BSM (engine bay fuse box) repair.

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My Name: Ozvtr

Posts: 1043
Joined: Wed Jul 10, 2019 3:31 am
Model: C3 2002-2005, Original shape model
Year: 2003 (53)
Engine Size: 1.4 (8v)
Fuel Type: Petrol
Mileage: 80000
Trim Level: Other
Gearbox: Automatic PRND
Engine name: TU3 (75 PS)
Location: Brisbane, Australia.
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Well the friend who bought the second C3 from me said that the blower fan had stopped working. So I gave them a spare fan speed controller to try. The car has auto style air conditioning with an electronic fan controller. These speed controllers are prone to failing and is the usual suspect if the fan stops.
I must add at this point that I think that the design of the air distribution box in the C3 is S#!t. It is very inefficient and tends to block air flow instead of distributing it! Consequently there is a lot of electrical pressure on the fan and it tends, therefore, to draw a lot of electrical current. This tends to damage or cause to fail a lot of the components in the electrical path of the fan. This includes electrical connectors, controllers, switches, relays and resistors! I think there may be a few people here who have experienced this first hand.
However, replacing the controller didn't fix the problem, still no fan. They looked at the fuse in the BSM and attempting to remove the fuse resulted in it disintegrating! They replaced the fuse but still no-go. Ok time for me to have a look.
The electrical power comes from the battery to the BSM, through Fuse #18, through a relay in the BSM and out of the BSM directly to the electronic controller. Hmm simple enough. First port of call, the connector on the controller.
The controller is located on the air distribution box near the drivers left foot.
bsm 016.jpg
bsm 015.jpg

The engine needs to be running for the fan to work, so start the engine. Pull the connector out of the controller and test for 12V with a multimeter. The fan speed doesn't need to be set, power is available as soon as the engine starts.
Well, no power there. Lets check the fuse in the BSM. The fuse is wobbly in the BSM! OK, the BSM has to come out and see whats going on.
bsm 017.jpg
Next pull the BSM apart.
Here is a picture of the circuit board inside the BSM.
What's wrong with this picture?
Hint: have a look at the top right corner, under the relay.
bsm 009.jpg
bsm 010.jpg
bsm 011.jpg
The heat from the current has melted and carbonized the fuse holder! I'm not going to go into Joule's law here but at high current even small resistances can cause massive heat build up and result in damage to components. The circuit fuse is rated at 40 Amps so I would guess the current consumption would average between 20-30 Amps.
How did this happen? I have no idea!! The metal contacts that grip the fuse look intact despite sitting in a puddle of melted plastic.
Is it repairable? Well, let's see.
I laboriously picked out the melted and carbonized plastic bits, stuck to the circuit board. Then de-soldered the metal tangs and cleaned up the area.
bsm 018.jpg
I then soldered the tangs back onto the circuit board.
bsm 019.jpg
bsm 020.jpg
It looks a bit 'naked' but electrically it's the same as one with the holder. It's just that the fuse is not supported but it doesn't have that much mass so it's unlikely to wiggle around.
Next, put everything back together and test it out.
Yep it all works now.
This fuse supplies only one circuit. Typically a 40Amp fuse might service a couple of circuits, telling me that's a lot of current for one device...the fan!
Can this be prevented? Well the only thing I can think is to reduce the fuse rating so that it blows the fuse instead of drawing high current. The only thing is that it might be blowing fuses all the time. :-(
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