Aux Fan - Adding a manual switch

[GDAus]

Over the years here I seen this question asked and I've finally got a bit of time to put together my version of the problem based on 30 something years as an electrical engineer ....

The Aux Fan has 2 functions, it's there mainly to provide extra cooling for the engine when the Air Conditioning is running. But BMW added the capability to act as a last line of defence when the engine is overheating. In its original form, the stock temp sensor is 98 ^oC and since the sensor is mounted at the bottom of the radiator at the coolest part, it takes some time to react to higher than normal engine temperatures.

My thinking is that, it's better to stop the engine getting up to a critical temperature, rather than trying to pull it back down from the brink. During the first Australian Summer I had with my E21, even though I flushed out he cooling system a couple times, it still wasn't happy idling or grinding along the peak hour traffic. It was getting a bit over the ¾ mark on the temp gauge without the aux fan kicking in and I thought this wasn't real good.

The standard fan is a beefy unit, nicely designed with a duct for the fan to increase it's efficiency. Of course, if you don't have working A/C or chronic overheating, the fan wont have worked much and will need to be overhauled to clean and re-lube the bearings. Tested mine as it was and it was pulling a bit over 11 amps at idle speed. After cleaning etc it was spinning freely and the current had dropped to 8.1 amps at idle and pushing more air.

NOTE: the nut on the Fan Blade is a LEFT-Handed thread! It needs to be undone clockwise.

Aux Fan01.JPG Aux Fan03.JPG
The motor has an end face commutator so it just pulls apart without the brushes getting caught etc.

The first thing I did was to add a switched bypass in parallel to the temp sensor on the bottom of the radiator (there are lots of possible ways to do this, but to me, this is the simplest). Secondly I found a temp sensor with a lower opening temp to replace the original. The 91 ^oC sensor I used will kick in the Aux Fan at about the ¾ mark on the Temp Gauge if I'm not paying attention!

Connecting to the temp sensor in the radiator using a piggyback spade connector or making up a Y splitter is pretty simple.

This is 3 ways of doing it. Piggy-Back Cons011.jpg

I had the gear to make up the Y splitter so that is how I did it and this shows the Radiator end. The parallel connection is made to the Black/white wire. The other end I ran up the existing loom .. under the battery tray ... around the fuse box and through to the cabin along the existing wiring loom .. not my neatest installation ever, but it was mainly to get me through the summer. Still working perfectly after about 9 years

Aux Fan Switch Bypass01.JPGAux Fan Switch Circuit 01.jpg

Using an illuminated switch, if it's wired correctly, will mean the switch light will also act as a warning light to show the fan has kicked in if the temp sensor switches on. (of course it will also come on if you have working A/C when the A/C comes on!).

Doing it like this means that you are switching the Aux Fan Relay which in turn switches the higher current for the fan. The current needed to pull in the Aux Fan relay is only about 0.1~0.16 Amps for this class of relay, so the additional switch and wiring don't need a high current rating.

The positive feed for the switch can come from either the Start/Run (Green Wire) or Accessory (Violet wires .. that's what BMW calls purple) Buses. Using either of these means you can't accidentally leave the Aux Fan running if you turn off the ignition and leave the car.
I've used the accessory bus so I can turn off the engine and allow the fan to run for a while on a really hot day just to take the edge of the temperature spike that happens when you shut down an engine.


Cheers

[GDAus]

An extra pic of the fan


Aux Fan02.JPG

[320iAman]

I like the idea of a 30 amp switch in the cab of the car lighted or unlighted , the switch will bypass the thermocouple and the gives the driver control when a cooling boost is needed.

Randy