The fan is on a viscous coupling. You can see the vanes in the middle. The vanes sense the temp of the air coming through the rad. As the engine works hard, the coolant temp rises, and so does the temp of the air flowing through the rad. The vanes on the fan sense this, and at a given point the fluid in the vanes expands and engages the fan, which until then had been turning freely. It is arranged this way for fuel economy - you lose a few horsepower when the fan kicks in.
The good news is that if you can feel the fan kicking in and out, then at least it's working. The viscous coupled fan doesn't last forever and many old vans have worn out fans but don't know it. In a cool country like the UK you may never notice - then you go to Europe in a heatwave and climb up the alps ... and overheat. The Merc cooling system is designed for all climates and has a lot of spare capacity.
The other issue is the state of your rad. Again, they don't last for ever, and get clogged up with road dirt, and the vanes get damaged by stones etc. The vanes on the Merc rad are quite close together, so tend to easily get clogged. I fitted a new rad last year and it was interesting to see that there was more space between the vanes on the new rad. If you have a dirty rad, then it's efficiency is reduced, and so the temperature will climb quicker, cutting in the fan. It's a positive feedback loop - and each component of the system needs to be in good condition. So inspect your rad, and have it blown out with a high pressure airline from back to front. If a lot of crud comes out, then that's good!
The way to cut down the noise is by insulation. Most classic Hymers need the dashboard sorting out after 20 odd years - it's a common thing. If you can see the windscreen move slightly in relation to the dash when the van rocks from side to side, then it needs doing. There have been plenty of articles about this. When doing this job, it pays to have the new dash well insulated - thick soundproofing on the underside, and good carpet on top. I also have an extra jacket on the inside engine cover. I did mine 2 years ago, and am very pleased with the results. I reckon my van now drives as quietly as practicably possible.
The good news is that if you can feel the fan kicking in and out, then at least it's working. The viscous coupled fan doesn't last forever and many old vans have worn out fans but don't know it. In a cool country like the UK you may never notice - then you go to Europe in a heatwave and climb up the alps ... and overheat. The Merc cooling system is designed for all climates and has a lot of spare capacity.
The other issue is the state of your rad. Again, they don't last for ever, and get clogged up with road dirt, and the vanes get damaged by stones etc. The vanes on the Merc rad are quite close together, so tend to easily get clogged. I fitted a new rad last year and it was interesting to see that there was more space between the vanes on the new rad. If you have a dirty rad, then it's efficiency is reduced, and so the temperature will climb quicker, cutting in the fan. It's a positive feedback loop - and each component of the system needs to be in good condition. So inspect your rad, and have it blown out with a high pressure airline from back to front. If a lot of crud comes out, then that's good!
The way to cut down the noise is by insulation. Most classic Hymers need the dashboard sorting out after 20 odd years - it's a common thing. If you can see the windscreen move slightly in relation to the dash when the van rocks from side to side, then it needs doing. There have been plenty of articles about this. When doing this job, it pays to have the new dash well insulated - thick soundproofing on the underside, and good carpet on top. I also have an extra jacket on the inside engine cover. I did mine 2 years ago, and am very pleased with the results. I reckon my van now drives as quietly as practicably possible.
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