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Citroen Fan Resistor Module Repair
A common fault in the heater system on Citroen, Peugeot & Renault vehicles is the so called ‘smart sense’ heater fan module. It uses a computer controlled Pulse Width Modulated signal to vary the voltage applied to the fan motor.
A customer brought in one that was not working – he had replaced it with a new one, but having several Citroen vehicles, wanted to use the repaired one as a spare for the others.
A good visual inspection quickly showed the fault area.
These are the 2 terminals that power the fan motor. As you can see from the size, they carry a lot of current. However, if you look closely, they have been overheated – even to the extent of melting the solder holding them onto the circuit board and actually pulled down from the board!
No doubt arching due to a poor connection occurred and resulted in these burnt, pitted terminals.
It was decided the best course of action was to remove the connector entirely, clean up the terminals (I had no others that would replace them), resolder them and look for any other damage done to the module.
After straightening, cleaning and repairing the terminals, they were reassembled into their connector.
The connector was then resoldered onto the circuit board.
That’s one problem solved, but was any other damage done?
Further investigation showed the power Mosfet was faulty and after removal from the board, evidence was shown on the heat sink. A burn mark indicated that the Mosfet had failed due to excessive heat.
You can imagine the amount of heat that is produced by this Mosfet by looking at the size of the heat sink!
It was decided that a modification was in order. The original Mosfet had the specifications of 55V, 75A. An upgraded Mosfet of 55V, 110A was installed. This should help with higher current loads.
Removal of the thermal compound was quite a task as it had been baked on by the excessive heat.
The Mosfet needed to be removed from the heat sink during soldering as the heat from the iron was dissipated and not allowing the solder to melt onto the board.
Once this was done a good solder joint was completed. After the excess pin length was trimmed and new thermal compound added, the Mosfet could then be clipped back into place.
But the question remained, what caused this fault in the beginning?
A cabin or pollen filter is installed in the heater/cooling system to provide clean air to the passengers inside the vehicle. It has been stated that the air inside a vehicle is actually 10 times worse that fresh air outside the vehicle. This is due to recirculated air inside the cabin that encourages airborne contaminants including fungus spores, dirt, pollen, dust (or any other nasty stuff small enough) that can cause health issues like asthma, allergies, nausea, headaches and dizziness, just to name a few. Therefore regular replacement of the cabin filter is vital for passenger health. However, more is involved – it also is vital for fan motor health! What do I mean?
As in the case with this damaged fan module, the cabin filter had not been replaced for a long time. Due to a blocked filter, the motor had to work harder to draw air into the cabin. This mechanical work is converted into higher current draw and eventually damages the Mosfet. I have even seen cases when the motor itself burns out due to the excessive current and heat being produced.
The customer was advised to check the female side of the connector for damage on the motor terminals as well as the need for regular replacement of the cabin filter.
If you are interested in this repair, you can see a video on my channel following the link below:
This article was prepared for you by Mark Rabone from Australia.
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Note: You can check out his previous repair article below: