Radiator Fan/AC Diagnosis?

dbonawitz · July 17th, 2017, 11:21

Looking for some experienced help with radiator fan/AC issue?

I have replaced nearly every single sensor/component trying to diagnose my Air conditioning system. I have referenced the A4 Air Conditioning Control Circuit Troubleshooting guide.

First thing to check is if you get the low speed radiator fan when you turn on the AC switch.
I get nothing.

I have replaced both fans, the Fan Control Module, The fuse panel on top of the battery, the thermal fan switch (screwed in radiator), the coolant temp sensor (back of engine), the ambient air sensor (in cowl area near windshield wiper mechanism). I have even cleaned all the electrical circuits to ground (under battery and cowl area).

I have tested the fan system using a jumper wire. I find that the fans do turn on low (jumper 1-2), and high when the ignition is on (jumper 2&3). So coolant fans and circuits show they should be running properly. All posts lead to a bad coolant temp switch. I bought 2 switches, and tested them in boiling water. They showed continuity near 200°F.

When put in the car, the fans do kick on low speed when the coolant temperature reaches around 99°C.

Yet- When I press the AC button, the fans do NOT automatically turn on low speed.

What is the circuit path that gives the fans power when the AC switch is pressed?
What else could be bad? The AC switch itself? The computer?

steve6 · July 17th, 2017, 12:10

Quote:

Originally Posted by dbonawitz

Looking for some experienced help with radiator fan/AC issue?

I have replaced nearly every single sensor/component trying to diagnose my Air conditioning system. I have referenced the A4 Air Conditioning Control Circuit Troubleshooting guide.

First thing to check is if you get the low speed radiator fan when you turn on the AC switch.
I get nothing.

I have replaced both fans, the Fan Control Module, The fuse panel on top of the battery, the thermal fan switch (screwed in radiator), the coolant temp sensor (back of engine), the ambient air sensor (in cowl area near windshield wiper mechanism). I have even cleaned all the electrical circuits to ground (under battery and cowl area).

I have tested the fan system using a jumper wire. I find that the fans do turn on low (jumper 1-2), and high when the ignition is on (jumper 2&3). So coolant fans and circuits show they should be running properly. All posts lead to a bad coolant temp switch. I bought 2 switches, and tested them in boiling water. They showed continuity near 200°F.

When put in the car, the fans do kick on low speed when the coolant temperature reaches around 99°C.

Yet- When I press the AC button, the fans do NOT automatically turn on low speed.

What is the circuit path that gives the fans power when the AC switch is pressed?
What else could be bad? The AC switch itself? The computer?

Does the a/c system actually kick in without the fans? Have you changed the hvac switches there in the dash?

BobnOH · July 17th, 2017, 12:13

Mk IV Manual AC electrical & cooling fan troubleshooting and repair

dbonawitz · July 17th, 2017, 12:58

Thanks BobnOH & Steve6.
All tests were performed sucessfully as listed in the A4 Coolant_Fan_Testing. The fans are new, and function properly as a system when you bypass the radiator temp sensor, and jumper 1-2 and 2-3.

What the A4 Coolant Fan Testing does not tell you is what to do when it passes all those tests, but the fans till do not work when you press the A/C switch.

Steve6- The A/C system won't kick in because I will need to put refrigerant into the system. The pressure sensor (also new) was replaced too.
But the first thing it states in the A/C is to check the fan operation.

I did have a thought-Could the A/C switch only actually apply power to the coolant fans when enough refrigerant is in the system?

dbonawitz · July 17th, 2017, 12:59

I have not changed the hvac switches in the dash, but that was my next step. Just wondered if I'm on the right track.

steve6 · July 17th, 2017, 13:05

I don't think the fans will kick in unless the a/c system goes into activation mode though, it just stays shut down, fans included?

dbonawitz · July 17th, 2017, 13:36

That would be my question! Exactly.
Do the fans only kick on with the A.C. switch when enough Freon is present?
That would also make an assumption that in the A4 fan diagnosis, that the A.C. system would need to have enough of a charge, which it does not state. Maybe an amendment is necessary?

dbonawitz · July 26th, 2017, 18:58

Hey guys. Finally got to the bottom of it all today!
Turns out the "new" AC high/low pressure switch was bad. Swapped it out with one off my Jetta, and the fans kicked on, and AC compressor kicked on.

So, anyone who may read this in the future:
If you press the AC switch, and it lights up yet you don't get the radiator cooling fans coming on (turning low), make sure to check the AC high/low pressure switch. BTW-the switch CAN be taken out while the AC is pressured up because there is a schraeder valve under the switch.

Thanks again Steve6 and BobnOH for your replies!