Renault Scenic II fan fails quite often. In some cases you may notice that not all speeds selected from dashboard works, or works occasionally. But most often fan stops working at all. Of course first step to check is fuses. If fuses seem to be OK then the only way to look is at fan speed controller (sometimes called fan resistor).
The easiest and most expensive way is to call service leave them to do repairs. But if you have some engineering blood, you can do it yourself by spending less than hour of your time and probably only couple bucks.
If you decided to repair by yourself than prepare to dig pretty deep in to your car’s panel. How to reach speed controller you can follow this video:
Once you have blower resistor in your hands the first thing I would recommend to do is to make sure that this the source of the problem. Who knows, there might be other reasons why fan does not work. There might be a fan motor failure and so on. To do so, remove fan resistor plastic cover and then short two leads with thicker wire like shown in the picture bellow.
Once you shorted those two pins, assemble the resistor and connect to your car – don’t assemble panel yet. Turn the ignition and your fan should blow at maximum speed always. If you get this result, then you can be sure that fan regulator (resistor) is faulty and you can proceed further.
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You can replace the regulator with new one which cost from $30 to $100 and you still would save lots of cash comparing to what service would charge you. But if you have time and patience, you can try to save even more. You can replace main transistor in circuit which is probably blown. Transistor costs about $2 and if you succeed then your fix would cost practically nothing.
First of all you should remove old transistor which is clipped to radiator. Remove clip and desolder it with desoldering wick (don’t use solder sucker as they may create electrostatic discharge and destroy MOSFET). Use powerful soldering iron as there is quite some thermal mass especially on the middle transistor leg. I have used 100W soldering iron.
When you desolder transistor you can try to test it if it’s faulty. In most cases you can use multimeter. Multimeter should have diode test function and minimum voltage for diode test should be 3.3V. Follow the test procedure here.
Desoldered MOSFET transistor most likely will be HUF75344G3 which is power MOSFET with main characteristics: 55V, 75A and 8mΩ drain-source resistance when open. It comes in TO-247 package. You may replace with same transistor or look for similar. Voltage and current can be higher. Resistance can be same or smaller. For instance IRFP064N is great replacement which comes with same max voltage 55V, same resistance 8mΩ but higher drain current 110A which gives even safer margin.
Before placing transistor to place, first of all apply thermal paste where it touches aluminum radiator. When soldering be sure to prevent transistor from electrostatic discharges. Ideally you should use grounded electrostatic wristband and grounded electrostatic mat. If you don’t have any of these at least try not to touch transistor leads while handling.
After part is soldered and fixed, assemble plastic cover and test it in your car if it works. If fan works as expected, then assemble all panel parts and enjoy the result.