BMW AC Fan wiring upgrade

AC Aux fan wiring upgrade page



Its hot here in California and anything I can do to get more effective AC use is a good thing. I noticed that during stop and go traffic or low speed driving that the AC was not as cool as with higher road speeds. I wanted more air flow over the AC condensor and the low speed setting of the auxilary fan was too slow for stop and go traffic. So I bridged the resistor just in front of the fan near the front grille. Seemed to help. It worked fine and I periodically checked to make sure things were ok with the fuse box. Then after a hot day and long run of AC running I checked the fuse box and discovered the fuse for the aux relay was melted and the fuse had popped appearently sometime in the afternoon but didn't know it since I can't hear it from inside the car generally. The heat generated from long use melted the fuse and a little of the fuse box. Its a 30 amp rated fuse. My solution was to control the fan with another relay thus eliminating the high current and heat flowing through the fuse box. Bottom line is that instead of 30 amps and a lot of heat it now only has to handle 110mA (.110 Amp) by only driving the relay. It works great and the fan is full speed. Again, this is only for when the AC is in use. No more excessive heat in the fuse box.


Too much current resulted in too much heat and eventually melted the fuse before popping it. Was stock 30 amp rating.


New relay and wiring installed beside the left side highbeam light:
New power wire (large blue wire - 10gauge) on relay terminal 30 comes from battery positive terminal through a heavy duty fuse case with 30 amp fuse.
Old power wire (black with blue stripe going into relay terminal 86) now only controls relay and is very low power.
Remaining portion of same color wire then connects between the relay terminal 87 and the fan plug.
Black short wire with yellow connector provides ground for relay terminal 85 and secured at relay mounting screw which is headlight assembly mounting.


This photo shows the new current required from the original fuse location to control the fan. Reads 110mA (.110A) and used to be 30A!


New lower power fuse in stock location. Could have been much lower rated but its what I had handy. Only needs about 1amp and used to be 30amp. Fuse terminals in the box were still ok and no changes were needed.

Results are that all the heat and current normally in the fuse box generated by the Aux fan for AC use is now outside the box and all the original wiring only needs to control the relay coil which is very low power. No more melted fuse box and the fan runs full power.


Another helpful item to assist in better cooling is adding a heat shield to protect the AC compressor for the exhaust manifold heat. I made up one from aluminum purchased from a metal supply store and cut to fit. The holes you see match to threaded recesses on the block. The smaller holes toward the top tie the shield to an AC hose and prevents the shield from vibrating on the exhaust. It mounts under the exhaust and over the compressor and installed from under the car. Sorry there isn't a photo of it in place.



Useful related links:

My 89 635
How to install a performance chip
How to add a central locking switch
How to mount 6x9 rear speakers and a subwoofer
How to cure a fuel tank leak
How to make a front end frame lift
How to fix the sunvisor mirror light
How to make a cup holder for your 635
BMW Clean Car Contest at Huntington Beach, April 2002
633/635 (E24) Forum on the Web. Oustanding source of info from enthusiasts.
5-speed conversion process for 2002's.
Mesa Performance Parts - BMW/Porsche Parts



Where do you want to go?
MAIN | Cars | Prior Cars | Mini Cars | Oddball Cars | Crash | Travel | Misc | Dogs | Car Jokes


peter@lexam.net