With a bit of research with the help of the VW T4 Forums, I see that the gauges are a bit of a nightmare on my age of van (1994), and they have a mind of their own.
So in an effort to give me some piece of mind, I decided to change a few things in the cooling system of the van. The first thing I changed was the radiator fan switch, as I wasn't convinced the fans were coming on when the coolant temperature got high.
This is how I did it.
What does the fan switch do?
The fan switch is located on the left hand side of the radiator as you stand facing the engine with the bonnet open. Its job is to detect the water temperature and if it is too hot, then the switch will activate the electric fans to help cool the water.
I got the switch from GSF, and it cost about £15. The switch activates at 87 degrees Celsius - which switches the fans on at low speed, and then again at 93 degrees it will switch the fans to fast mode!
I managed to change the switch without putting the radiator into "service mode" - which means removing some of the panels at the front of the van, and swinging the radiator onto the bumper.
The first thing to do is remove the electrical connector. Once this is out the way, I removed the power steering reservoir from its mount, and moved it out the way. This gave a bit more room to access the switch. (see pic)
Replacing the fan switch
I then used a pair of mole grips and locked it onto the switch, applied some pressure and it came loose without to much difficulty. I had a rag and the new switch ready for when I unscrewed the switch.
As soon as the switch came out of the threads, I quickly plugged the hole with a rag, and then grabbed the new switch, removed the rag and screwed in the new switch - all within a few seconds. This minimised coolant loss.
I tightened the new switch by hand, and then nipped it up with the mole grips - job done! All that was left to do was replace the power steering reservoir back in its mount and reconnect the wiring connector onto the new switch.
Testing the fan switch
All I did to test the new switch was run the van up to temperature at fast idle, and wait for the fans to come on - which they did, and then promptly switched off about 30 seconds later, as the water cooled down again.
This is exactly what should happen, the switch should cycle the fans on and off depending on the temperature of the water.
Fantastic result - job done!
Other common problems: Radiator Fan fuses
Sometimes the fans might not be working because of the fuses. This is a common problem too. The fuses are located above the passenger headlight, in a small black box which is about the size of a matchbox.
Even if they look ok, its worth checking the continuity with a multimeter as they can have hairline fractures in them.