I'll be your first customer as I always wish to diagnose what's wrong before replacing any parts & I've always asked how to diagnose the FSU:
- Diagnostic hints requested for an FSU
To delve a bit into history, my first hedgehog failed on 03-26-2008
, where Doru kindly pointed me to post 138 of the canonical "elvis" FSU thread
You'll notice, my first request was for a diagnostic procedure (as I strive to never replace things before proving them bad):
But, apparently, most people "suspect" it's the FSU; they replace the FSU; and then they move on with their lives.
However, I like to diagnose things, so I started trying to figure out how to diagnose it.
When no good diagnostic procedure was forthcoming, I realized most people consider replacing the FSU their diagnostic procedure. If the new FSU works, then they consider the diagnostics done.
However in my case, the new FSU didn't work (it worked for the first five minutes when cold, and then failed when warm). Early on, I tried to figure out a good diagnostic procedure (starting, in post #16 of the canonical FSU autopsy thread
(and in post #7 of this related thread
); plus, I tried to determine what's the most reliable FSR
out there today).
To further understand the FSU, I autopsied mine, and posted the results here
Since then, I've had two more blower resistors fail on me, with the resulting summary being:
- The original GKR lasted from 2002 to 2008:
- Symptoms were the fan just stopped working
- The acm replacement FSU lasted from 2008 to 2013
- Symptoms were erratic behavior & overnight battery drain
- The Valeo/Sitronic/BMW/acm replacement was half dead on arrival
- Symptoms were it worked for 5 minutes and then quit
- Same thing, day after day; so I returned it
- The Valeo/Sitronic/BMW/acm replacement looks the same
- I haven't put it in yet, as I wanted to fully test the system
Of course, the first thing one can check is the fuse; but bear in mind, that's not as easy as you would like it to be:
And, I checked the 40 Amp yellow Fuse F76:
- What is the secret to removing the diabolical connectors hindering access to Fuse 76
In preparation for a good diagnostic test, I built a sturdy test connector so that we could measure voltages & very high current (up to 40 Amps) safely & easily as detailed in this thread:
- How to build a test jig for fsu parasitic battery drain and HVAC anomaly diagnostics (1
Note: My meter only goes to 10 Amps (but the test jig can easily handle 40 Amps)!
Here's a picture of my test jig connector:
Full details on how to build that test jig connector are in the referenced thread; suffice to say I tore down my original GKR FSU & sucked the solder out of the five pins to pull the connector off the circuit board and then I put Romex cable into the holes (they fit perfectly). The Romex cables should be able to handle the 40 amps at 12 Volts, if necessary.
Following in your footsteps, with the engine running, and the FSU not installed, I confirmed your observation that the AC compressor starts running (even with the fan off) simply by pressing the snowflake.
Since I wasn't running current through the wires, and to keep resistance low, I did NOT use the test jig above:
Instead of the test jig, I simply stuck nails into the FSU Harness connector, and then clipped my DMM leads onto the nails.
With the key out of the ignition, and the leads in place as shown above, my results were:
Note: FSU pin #1 itself has 0.3 ohms to its cooling tines but FSU pin #5 is not connected to FSUcooling tines (1.2MOhms).
- FSU Harness pin #1 to FSU Harness pin #5 = 1.0 Ohms
- FSU Harness pin #1 to chassis ground = infinite Ohms
- FSU Harness pin #5 to chassis ground = infinite Ohms
Note: FSU pin #2 is tied internally to FSU pin #5 (0.4 Ohms between them).
- FSU Harness pin #2 to chassis ground = 12.41 VDC
- FSU Harness pin #2 to FSU Harness pin #4 = 12.41 VDC
- FSU Harness pin #2 to FSU Harness pin #5 = 0 VDC
- FSU Harness pin #2 to FSU Harness pin #1 = 0 VDC
Then it was time to test the HVAC controller
- FSU Harness pin #3 to chassis ground = 0 VDC
by turning the key to position 2, and testing the voltage from FSU Harness pin #3 to chassis ground:
With the key in position 2, and the FSU not installed, I tested the HVAC controller as cn90 suggested.
- With the voltmeter on FSU Harness pin #3 and chassis ground, and with the snowflake off,
- I ramped the indicated fan speed from 0 dots (fan off) to 16 dots (full ramp).
- Then, I pressed the snowflake to turn it on,
- and then ramped the indicated fan speed down from 16 dots (full ramp) to 0 dots (fan off).
When I got to the 16th voltage (a full ramp on the HVAC display), I then turned on the snowflake, and counted back down by pressing the fan speed down button, one dot at a time.
Since the voltages were exactly the same, with or without the snowflake, I only show one voltage for each ramp dot in the table below.
- Ignition in position #2; snowflake off (or on); FSU Harness #3 to chassis ground:
- Fan indication @ 0 dots = 0.104 VDC
- Fan indication @ 1 dot = 1.996 VDC (delta = 1.892V)
- Fan indication @ 2 dots = 2.307 VDC (delta = 311mv)
- Fan indication @ 3 dots = 2.621 VDC (delta = 314mv)
- Fan indication @ 4 dots = 2.936 VDC (delta = 315mv)
- Fan indication @ 5 dots = 3.250 VDC (delta = 314mv)
- Fan indication @ 6 dots = 3.540 VDC (delta = 290mv)
- Fan indication @ 7 dots = 3.880 VDC (delta = 340mv)
- Fan indication @ 8 dots = 4.120 VDC (delta = 240mv)
- Fan indication @ 9 dots = 4.430 VDC (delta = 310mv)
- Fan indication @ 10 dots = 4.750 VDC (delta = 320mv)
- Fan indication @ 11 dots = 5.060 VDC (delta = 310mv)
- Fan indication @ 12 dots = 5.380 VDC (delta = 320mv)
- Fan indication @ 13 dots = 5.770 VDC (delta = 390mv)
- Fan indication @ 14 dots = 6.080 VDC (delta = 310mv)
- Fan indication @ 15 dots = 6.320 VDC (delta = 240mv)
- Fan indication @ 16 dots = 7.660 VDC (delta = 1.34V)
- Fan indication @ MAX AC = ?.??? VDC