Auxillary fan wiring problem

[DaveSmed]

The purpose of the diode is to prevent voltage from backfeeding through the A/C switch from the temp switch- It makes it so the A/C doesn't kick on when you overheat.

That could be missing completely, and you should still see 12v at the temp switch on the rad.

Do your power windows and HVAC blower work?

[bmwpower]

Ok, I've been staring at the diagram for a couple hours now, going through the circuits trying to figure things out. Thanks xLibelle for posting this.

Question: I noticed a "diode" in the diagram. I also noticed that it appears to be a common replaceable part from vendors. Not sure I've ever seen it in the fuse box.

[xLibelle]

i guess you could use more words from me, than to just post a picture... just thought i would post this first and come back when i have time to analyze the problem.

[DaveSmed]

...or you could figure out where the open is in between fuse 19 and the thermo switch on the radiator.

Also, given the fuse is good on 19, is it getting power with the key on?

[jlevie]

With the A/C system removed, the switch bumps the idle speed and enables the aux fan. Unless you are going to run a 75C thermostat, the 80/88C thermo switch will trip too low on the 80C side and too high on the 88C side. You really want a thermo switch that will bring the fan on in high speed mode a couple of degrees above the thermostat rating. That keeps the fan from engaging while the car is being driven at speed, but still engages quickly once you get to pit lane speeds or when idling.

[bmwpower]

In this case get an 82C thermos switch from BavAuto and a 30A generic relay and fuse from any auto parts house. Tap power for the fan from the power distribution block on the firewall, run it through the fuse and then to the relay. I recommend installing a switch to allow manual engagement of the fan.

Using the existing thermo switch isn't a great idea as its low limit is 91C and the high limit is 99C. That's quite a bit higher than you want the fan to come on if deleting the engine fan.

This brings up a good point...

How about getting the 80C/88C thermo switch and using the A/C switch to activate?

What else does the switch do if I turn it on? I know it usually bumps up the idle 200 RPM or so, just don't remember if there is anything else electronic that it does (ECU, etc.). Also, is it a moot point if the compressor is removed? I will have to check unless someone else knows.

[jlevie]

In this case get an 82C thermos switch from BavAuto and a 30A generic relay and fuse from any auto parts house. Tap power for the fan from the power distribution block on the firewall, run it through the fuse and then to the relay. I recommend installing a switch to allow manual engagement of the fan.

Using the existing thermo switch isn't a great idea as its low limit is 91C and the high limit is 99C. That's quite a bit higher than you want the fan to come on if deleting the engine fan.

[bmwpower]

The aux fan's purpose is to provide additional cooling for the condenser and radiator when A/C is in us and the car is idling or in stop and go traffic. Cars w/o A/C don't have aux fans (it mounts to the condenser) as the engine driven fan is more than adequate for cooling just the radiator.

My understanding is the aux fan also comes on HIGH fan speed when the coolant temp at the thermo switch is high (forget the threshold temp).

My car has the aux fan. I removed the condensor, removed the aux fan from the condensor and reinstalled the aux fan in the core cavity using the 318i bracketry. Plans are to not reinstall the engine driven fan. Again, track only car.

So no A/C or engine fan/clutch, BUT aux fan installed running off thermo switch on radiator.

[jlevie]

The aux fan's purpose is to provide additional cooling for the condenser and radiator when A/C is in us and the car is idling or in stop and go traffic. Cars w/o A/C don't have aux fans (it mounts to the condenser) as the engine driven fan is more than adequate for cooling just the radiator.

[bmwpower]

The way I read the wiring diagrams, power is only present at the thermo switch when the A/C button is depressed.

Really? That doesn't make sense. What happens when the coolant temp reaches the high temp threshold and triggers the switch? Power should always be present at the thermo plug regardless of the A/C switch. Bentley never mentions the A/C switch in the testing procedure, but I have not looked at the wiring diagram yet. Otherwise, when the high temp is triggered, the fan won't go on unless I have the A/C on.

[jlevie]

The way I read the wiring diagrams, power is only present at the thermo switch when the A/C button is depressed.

[bmwpower]

The fan is only enabled when A/C is selected.

It's also supposed to come on when the thermo switch plug is jumpered as well, which it's not. Not getting 12v at the plug for some reason.

...unless you mean I'm only supposed to get 12v at the plug when the A/C is on.

[jlevie]

The fan is only enabled when A/C is selected.

[bmwpower]

I have an extra AC switch if you want it. When you press the AC button, the fan should come on. from your post it sounds like the switch is broken, but if you hold the switch in, will the fan come on? It could also be the relay, but I can't remember where the relay is... :/

I guess I didn't hear the fan before when hitting the switch, but yes, I just tried it and the fan comes on. Again, nothing when jumpering the thermo switch harness.

[gearheadE30]

I have an extra AC switch if you want it. When you press the AC button, the fan should come on. from your post it sounds like the switch is broken, but if you hold the switch in, will the fan come on? It could also be the relay, but I can't remember where the relay is... :/