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Discussion Starter · #1 ·
Hi
I would like to pick the collective brain on this issue.

The car overheats in slow or stationary traffic, with or without A/C on, but generally when the ambient temp is high (TX!) Otherwise the whole system works normally.
I have checked all fuses and known voltages as per multiple sources and Bentley. Generally the fans work per normal operation. What I heard once, was a "bird in death throes" under the bonnet and at that point the fans stopped working and the heat runs up very quickly. In this state the fans do not operate at all. After the temperature drops and the whole lot is cooler, then the system returns to normal.

There is some history. Initially the heat caused cycling off and on of the A/C and this killed the compressor.
I replaced the coil.
Fans work at slow and fast speed when jumped from 12V.
I replaced the coolant sensor (green 4 pin)
Coolant good with no leaks
Replaced the pressure sensor on the A/C
Replaced the Fan Control Module
Replaced water pump and thermostat with a timing belt change about 8k miles ago.
The A/C refrigerant was checked and topped up by garage and the same day the car overheated.

I feel that something is breaking down under heat but at a loss of what it could be.

Anyone have ideas for me?
thanks
 

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Not sure if they MK4 is the same (even though I have one), but the Passat uses a 2nd sensor for the fans. It's on the lower radiator hose (it's blue on the Passat).
 

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Discussion Starter · #3 ·
Not sure if they MK4 is the same (even though I have one), but the Passat uses a 2nd sensor for the fans. It's on the lower radiator hose (it's blue on the Passat).
I don't believe they do, but I will double check this, thanks.
 

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Discussion Starter · #4 ·
I do not see any secondary sensor, I know where it lives on my Passat and I don't see it on the Jetta nor in Bentley. What is the chance of the temperature sensor causing this? Theoretically it stops the A/C engaging when cold, so I cannot see this causing the car to overheat.

thanks
 

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The regular temp sensor is the same as the Passat, so I doubt it.
 

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Discussion Starter · #6 ·
TL: DR new FCM was faulty. Lesson for me is to do “subtractive” fault finding on vehicles.

Long story!
My problems started 9 months ago. AC was cycling in the hot weather. Started the internet search and checking things but did not find root cause. At some point the coil died on the compressor. I had already started buying the most likely items and I had already swapped the water pump and thermostat in a TB change.

The first item to arrive was the fan control module (FCM) so I swapped it out. A bit of a PITA location wise so after I swapped it, I left it alone. I was convinced the coil was the problem, but was unsure of the reason for why it had burnt out. That was a very unfriendly swop but I managed and the AC worked.

There were a few occasions that the car overheated when my son was driving but it always started working normally again. The fans just stopped dead and the engine temp rose quickly after that. The list above was were things I did but as the car was working normally except in extreme cases of heat it was difficult to figure what exactly was going on. It took me driving the car to experience exactly what the heck was happening.

PZ that part you mention is there in the radiator! It is a triangular connector with three pins. It is not in any Bentley or Chilton docs for this model car, that I can see and it is well hidden. I had been driving the car and it had displayed the usual symptoms of no fans in the heat AND I WAS HOME... so I got hunting. I was covered in grease from looking and I thought I might as well just add the old FCM back and see. Nearly lost a hand as the bastard started as soon as I had ‘hot’ plugged the original FCM in.
So far had a few days of driving and next week we will see what exactly happens in the heat of the day.

Thanks for your help.

this below was useful too for checking the fans.
OFF INTERNET SOMEWHERE said:
Here's a procedure that should be in the DIY section as well as the Bentley manual:

After testing the thermal switch and motors as outlined in the Bentley manual,
unplug the wiring harness connector--the one with two fat red/black wires and a smaller red wire and red/black wire--from the control module.
Refering to the diagram on the side of the controller, viewable from the left side, jumper the pin in the the connector that corresponds to "30a" (12 volts) on the controller to the pin in the connector that corresponds to "1" on the controller. The fans should run at slow speed.
Then, jumper the the pin on the connector that corresponds to "30a" on the controller to the pin on the connector that corresponds to "2" on the controller. The fans should run at high speed.
The last thing to check is the voltage from the thermal switch to the controller. (diferent coolant sensor on BBW but follow logic) Jumper pins 2 and 3 on the thermal switch connector. This puts 12 volts on the red/orange wire that goes to pin #7 on the other connector on the controller (the one with 13 small wires). Measure the voltage at pin #7. You should read 12 volts. I did.
BTW, the voltage on the pin on the connector that corresponds to the "30b" on the controller also measured 12 volts.
 
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