Coolant temperature sensor - check
Disconnect the electrical wire from the temperature sensor and measure the resistance between the sensor contacts and ground. At low coolant temperatures, the resistance is very high and becomes smaller the hotter the engine. The temperature indicator on the instrument scale can also be damaged; in order to quickly check it, disconnect the electrical wire from the indicator again and, with the ignition on, short to ground. The arrow of the temperature indicator should sharply deviate to the maximum position. The earth connection should be as short as possible so as not to damage the indicator. Switch off the ignition.
Removing the T4 radiator . Description of the Volkswagen Transporter cooling system .