, through the fan control module, through the ambient air temp sensor in the rain tray, through the temp sensor that's located in the middle of the radiator hose on the top right of the engine, and then through the hi/low pressure switch on the dryer before going to the plug on the back of the compressor.

The ambient air temp sensor and the sensor in the radiator hose can be jumpered by simply unplugging the wires and inserting a paper clip bent into the shape of a "U" to bypass the sensor itself. You're just trying to rule out an open sensor. Take care not to let you jumpers touch anything.

The hi/low pressure switch can be jumpered the same way, but it has 4 wires, not just 2 like the other sensors. Google how to jumper this one and it'll tell which ports to jumper as the are numbered on the plug socket itself.

If any of these sensors are bad (open), your compressor should engage once you bypass the bad one. Again, this process assumes everything else is ok. However, if you are sure everything else is ok, and the AC still doesn't come on, take a look at the main coolant temp sensor. If I recall, it's in the same radiator hose as the other on you jumpered already, but it's closer to the engine and has 4 wires.

This turned out to be my problem. It was reading temp according to the gauge fine, but after giving up for a few days trying to diagnose the AC, I got a MIL for the CTS. When I replaced it, the AC started working again. The circuit for the AC compressor has a wire coming from the ECU, and it's my theory that the AC was being disabled by the ECU due to the failing CTS. Maybe there's a way to ohm this sensor to check it, or maybe there's a soft code stored in the ECU that you can check for.

I found a schematic for all of this by searching "ambient temperature switch" in this forum, but the schematic is gone now because of the photo bucket issues.

Sorry for being long winded, but maybe this will help. Also, I had to rely on my failing memory for all this, so I apologize for any errors in my recollection.
William Bill
-2002 Jetta TDI
-2002 ford F250 7.3L
-1984 Mazda B2200 2.2L NA Diesel
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