Coolant temperature gauge problem

[Oskar1997]

Hi,

Looking for some advice to a problem I have come across on my 2003 Audi A4 B6 1.9 TDI (AVF).

One day my coolant temperature gauge on the dash kept going from 50°C to 90°C back and forth when the car was at operating temp occasionally to 130°C. 

I opted to change the coolant temperature sensor but when I was changing it noticed the connector was loose and not properly connected to the sensor (sensor basically slipped off with minimal effort and without need to unclip). I changed the sensor + o-ring+ plastic clip. When trying to connect the connector there was no audible "click" which meant the connector was broken. 

I bought a new connector and replaced it by de-pinning the terminals on the old connector and making note of the wire colours and where each one goes in slots numbered 1-4. I plugged the new connector to the new sensor and it was looking great and I thought the problem was fixed.

Now when I drive the car, it gets up to operating temperature as normal. However just before it gets to the 90°C mark the gauge becomes erratic by jumping from 50°C to 90°C and sometimes even to 130°C just like previously. The coolant level indicator also comes on in the middle of my dash intermittently. After the car is at full operating temperature the coolant temperature gauge just sits at 50°C without any movement at all for remainder of journey.

Looking at my OBD2 and monitoring the ECT when driving the temperature is around 85°C-90°C at all times being reported by the ECU but the dash shows 50°C and coolant level light comes on despite there being enough coolant after checking.

The car also drives normal, no rough idle at all cold or warm, normal MPG. No signs of anything when driving apart from the temp gauge and coolant level warning. 

Anyone have any thoughts on this one?

Many thanks in advance

Edited April 10 by Oskar1997

[KevinAudi80]

And how about the condition of the thermostat? The best way is to use a Vag to check the temperature. It could be the sensor, it could be the harness, plus the water pump may not be working. I am betting more on the sensor in this case. 

[Steve Q]

I'd definitely be considering the thermostat 

[Oskar1997]

On 4/12/2024 at 9:36 PM, Steve Q said:

I'd definitely be considering the thermostat 

When I drive with the computer plugged in it shows that the engine coolant temperature gets to normal operating temp in a normal amount of time. Do you think the thermostat would be worth considering despite this?

[Oskar1997]

On 4/12/2024 at 4:32 PM, KevinAudi80 said:

And how about the condition of the thermostat? The best way is to use a Vag to check the temperature. It could be the sensor, it could be the harness, plus the water pump may not be working. I am betting more on the sensor in this case. 

When I drive with computer plugged the engine coolant temperature suggests the thermostat is okay. Normal operating temperature reached in normal amount of time. I have bought a 2nd new sensor just to make sure and same result.

I am worried it could be the harness like you have mentioned. The sensor has 4 wire running from it. Do you know what each wire is for? Could it be that a wire that feeds to ECU is okay and supplying the correct info (might explain normal temperature on the computer plugged in when driving) and another wire supplies info to the temperature gauge in the cabin and it broken somewhere? 

In respect to the water pump, are there any signs to look out for that would indicate its failed?