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Matt, Mechanical Engineer
Category: Mercedes
Satisfied Customers: 22895
Experience:  BEng hons Mech engineering, in auto industry 22 years
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I have 1988 420sl and want to replace the viscous fan with

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I have 1988 420sl and want to replace the viscous fan with a kenlowe unit
My question is where do I put the inline coolant sensor, top or bottom hose and what values should it have ie 82-68 degrees c to maintain the engine running temp of 80 ie the thermostat opening temp?
The sensor above is an Intermotor 50100 but there are lots of others with different ranges.
The coolant in the top hose will be at 80 or above or the thermostat would not have opened, what is the ideal temp of the coolant in the bottom hose going back in to engine to soak up the heat?
Many Thanks

I'm assuming that the sensor is to turn the fan on

and the sensor should be in the top hose as thats the exit from the engine now every engine and cooling system is different so you may need slightly different figures but usually you set the fan to come on at 88-90°C

the mechanical thermostat and radiator size is what controls the running temperature of the engine so providing these are in good condition and the fan runs then the engine temperature will stay under control

as the fans should only be switched on when you're travelling below 25 mph

and the typical temperature drop over a radiator is about 15-20°C so 88° into the rad should give 68° on the bottom
Customer: replied 4 years ago.


If the switch is in the top hose it will switch the fan on regardless of vehicle speed and therefore take no account of natural air movement/cooling.

Surely the bottom hose is the more logical position as the fan will only be switched on if the radiator has not done enough without help?

Hello Andrew

I agree that top hose placement will switch the fan on as you say

however its about a speed of response as placing the switch after the rad will mean its reacting once things are too hot rather than pre-empting that the radiator will need more cooling

and what you want to achieve is a stable engine temperature without sudden rises and falls and temperature under and overshoots
Customer: replied 4 years ago.

Okay I accept what you say, the fan I am using is 2 speed unit, I will probably put a lower temp sensor in the top hose for the low speed and a higher temp sensor in the bottom hose for the faster fan speed.

The original viscous coupling is not an on/off switch I believe it slips less/drives more as temp of air through radiator rises, therefore reacting to the situation rather than just overcooling.

Thanks for your thoughts.

Hello Andrew

the original viscous fan unit work in two ways

firstly the viscous unit allows the fan to slip so its not turning at full engine speed ( can't run faster than about 2500 rpm)
and as the engine bay heats up the viscous unit slips less so the fan is more effective as it all heats up

typically a viscous fan unit though is overcooling the engine and costing power to drive
hence electric units that only come on when required are a superior solution
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