Heater fan question - The Unofficial BMW M5 Messageboard (m5board.com)

DP400 · 11th January 2007, 11:21

I seem to have a problem with my heater and was wondering if its just me being a clot, or if there is something wrong.

Correct me if im wrong by the fan speed should be controlable by the dial when its inbetween the "off" and "full blast" settings, but when my dial is in the adjustable postition the fan does not come on at all 9 times out of 10.

Ive only had the car for 5 days and this is the first car ive owned with climate control so I'm still getting used to it but I can see that I'm doing anything wrong.

Does it sound like there is a fault and if so, does anyone know what it might be?

Thanks in advance for any suggestions.

milesr3 · 11th January 2007, 15:33

This is what I meant about the climate control appearing to have a mind of it's own...

I don't understand the logic behind it, but it seems that you have to have the fan on full blast until the temperature inside the car reaches the temperature set on the dials. Then the fan speed control works as you would expect and you can back it off.

You'll notice the fan speed increase when you hit the demist button too.

At less than the full blast setting you can hear the fan running, just a lot slower. If you adjust the temperature range you'll hear the motors redirecting the air to the correct vents. Sit with the ignition on, but the engine stopped and have a play.

Richard

raymondw · 11th January 2007, 21:50

This sounds like a failing the final stage unit that drives the heater fan. This part also is known as the 'sword''and is located underneath the IHKA unit in the instrument cluster. Acces is possible by removing the cloth-trim parts on the right side (in the UK driver side). Two electronic unit's can be found. The control unit and the final stage unit. It's the latter one that has failed.

In detail. The sword consists of four MOSFET transistors in the so called 'H-bridge' circuit. Although these parts are conformally coated against environmental influences, the conformal coating works as an isolator and ultimately leads to excess heat within these devices. Unlike normal transistors, the resistance of a MOSFET increases exponentially with increasing temperature with more dissipation as result. Finally, these parts will fail.

Replacement is possible but requires the removal of the coating which is difficult. Used unit's can be bought from theb scrappers, but usually these have the same fault.

Sebring NL · 11th January 2007, 22:37

Nice to know Ray.

Will be taking care of my broken blower motor soon, and hope my 'sword' is still in working order..

Let me guess...Its hard to reach, costs a fortune and there are 4 possible parts.

davidoli · 11th January 2007, 22:58

Didn't the hedgehog come first and the sword later.......

IIRC Richard's M5 is a 1991 3.6.......won't it have a hedgehog thermistor.....

Correct me if I'm wrong , I won't be upset !

milesr3 · 11th January 2007, 23:07

My ex-car was a 1991 1.6 with climate control.

Are these MOSFETs available separately or is there an equivalent contemporary replacement device? I used to have access to books for looking this kind of thing up, but sadly my days of messing about with semiconductors are long gone...

davidoli · 11th January 2007, 23:10

It was bloody quick for a 1.6 though Rich !

milesr3 · 11th January 2007, 23:21

That was because of the grounding wires, the aftermarket chip with a 9,000rpm limit and removing the plenum MAF and airbox to improve induction.

Thank-you for pointing out that my 3.6 wasn't actually a 1.6 David

My finger must have got stuck on the '1' key. Can't edit it now 'cos you've quoted it...I'll be forever remembered as the one who thought his M5 was a 1.6.

p.s. DP400 - Only joking about the mods.

If you cut me in half it says "stock is best" in the middle, like a stick of rock.