Cooling fan issues/temperature gauge

[zeroasylum]

Bit of a story so here goes

Car is an Mk4 GOLF 2002 WITH 1.6l engine . Engine code AVU

The car had thrown a fault code with the temperature sender and the fans were coming on straight away when the car started and then not turning off for about 10 minutes after stopping. Also the temperature gauge was staying at zero. The car was not overheating but clearly had an issue.

Diagnosing a faulty sender unit and with some help from others on here with part numbers and anther issue which is not fixed I replace it on Sunday with the Green temperature sensor unit part number 059919501A.

After replacing it I started the car and noticed the following. The fans came on like they had been. And leaving the car on for a while the temp gauge did not move. So I switched it off and inspected the fan relay box on the passenger side inside wing as well as the grey 2 pin thermostat down there. I disconnect the cables and checked for corrosion on the connectors but none was seen. I reconnected everything and restarted the car. This time the fans did not start but the temp gauge did come up and continued to rise to the mid-point around 95 degrees. I thought it was fixed and they would kick in but then no fans came on at all and on switching on the AC to check the fan trigger off the pressure switch they did not come on.

So my conclusion was the relay box was an issues or fan grey thermostat which I assume triggers the fans to come on (please correct me if I am wrong as I have ot consulted the wiring diagram). So back under the car checked cable connections and all were good. I then decided to do and old trick and give the relay box a light tap just encase the relay was stuck.

On restarting the car fans came on and temp gauge rose to the middle. They then shut off when it dropped below the trigger point and back on again when the temperature raised again. AC fans trigger also worked. All seemed to be good.

However in the last few days the fans are working but the temperature gauge will not raise and stays at zero. The car is not over heating but is the issue with a stuck closed thermostat or something else. Are the fans coming on and staying on therefore over cooling?
I think it could be the fan relay or the grey temp sensor (I don’t think the green one would die that quickly) or the thermostat.

Advice welcome as I am not sure now. The car has been taken on several trips over 20k and sat in traffic but is not overheating or displaying any running issues?


Many thanks
Zeroasylum

[epitrochoid]

It might be the thermostat. they are made of plastic, which breaks when it fails and jams open.

[zeroasylum]

Thank you for the reply. I had already diagnosed it when I saw your post but you were right on further investigation it appears to be the thermostat that is stuck open so have a new one including a new housing and going to install it this weekend. I wish the used less plastic. I work on a lot of classic cars and metal housing and hose connections are so much better