Audi 2.7 TDI Radiator Fan Control with Auxiliary Heater

Hi, here's my problem:

The vehicle is an Audi A6 4F Avant with a manual transmission, front-wheel drive, a 2.7-liter TDI (BPP) engine, and a Webasto auxiliary heater installed by the factory.

The following happened:
- Traffic jam, air conditioning off, coolant overheating (see also error log), a coolant hose has come loose, resulting in a loss of several liters of coolant.
- Only one error occurred:

00739 - Coolant circulation pump (V50)
001 - Upper limit exceeded.
Environmental conditions:
Error status: 01100001
Error priority: 3
Error frequency: 8
Lost Count: 20
Mileage: 251,189 km
Time: 0.
Date: 2015.04.22
Time: 08:48:35

Environmental conditions:
Temperature: 100.0°C

- The system was sealed, refilled, and tested as follows: The engine was warmed up, partly at idle, partly at higher RPM, and partly through driving. The air conditioning was turned on intermittently. Data was logged in the engine control unit, block 19 (coolant temperature, control of the air conditioning fan and coolant fan), details are in the Excel sheet.
Observations:
1. An increasing coolant temperature does NOT trigger the fan to turn on.
2. Turning on the air conditioning immediately activates ALL fans.
3. The control system is also dependent on the coolant temperature.
4. When the fans are activated, the coolant temperature drops, which means that (some of the) fans are working.
Questions:
1. Is it possible that the fan control is being managed by a different sensor than the one that was logged? Which sensor is it, and where is it located?
2. Ab welcher Temperatur sollte geregelt werden? Würde etwas im Bereich von 85-95 Grad erwarten, aber sicher nicht deutlich darüber erwarten. K"Is this correct?"
3. How can we further narrow down the error?
The complete log file (extracted after the measurement run) is also attached.

Here's the analysis of the Excel file (which seems to be too large for a direct upload).

Regards,
had

Hello,

Have you already checked the most obvious thing, whether the

Quote:

coolant circulation pump (V50)

is working?

Best regards, Rainer.

Hi,

Yes, the actuator diagnosis was positive, and you can also hear the pump running.
The pump doesn't explain why the high temperature isn't triggering the fans to turn on.

Regards,

Had.

It's possible that a dual sensor is also used here for the coolant temperature. These sensors are prone to failure. Check if the values are plausible and that the engine control unit and the instrument cluster are displaying the same temperature!

Was the cooling system properly bled after the coolant loss?

Best regards, Rainer.

Hi,

As far as I know, this dual sensor provides two signals: one to the engine control unit (presumably the signal that is read), and a second signal that is used to control the needle in the instrument cluster. Both readings were consistent and accurate.

Regarding bleeding: Yes, it was bled. However, even air in the system shouldn't cause the observed problem (the temperature sensor correctly indicates a high temperature, but there's no activation). Furthermore, this was irrelevant when the problem first occurred, which led to the vehicle being stranded.

Regards,

Had.

Hi Hadi!

Regarding your last sentence: No, it's not irrelevant, because in the case of a failure, for example, a head or cylinder head gasket could have already allowed air to enter the cooling system beforehand - obviously?

Does the entire radiator become hot with the same temperature when the engine is being warmed up or running hot?
You can also check this quite well with an infrared thermometer, in addition to the finger burn test on the lower radiator hose.
There's likely a sensor located on the lower radiator flange that controls the pump and fan (or contributes to their control). If it remains cold in this area or the sensor provides incorrect readings, the fan won't activate, which is understandable.
Please check this beforehand, if it hasn't been done carefully already.

Best regards from Munich!

Hi,

Okay, that would be consistent with the behavior. I'll see later how this helps me.

Best regards from Gilching.

had

Quote:

Please check this beforehand, if it hasn't already been done carefully.

In addition, the engine features a proper water pump (a mechanical coolant pump) and a thermostat (a coolant regulator), both of which are primarily responsible for maintaining the engine's thermal balance.

The electric pump seems to be more of an auxiliary pump, for example, for operating the auxiliary heater.

Your first task would be to determine what temperatures are present in each location, or, as DieselBär already mentioned, to take measurements.

Possibly the thermostat is stuck, the impeller of the water pump is loose, or the head gasket is damaged... anything is possible.

The V50 still has a valve unit installed. It's worth checking that. As far as I can tell, the unit is located in the water collection tank. There's also a need to bleed air from the heat exchanger in that area.

For older models, it is recommended to replace the expansion tank as well when addressing this issue.

Hey, here are a few points:
This car doesn't have a dual sensor, which means the temperatures displayed by the engine control unit (ECU) and the instrument cluster always match.

At Klima, all the fans are running so that the system can actually cool effectively.

The pump in question is used for waste heat recovery. It is only installed in models with the comfort climate control system. It has nothing to do with engine cooling.

If the pressure becomes too high, the valve in the lid of the expansion tank should open first. This could be due to a defective valve, a faulty hose, or the pressure building up too quickly for the valve to release enough volume.

I need to check if there is a separate sensor for the radiator fan.
However, the logs didn't indicate a compelling reason to turn on the fans. 100°C is not necessarily a critical temperature in a pressurized system.

There is another G83 sensor that measures the temperature behind the radiator.

Hello,

I strongly suspect that I lost water because a clamp wasn't properly installed, causing a hose to detach due to the increased pressure.

"Where exactly is this G83 located? When standing in front of the car, you can see a connector that's roughly in the middle of the radiator from the outside. Is it there? Do you perhaps have a diagram?"

Regarding the temperature: The temperature doesn't seem to be supposed to rise to 100 degrees, at least that's what I see in this temperature entry for the V50. Therefore, I would expect the fans to be activated. However, they are completely stable at 10.3% (at temperatures from 20°C to 100°C).

Regards,

had

A coolant temperature of 100°C is quite normal. The coolant is under pressure, so it won't boil at that temperature.

Best regards, Rainer.

I only have a schematic diagram, and it's not helpful.
However, if it's the water cooler and not the air conditioning unit, I wouldn't know what else could be located there besides the G83.

Hi,

@Christians: Could you please double-check the existence of the G83? I just can't seem to find it.
"In the meantime, I've also obtained the wiring diagrams. The G83 module is included for the 2.7L engine produced from September 2008 onwards (but with a different engine code). However, I can't find a G83 module listed for my car (2007 model, engine code BPP)."
However, the "Parts Guys" already included the sensor in the BPP.

Regards,

Had.