Thread: AUX fan woes
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Old 09-15-2020, 07:51 PM
oldskewel oldskewel is offline
Join Date: Nov 2014
Location: SF Bay Area
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Originally Posted by Overboost View Post
...If I ever doubt my fan is operational, I simply unplug the AC pressure switch shown below with engine running and AC on and the fan will run full song after 5 seconds. This will at least let you know you have an operating fan but may not diagnose your DME controlling it dependant on engine temp. For that you need the cable you have ordered and a copy of INPA installed on a PC. My guess you have a fan out or going out and the quick AC pressure switch test will let you know. Mine is a 3.0L M54 model but I would guess your diesel M57 would have the same AC pressure switch.
Nice to have more info.
My 2001 3.0i looks just like yours, but nowhere near as clean.
I just ran that test on mine, measuring the fan control signal as I have been doing, and confirm it works as you say.

So yes, that is a very easy test. Maybe THE easiest, as long as you know your AC works.

Just turning the AC on should make the fan spin, even if very slowly. Disconnecting that plug should ramp it up to almost full speed within a few seconds.

Specifics on the testing I did:

AC Pressure, as measured in Foxwell (units = Volts):
engine cold, about 70*F ambient.
Ignition ON, engine stopped, AC off, cold: 1.25V
... connector removed: 5.00V
Engine running: 1.24V
Engine running, AC on: 1.76V, rising
... connector removed: 5.00V

When the AC was turned on, the fan control signal dropped to 12 .. 11V, as before.
When the AC pressure sensor connector was removed, it very quickly ramped down, at about one Volt per second, ending at about 2.0V, which should be almost top speed for the fan.
2001 X5 3.0i, 190k miles, AT, owned since 2014
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