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Thursday, July 16, 2020

Doug's Domain

Doug Vetter, ATP/CFI

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July 25, 2006

Auxiliary Cooling Fan Troubleshooting

After coming home from the dropzone last weekend, I decided to wash the bugs off the front end of the E46 before putting her to bed. While I was cleaning the grills I noticed the aux cooling fan started up, smoothly spooled up to a modest RPM and then -- presumably as the coolant temperature dropped -- ramped back down just as smoothly and turned off. At that point it dawned on me that I could not remember when I last heard the aux cooling fan operate on the E36. Since I'd heard it's supposed to run when the A/C is on and it most certainly has not been running (at least when I've been looking), this got me to thinking -- maybe it's broken and has been for a long time.

(Image: Aux Fan Block Diagram)I got up early today to enjoy the uncharacteristically crisp morning air and do a bit of troubleshooting. I reviewed the Bentley manual, which revealed the troubleshooting procedure. NOTE: the wiring color code in the troubleshooting section is WRONG, though fortunately the schematics in the rear of the manual are correct. I had to go through this procedure twice to get it right because of this discrepancy. Lesson learned? When working on BMW's and looking for a ground, think dirt (yes, "brown"). Anyway, here's the procedure:

  1. Disconnect the connector from the coolant temperature sensor (located at the top right of the radiator). Insert three pieces of wire (I used some solid copper from a piece of ethernet cable) into each of the three pins of that connector, and, with the ignition key in the "ON" position but the engine stopped, test for power. The center pin is ground, but to elminate the ground as a possible fault, I used a frame ground. I found 12 volts at pin 1 and 3 (the outer two pins).
  2. I then bridged the center pin of the coolant temperature sensor (brown wire, or ground) first to pin 1 and then to pin 2 to simulate a high coolant temperature and turn on the fan manually. As the aux fan on the E36 is a simple two speed unit, closing either circuit sends power to low and high speed relays located in the fuse box. The relays are needed to deal with the high power (30A) circuit required to power the fan.
  3. I pulled both the low and high speed relays and checked for power on pins 30 and 86 (these numbers are imprinted on the relay contact blades). I found 12 volts on both pins of the low speed relay but only on one pin of the high speed relay. There should be power on both pins on the high voltage relay, but the Bentley has been wrong before. Not sure if this is significant.

Fortunately, as I intermittently activated each circuit, the fan ran in low and high speed modes. That meant the fan and much of its control circuitry was okay -- and good thing too....the aux fan is $670(!) according to realoem.com. This test confirmed what my technician told me earlier, the fan is almost perfectly quiet running in low speed. And in fact it's not terribly loud running in high speed either. If you look at aerodynamic design of the blades this is no surprise -- they're designed with swept airfoils to negate the air turbulence that we hear as noise. This means it's likely that I wouldn't hear it over the engine or the engine driven fan -- I'd have to look at it instead.

Why did I go on this little detour to test the aux cooling fan when I was about to replace the A/C compressor? The A/C system needs a reasonable airflow through the condenser (a radiator-like device mated to the front of the engine coolant radiator) to dissipate the heat generated during the compression phase of the vapor cycle. If airflow is insufficient, the condenser fails to do its job and the pressure does not bleed down as much as it should. The compressor must then work against this higher-than-normal pressure to get its job done. This can cause the valves in the compressor to chatter -- and that's exactly what my compressor has been doing for at LEAST three or four years. Could the aux fan or one of its control circuits been broken that long? Certainly.

The BMW docs indicate that on the six-cylinder E36, three things turn on the aux fan:

  1. A request from the IKHA control panel / module
  2. High coolant temperature
  3. High refrigerant pressure

The point to bring home here is that the fan does not automatically turn on when the A/C "snowflake" button is pushed, nor will it run, apparently, unless there is insufficient airflow over the condenser. Since I drive mostly on the highway and the speed helps maintain reasonable coolant temperatures as well as refrigerant pressures without the need for even the engine driven fan, it's no surprise I haven't heard (or seen) the aux fan run lately.

Toys for the Birthday Boy

This morning I reached the mighty old age of 37. This means two things. First, I'll have to increase the minimum age on female prospects to 21 to remain "respectable" (or whatever I was when I started dating). :-) Second, it means I'm fully justified in buying some additional toys -- in this case, some essential tools I'll need to work on the cars and the airplane.

I just went to a local tool house called Eppy's. They have a great selection of reasonably-priced tools including SK. I used to work with hand tools every day in my previous life as an electrician, so I know how to abuse a tool. We beat on SK ratchet sets and they worked like the proverbial Timex, so choosing SK was a no-brainer.

This past weekend I wound up buying two torque wrenches (3/8" and 1/2" drive) with full metal handles and pivoting heads for getting into those odd places, a range of Torx sockets for the interior, a shallow 36mm socket for the oil filter cannister, one impact socket for the wheel studs (yes, I'm planning a full suite of air tools, too), and a giant 32mm wrench essential to remove the engine-driven fan. This ran about $600, and I'm not done yet, but I figure if I can get my toolset up to snuff quickly enough I can do the brakes on my own this time and nearly offset that amount in labor savings.

Truth be told, I considered going "high end" with Snap-On, at least for the torque wrenches, since I have worked with their stuff before and like it, but I don't intend to use these tools every day and I felt my tool dollars would be better spent on a wide range of tools rather than one or two items. Plus the local Snap-On rep never called me back. Way to push product, dude.

Tools and Equipment: $600, Mileage: 130552.