Procedure to Upgrade the Aux. Fan Switch 80/88 C (1989 325i Cabrio, M20, B25)

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This upgrade will allow your 2 speed auxiliary cooling fan to come on at a lower temperature than OEM. The OEM switches closed and the fan started when the radiator reached 91 degrees C. The upgrade switch allows the fan to start at low speed at 80 degrees C and then kick up to high speed at 88. I live in Arizona which makes this upgrade mandatory. It's easier to do this upgrade at the same time you completely flush and fill your coolant

Parts, Fluids and Tools Required
New auxiliary fan switch 80/88 degree C (Bavarian Auto part 61-31-8-361-787)
New 14x18 aluminum or copper seal ring for switch (BMW part 07-11-9-963-201)
1/2 - gallon of BMW Coolant 82-14-1-467-704 mixed 50/50 w/ distilled water
1 - 12 oz bottle of Redline Water Wetter additive ($8, but you only use 2 oz of it)
1 - 10 gallon plastic tub preferably 8”D x 20”L x 15”W
2 - “Rhino” ramps (better than jack stands as you’ll get the car higher off the floor)
Liquid Wrench spray penetrant
8 mm socket
13 mm socket
22 mm deep well socket
23 mm (or 15/16”) deep well socket
Torque wrench (for new switch only)
6” funnel
Old bath towels and maybe a roll of paper towels
Garden hose with spray nozzle
Safety eye glasses
A couple pairs of disposable latex gloves

Draining some coolant from the radiator
This is done in order to lower the level in the radiator to just below the switch location.
Spray ”Liquid Wrench” or an equivalent penetrant the evening prior to the bleed screw on top of the thermostat housing.
Use “Rhino” ramps to raise front end and check that plastic tub will clear.
Engine is to be cool to the touch.
Put on, and keep on, your safety glasses.
Have garden hose sprayer ready on the side to wash down contaminated surfaces including you.
Cab temperature control should be set all the way to Hot and fan switch on low (#1) speed.
Turn ignition to on, don’t start engine (this will aid in draining the heater core).
Remove cap from expansion tank and keep close by.
Place drain tub under driver side of radiator.
Remove black plastic radiator drain plug (13 mm) and drain approx. ½ gallon radiator coolant.
Replace plastic radiator drain plug tight but don’t crank on it as you’ll snap it.
Turn fan and ignition off.
Set drain tub off to the side until ready to transfer old coolant into empty jugs for proper HAZMAT disposal.

Installing New Switch and Re-filling
Locate the old 91/99 degree C switch on the passenger side of the radiator.
Unplug the plastic 3 wire plug connector being very careful as these can be quite brittle.
Using the 22 mm socket loosen the switch to the point you can back it out by hand.
As you unscrew the switch look for any leaking coolant and if you get some then screw it back it hand tight and go back and drain a little more coolant from the radiator per the above.
Remove the switch and discard the old seal ring.
Screw in the new 80/88 degree C switch hand tight with a new aluminum or copper seal ring.
Torque it to 15 N-m (11 ft-lb) and plug the 3 wire connector back in.
Turn the ignition to on and the fan on #1 speed (this will aid in filling the heater core).
Open the bleed screw with your 8 mm socket but don’t fully remove it.
With the funnel in the expansion tank add approx. 2 oz of Redline’s Water Wetter and then start filling the system very very slowly with the 50/50 mix to avoid creating air pockets.
You’ll probably get at least 1 ½ quarts in before you hit the mark in the expansion tank or you see coolant bleeding out the bleed screw (which ever comes first).
If you see bleeding before you hit the tank mark then close the bleed screw (not torqued) and wash down around it with the garden hose.
Start the engine and hold throttle at 1500 RPM max until the thermostat opens (temp gauge should be just under the 12:00 position).
You’ll know the thermostat has opened as you’ll notice the expansion tank level will suddenly drop as the coolant is now allowed to enter the radiator.
Open the bleed screw and continue filling expansion tank until you hit the mark in the tank or you see coolant and most likely air bubbles bleeding from the bleed screw (which ever comes first).
You’ll probably get just under another ½ quart in the system at this stage.
Assuming you see sputtering from the bleed screw before you hit the tank mark then continue to bleed air out until the bubbles stop and then tighten it back down hand tight and wash down around it with the garden hose.
While you were watching the bleed screw you need to watch the tank as well. If you see the level beginning to rise higher than the mark then quickly put the cap on hand tight.
Turn engine and ignition to off.
Check the level in the tank and top off to the mark if necessary.
Take a 45 minute or so break and allow the engine to totally cool down.
Re-check the level in the tank and top off to the mark if necessary.
Start the engine and hold throttle at +/- 1500 RPM and repeat the same steps again.
You’ll probably only have to add less than a quart this time for a total replacement of the ½ gallon that you drained initially.
Close the bleed screw tight but don’t crank on it or torque it as you’ll snap it.
Close the expansion tank cap.
Take a 20 mile tour of your town or city in city traffic with a jug of pre-mixed coolant and your 8 mm socket and ratchet.
Watch the temperature gauge that it doesn’t creep higher than 12:00. If for some reason it does it’s because you’ve got poor circulation caused by air pockets and cavitation.
Find a safe place to pull over and park so you can bleed again and top off to the mark.
Finish your tour and head home.
The next morning check the level in the expansion tank and top off as necessary to the cold mark (you might not need to any at all).
Put the jug of coolant mix upright in your trunk or behind your seat for a few weeks as a precaution only as the weather and driving conditions may be more extreme (hotter) compared to your 20 mile trial tour.

That’s all there is to it. It should have taken a total of about 3 hours including the cool-down break up until the time you left to take it for a test drive.

Article written by a long lost member - 89Cabreeo


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