What exactly is an FSR ??? | |
While I'm here I thought I'd explain what this 'Resister' thing actually is.
In the early days of auto electrics, stepped fan control was via a series-string of big power resistors built into an almost complete circle. +V power came in at one end and a rotating contact selected the voltage available at any of the junctions between the resistors. If the contact went to before the first resistor the the full voltage went to the fan, if contacted after the first resistor some voltage is turned into waste heat by the resistor and a lower voltage goes to the fan. Turning the contactor further along the series created more waste heat and lower voltages = lower speeds. Turned past the end of the resistors no voltage is connected = Off. Because they got very, very hot, often the resistors were placed in the engine compartment and operated by an extension shaft from the dashboard.
In the 1980s the big resistors were replaced by much smaller ones to produce low power voltage steps (0, 2-->8V) that were then applied to an electronic throttling device to feed higher power to the fan. Electronic throttling devices are called Transistors. Bi Polar Transistors can switch quite high voltages, but their control input must be between 0 and 0.7V so a small integrated circuit translator is needed between the control resistors and the power transistor. These Power Transistors also get hot as they reduce the voltage, but not as hot as the older power resistors, AND all the heat is produced in a single, small, fixed location. To transfer the waste heat away from the Transistor into the air a large metal plate with lots of pins (to provide a large surface area) is clamped onto the transistor body. This is what the BMW 1990s to 2010s FSR is. The control knob works the first stage resistor, the translating IC is intermediate and the Power Transistor is the actual, Final Stage Resistor.
Much more efficient Field Effect Transistors that can handle large amounts of power became available in the late 1990s. And along with their introduction a new method for speed control became popular - deliver small (micro second), full power pulses to the fan motor and allow it to coast between pulses. Shorter pauses = faster speeds. The mass of the motor and fan blades will smooth out the speed and the transistor always switches from full Off to full On, therefore greatly reducing waste heat. This is what BMW uses now.
The original 'hedgehog' FSR has a voltage converter IC (Elmos 10901D) with simple over heat monitoring, an over current detector that shunts the voltage control signal to ground (thus turning off the fan and signaling the controller) and power transistors mounted underneath the circuit board (3 pins in red circles) under the black resin. Over time the heat cycling breaks the solder joints with the sensors and transistor control pins, etc leading to full-on continuous running or total failure. Its heatsink has just a few, long, large heat exchanging pins.
There have been a few versions of the circuitry over the years, from BMW and others.
The currently available, upgraded version includes a microcontroller to better manage the system, including shutting down if the transistor starts to overheat. The transistor is flat mounted through a hole in the top of the circuit board with an all copper connection to the heatsink which on this model has lots and lots of smaller pins to provide ~2 times the heat exchange surface area. This 'improvement' has just one small problem, the microcontroller is mounted close to the heatsink and so it gets hot ... and fails in all sorts of 'interesting' ways.
Around 2014 an American company produced a hybrid FSR (APSX WB FSU). Its microcontroller accepted voltage step input and sent pulse/coast signals to a pair of FETs which required no heat sink pins. Failure of these is rare. These are no longer available With todays cheap electronics I think a similar DIY solution is viable. But I don't have enough information about the type of voltage signal used (bi-directional) nor the power rating for operating the fan ... can anyone supply this? Does anyone have one of the hybrids to experiment with ?? Or maybe someone from APSX can make the details public ???
Because European vehicles operate with (to them at least are) extremes of temperature (cold winters and hot summers), vehicle fans are usually run at moderate to fast speeds with very little waste heat generated in the FSR. So not a problem for BMW. Except not all BMWs operate in that climate, for instance in Australia, after reaching a comfortable temperature, we mostly have the fans just moving the heated or cooled air slowly through the system so the FSR runs very hot, and doesn't last so long. I'm sure vehicles in the southern US have a similar profile.
There are many 'interesting' failure modes for the FSR with always On being the most common. Many of the controllers in a BMW monitor the operation of other controllers and Fan ON while engine is Off can result in wierd results. In my case the cabin air sampler fan never stopped, the indicator flashes at lock and unlock didn't happen, air suspension often had to be reset and the battery got flattened randomly.
And now you know.
bye 2003 TD6, EGR delete, vortex crankcase breather update, performance chip, UHF 2 way, Android head unit, crash camera on dash, always-on rear view camera to mirror screen, LED DRL`s, Electric trailer brake controller. FSR replaced. Pulling 3T caravan all about Australia
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