# Slow Speed Fan Fix



## Four X (Nov 8, 2014)

There is a problem endemic for Mini's wrt the failure of the slow speed fan. There is a dropping resistor that is integral with the fan and its housing and this resistor fails. There are after market solutions ranging in price from $54 up to several hundred dollars for the original part. The problem exists because the rating of the resistor is too low , that is, its ability to dissipate the"waste heat" when in circuit is not sufficient and it overheats and burns out. The result of this failure is either no fan or high speed fan only at an elevated engine temperature when driving or on turning off. The radiator fan is also engaged with the a/c and as it only has high speed in the failed mode either does not come on or is at full power when extremes are experienced but in any intermediate phase the fan is not operating. The consequence of this is loss of efficiency and the danger of overheating the motor in more extreme ambient temperatures.

What is required to fix this? Answer: a new resistor that has the same ohmic value as the original but has a higher ability to dissipate heat and not burn out.

What is the value of this resistor? To determine this the fan was run at full power and the volts in and the amperes drawn were measured. By applying Ohms law to the numbers it was determined that the resistance of the motor was 0.8 ohms and the power was (from W=EI )determined to be 168 watts.

What value should the resistor be? Here an assumption had to be made and 1/2 the full power was chosen as the low power setting. To achieve this the resistance in the circuit had to be doubled and so 0,8 ohms was the value to add was aimed for.

The cross sectional area (QSA) of the original resistor was insufficient or its composition not suitable to dissipate the heat produced and not oxidise, and a "hot spot" developed and burned out the resistive material. This is evidenced by the blackened surface of the resistance element and the coating, originally green, having been destroyed.The QSA was found to be 0.57 sq mm.

A search of the internet for a suitable heat resistant replacement was instigated and nichrome wire was chosen. In Australia the only accessible source was Jaycar. "Used for wiring heating elements etc. 80% nickel, 20% chromium. - Resistance 13.77 ohm per metre.- Size 28 B&S (0.315mm).Pack of 4 meter"

The material was purchased and a trial and error set of experiments were conducted. 
Trial one; 4metre length inserted into circuit; result; no fan movement.
Trial two; 2 metre doubled length (2metre parallel) tried, again no movement.
Trial three; wire redoubled (1 metre 4 parallel sets) fan moved slowly. 
Trial Four; redoubled again (500m 8 parallel sets) fan working at a brisk pace. The Wire was quite warm.  By measurement of V and A the fan was found to be developing 75 watts and resistor dissipating, as heat 33 watts.
With these results it had to be accepted this was close enough and a method of utilising the 500 mm 8 strands on the existing ceramic former determined.

Not knowing the actual temperatures the wire would get to, it was decided to use silver solder (50% silver) as the bonding medium. The 500mm 8 wire strands were twisted together using an electric drill until tight and straight. The old resistance wire was removed, the end caps of the former abraded, flux applied to one end of the wire and the end cap and a small propane torch used to solder, at a suitable angle , the wire to the end cap. The 8 strand twisted wire was then wound, under tension, to make an even spiral around the ceramic former and terminated by soldering to the opposite end cap.

The fan was reassembled, connected to a 12 volt battery and tested. The numbers were as above. The fan was run for 30 minutes in the open ie not installed into the front shroud, and the temperature monitored of the winding and the plastic nest in which it sits. The top of the winding was 165 deg C, the lower end 152 deg C and the plastic nest 38 deg C. Some of the fan air flow is directed, via an inset duct, to cool the resistive element. When in the front shroud this cooling effect would be amplified. There is an overheat sensor proximal to the resistor that, in case of overheating would de-power the resistor and hopefully prevent ignition.

The nichrome wire during this test did not discolour warp or slump. Finally the new element was wound in Kapton tape and re-energised. The tape did not wither and remained intact.

I understand that some models of BMW also have issues with slow speed radiator fans and this may be of assistance to solving their problems.

The cost of the repair was $4.95, plus solder. The cross sectional area of the 8 nichrome stands is only marginally more than original resistor but has more bulk and surface area and 2.5 times the length, which equates to nearly 3 times the surface area for heat dissipation!. Hopefully, with the passage of time it will function well into the future.


----------

