Edited by Rick DeMeis
After those so very intelligent Body Control Module (BCM) engineers effectively eliminated most of the electromechanical relays in the modern day BCM, the next task given them was the elimination of the fuse. But is elimination of the fuse a solution in need of a problem to solve? Is it just a case of replacing a simple, but effective, component with a much more complex solution?
Today's BCMs (image below) are a mass of solid state switches and a whole bunch of fuses. In some BCMs there are as many as eight to 12 battery feeds feeding in the range of 60 to 80 loads. Each battery feed is fused.
This means that the BCM loads (lights, door locks, etc.) are driven from sets of drivers each protected by a fuse. There are some other loads that need their own fuses for safety reasons or just because the load current is too high to share. Other BCM's are fused by just one or two fuses (I am told). These modules rely on their solid state switches to provide the "fusing" protection in case of a faulted output.
An example of a Ford BCM
Fuses have been around since Cro-Magnon man. They are simple, take very little technology to manufacture (compared to a semiconductor), and cost pennies. In their simplicity, fuses are designed to prevent the wiring harness from turning into toaster wire in case of a short.
The fuse function is a simple I2R over time relationship. The higher the current the shorter the time to fuse, or open the circuit. The power dissipated in the fuse is proportional to the square of the current through the fuse. When too much power is dissipated by the fuse, it blows open.
The same can be said for the wiring harness it protects. Fuses are perfect for what they do in that they contain similar "fusing" properties as that of the wiring harness they are protecting, just a bit lower in current capability.
I2-t characteristic comparison
When it comes to fuses sticking out of a BCM it is a bit like the three rules to real estate: Location, location, location. If a module has fuses sticking out of it then it needs to be placed in a location that the car owner can access it. That headache then includes routing wiring harnesses and module orientation as well as restricting where on the module the fuses must be placed. All these restrictions and protections add up to cost and manufacturing hassle. The Ford BCM illustrated here had to have a flexible circuit board mounted on CEM3 fiberglass circuit board material to get the fuses on the "edge" of the module.
The insides of the Ford BCM showing the fuse connector array
Car manufacturers have been fairly innovative when it comes to locating these BCMs/fuse blocks. I have seen them under the dash board, under the hood, in the kick panel (right in front of the front door hinge area), and even under the rear seat. Some have hinges so that you can pull them down from under the dash board for easier access. Others are behind panels only to be found by consulting the user manual. More than once I have had my feet over the back of the driver's seat with my head under the dash looking for that errant fuse.
Then there is the riddle of what fuse number applies to what function! Fuse panels are no more labeled than the provision of an array of numbers. Truly, you can't tell the players without a score card. Without the manual it is like having a cabinet full of canned goods with the labels pulled off! All you have to go on is size. You just have to keep guessing until you find what you are looking for.