AudiAMM

Intro:
I'm new here. Helping my father out with his 2001 Audi TT. He said it has overheated on him a few times (car shuts off). I took it the other day and told him I'd take a look at it. I'm not a mechanic by any means, but I do have some experience (replaced head gasket/water pump on Honda Pilot, replaced alternators/temp sensors/thermostats, CV joints/axels, etc..). I feel comfortable doing work, but just need some direction on where to go from here. I have a bluetooth code reader, which has helped w/ diagnostics.

Summary:
The car seems to run completely fine if I turn the HVAC on. With the HVAC on, both radiator fans kick on. Without it, neither fan turns on (at least, up to ~195F coolant temp).
For my first drive, I went 20-30mins across town (~35mph avg speed limits). I had the heater on HI and the fans at full speed. I had no problems at all and coolant temp showed less than 165F pretty much the whole time. If I turn off the HVAC, the coolant temp starts to rise, then I turn the heater back on full and it starts dropping down to ~155F.

- Car has 99k miles
- Both radiator fans run when HVAC is on, neither run when HVAC is off (at least up to ~195F)
- I have not myself ran it hot enough for the car to shut down, so I don't know how hot it got
- I have replaced the coolant temp sensor (The one w/ easy top access, 10 min job)
- I have NOT replaced the thermostat (but I did pick on up and have it on hand)
- My father said it was in for routine service relatively recently when I asked if the coolant had been flushed

Next Step:
Where should I go from here?
- From searching, potential items I've seen: thermostat, water pump, coolant sensor, instrument cluster?, radiator clog
Thermostat seems to be a little more involved than I thought. How likely is it to fix the issues? or is this a quick replacement?

Thanks in advance!

Mister Bally

It seems that the high temperature sensor (two wire sensor, not exactly sure where it is on your engine). The service manual will show you where it is located.
Run a diagnostic scan using Ross-Tech vcds software for verification of the problem.

You can disconnect the sensor and short the two leads using a paper clip and the fans should run full speed. If not, the fan control module (underneath the battery) may be bad. You can bypass certain circuits of it using jumper wires and the wiring diagram (again, in the service manual) to activate the fans.

If the fan control module is bad, only use a genuine part from VW/Audi. The Chinese junk on Amazon will last a few months at most and could burn the car down. Pay the $170.00 for the right part. I'm going with the overheat sensor though.

AudiAMM

Thanks for the reply.

I not 100% sure which one is the "high temperature / overheat sensor". I didn't really get any conclusive results when searching for those terms. I found a few "temp sensor" related parts:
1) Engine Coolant Temp Sensor - This one is the most commonly mentioned temp sensor by far. We did replace this with a new, green one. Also, it has 4-pins, so it doesn't sound like this is the one you're referring to?
It looks like this:


2) Ambient/Outside Temperature Sensor - This is a 2-wire connector that appears to be located behind the bumper, but would that be related to this issue?

The other results are related to the cabin environment.

If I can get narrow down the sensor you're referring to, I can continue w/ the fan/shorting/control module tests.

I don't have access to any diagnostics tools beyond the common OBDII scanner. I'll check with a local tech and get an estimate on diagnostics.

AudiAMM

Update - I was driving home yesterday and the car shut off on me:
- Heater was on full blast during the drive (HI setting and full fans)
- It was a warmer day yesterday (about 70F) and I was on the highway
- I had my OBDII scanner on and the last reading for the coolant temp when it shut off was ~175F
- Temp gauge on dashboard was at 12 o'clock
- Electrical system was on, but no engine power
- Took about 20 minutes before it allowed me to start the engine back up (tried 3-4 times during that period)
- Threw a P0322 code (Engine Speed Sensor (G28): No Signal)
- No "obvious" signs of overheating (no smoke/steam, no burning smell, etc)

Mister Bally

The engine RPM sensor is the one sensor that when failed, causes the engine to not run. Your scan indicates it is bad.
FYI, mine in my D3 A8 with 4.2 V8 (2004 BFM engine code) failed hot only. Cooled down and car would run fine. Just be sure the connector is fine, no corrosion on the terminals etc.

If I were you, I'd replace the sensor.

AudiAMM

I got a new Bosch crankshaft sensor (Bosch 0261210147 Crank Position Sensor) and replaced it this weekend. I ended up taking the oil filter off for easier access and the harness connector on top was a tight squeeze to access, but it's done. Seems to have fixed the problem and I haven't had any issues so far!

Thanks again for your help!

jjt1234

Thks Mister Bally, Good Read!