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E34 M5 Resto Part IV – Fan Clutch Replacement

E34 M5 Resto Part IV - Fan Clutch Replacement

About 35 minutes into my first big road trip in the M5 (i.e., more than 15 miles), it became readily apparent that the fan clutch for the belt driven cooling fan had completely seized. How did I know? I’m not psychic, and the engine stayed nice and cool, staying firmly planted a touch below the middle of the gauge. However, at 65 MPH (slightly north of 3k rpm) on the highway, the car sounded like a fan powered swamp boat, with the cooling fan always running at engine speed — instead of doing its job of automatically adjusting speed as dictated by temperature and engine speed. The clutch, which is attached to the inside of the radiator fan, had completely frozen in the “always on” position. While this is much preferable to the alternative failure method — where the fan is always off and you overheat, potentially destroying the S38 engine — the constant whirring took a significant toll on my sanity on the 2.5 journey from New York City to the Eastern Connecticut shoreline. It also puts extra wear on the fan belt, and leads to the possibility of catastrophic failure at high RPM should something seize and the fan explode. In the latter situation, you can count on buying a new radiator and fan shroud at the very least.BMW cooling system fan clutch

Thankfully, replacement of the fan clutch is relatively simple. The only word of caution I received is to make sure that I ordered the Genuine BMW stamped unit (part no. 11527505302). While many others will work fine with the car (at ½ the price), it seems the Genuine BMW fan clutch is the quietest in terms of toning down the “swamp boat effect” — which I badly needed.  Also, unless you are extremely lucky, I would advise using a water pump pulley holding tool to aid with the removal as the reverse threaded nut (lefty-tighty, righty-loosey) causes it to tighten and lockup with use. In this manner, a spray of liquid wrench, wd-40, PB Blaster, or rost off  may prove helpful to loosen things up.water pump pulley holding tool

I, or course, thought I could make due without the water pump pulley holder, and upon receiving my new fan clutch from eEuroparts, attempted for hours to loosen the nut to no avail. The problem is that while the 32MM (~1.25”) nut is very easy to grab and manipulate in situ with a wrench, it spins the entire water pump assembly as well.tough nut to crack

I was told to give the wrench a few quick whacks with a rubber mallet, but that had no effect. My ego bruised, I ordered a holder — basically a piece of metal with holes that lock into the outer pulley bolts — and upon its arrival, the fan clutch nut loosened immediately. I was able to pull the clutch and fan blades without removing the fan shroud, and was astonished to find that not only had the viscous coupling inside the fan clutch failed, but that it had completely disappeared, with just a few remaining ball bearings jammed into position, keeping the fan locked in the “on position” that had been driving me insane.

The clutch and attached nut also felt loose — and so changing this out now was certainly good prevention against the aforementioned catastrophic failure.

Installing the new fan clutch is easy — if your fan blades are still fine, use an Allen wrench to loosen the three bolts holding it to the clutch, clean, and swap over to the new clutch.use an Allen wrench

Then, wrangle it back into position on the water pump pulley and tighten, again using the holder to keep the entire assembly from spinning.

The result? A pleasant “whoosh” upon startup and the fan noise quiets down almost instantly. Sanity (partially) regained.old vs. new

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