fsu testing

I have a 2002 325ci convertable and the fan has stopped working. is there a way to test the final stage unit?

I've never seen a test to determine if the FSU has failed, only symptoms that indicate that the FSU is malfunctioning.

my fan motor totally quit so I'm guessing it's the fsu as the fuses seem to be ok and rarely the the fan motor pack it in. I just wanted a test before ordering a replacement.
thanks for the feed back

Most folks just replace it. Just get an OEM FSU or you'll be doing again soon!

You can test it, but it's most likely bad if all of the fuses are good and the HVAC panel is working
FSU connections:
Pin 1 to blower motor (or a test load, light bulb)
Pin 2 to 12 volts
Pin 3 to 0-5 VDC (this is the speed control input from the HVAC panel)
Pin 4 to ground
Pin 5 to blower motor (or a test load, light bulb)

Vary the 0-5 VDC and the motor (or lamp) will follow if the FSU is good.

Don't buy one from ebay, they are mostly junk and don't last, get an OEM FSU.

There are at least two failure modes: erratic, uncontrollable speed fluctuations; no blower function.

A faulty FSU can also drain the battery.

Mine drained the batter. I would just get a replacement from your dealer and take the ~30 minuites it takes. Search for the FSU thread - good info there!

Originally Posted by windsoftime

I have a 2002 325ci convertable and the fan has stopped working. is there a way to test the final stage unit?

they all do that. FSU . good write up.

Originally Posted by JimLev

You can test it

That's nice information.

When I search for "fsu" in the bestlinks, I find:
- Understanding and TESTING the Final Stage Unit (FSU/FSR)

So I'll add your information to that set so that others can find it more easily in the future.

See also:
- What is the canonical thread on the BMW E39 FSU/FSR/Blower Motor Resistor (1) and what is the right brand FSR to purchase (1) (2) when your interior heating & air conditioning HVAC/IHKA blower motor goes haywire (1) (2) (3) (4) (5) (6) (7) (8) & how to test the FSU (1) (2) (3) & how to repair your own final stage resistor including an autopsy & wiring diagram schematic (1) (2) and why a bad final stage unit will cause the battery to drain overnight (1) (2) (3) & how to isolate and replace the E39 blower motor (1) (2) (3) (4) (5) & how not to replace the FSU (1)

Just reading the resistances won't tell you a lot.
Different ohm meters use different battery voltages which will effect the readings.
Once defective transistors are powered up they start drifting, shorting out, etc.