Aux fan works when jumped, changed switch and still nothing

[snowfox]

Hi all,

I’ve seen a bunch on this topic, but haven’t come across my specific issue yet.

My Aux fan does not run. When I jump it at the temperature switch I it runs great in BOTH low and high speed. I swapped out the switch today and still don’t get any fan activity when I turn the AC on.

My question - will the fan run in both high and low if the resistor is bad? Im knew to this, but it sounds like if the resistor was bad then it wouldn’t run in low speed when jumped. Should I change that out next or what other trouble shooting should I do?

[reelizmpro]

Then the fan and resistor are good. The problem is either the switch or the connection to it. This is assuming the cooling system is full and properly bled. Corrosion occurs and sometimes contacts need to be cleaned or replaced.

[snowfox]

I replaced the switch with a brand new one and still nothing. If it works jumping it then I assume the connections are good?

I believe cooling system is full and bled, but even if it wasn’t the fan should kick on with the AC button and it’s not.

[reelizmpro]

Don't assume that. Jumping the connector with a wire is different than 30 year old female terminals mating to male terminals. I've bought numerous switches because I assumed that and eventually had to replace those female terminals. I'm guessing they open up over time and fail to make proper contact plus corrode.

[snowfox]

Found this write up on another forum and this diode seemed to fix his problem, but no instructions on where it is or how to find it! Anyone have experience with this and a part number?

[varg]

It doesn't matter if your fan thermo switch is good or bad, if the fan works at all on low speed the AC will turn it on, the thermoswitch is bypassed in this scenario. Download a copy of the ETM, knowing how things are wired will help.

There is a diode in the line leading up to the fan relay to prevent the thermoswitch circuit from backfeeding and turning the AC compressor on, unfortunately I don't know where the diode is located. If you find it do post about its location so future searchers don't have to explore for it too.

[tomstin]

Does the AC compressor kick on? If not, the vent mixing sliders need to be in the correct position and the sensor to protect the evaporator needs to be functioning before the compressor and fans kick on.

There is also the circuit for an overheat or high temperature situation for the aux fan to come on. So, check those sensors.

[ZeKahr]

It doesn't matter if your fan thermo switch is good or bad, if the fan works at all on low speed the AC will turn it on, the thermoswitch is bypassed in this scenario. Download a copy of the ETM, knowing how things are wired will help.

There is a diode in the line leading up to the fan relay to prevent the thermoswitch circuit from backfeeding and turning the AC compressor on, unfortunately I don't know where the diode is located. If you find it do post about its location so future searchers don't have to explore for it too.

From looking at the ETM and from some forum sleuthing I’ve done, it looks like the diode is located inside the fusebox, underneath all the fuses and relays. He’ll have to open up the fusebox itself to get to it.

[snowfox]

Does the ETM show the part number for the diode? My AC works great, just not when your sitting still which I know is because the fan isn’t working.

[reelizmpro]

ahhh this is about the AC...I misunderstood your post.

Here you go, although a new diode didn't solve his problem at least you have more info.

https://www.r3vlimited.com/board/forum/e30-technical-forums/general-technical/363011-fusebox-diode

[hozzziii]

This is from an early production manual car.
I looked at the late production ones as well and with both manual or auto the aux fan relay also gets power from the black/white wire after the evaporator temp switch. Just the auto cars have a kick down relay which is another story.

By the looks of it, if your compressor is working then you have power at the black/white wire, so that's good.

My guess is you should check the black/white before the aux fan low speed relay.


We know the fan can work when you jump the low speed sensor wires, so the fan and low speed relay are good.

And as you can see the low speed relay gets its signal from both the low speed temp switch and the ac circuit. Hence why you need the diode so the low speed switch doesn't turn on the compressor.
Now your issue could be the diode, or a tear on the black/white wire.

I suggest you test for power at the 86 terminal on the fuse box where the low speed relay goes.
So get a multimeter, turn the car on, turn on AC, take the low speed relay out, check for power at the 86 terminal, you can use a strut bolt or the 85 terminal for ground, if you don't have power, then it's either the diode or the black/white wire.

If you do have power, then I'm confused.

[snowfox]

Hozzziii where is the 86 terminal? Sorry I am totally new to working on electronics.

[hozzziii]

Every relay has these pins or terminals on the bottom.

Those metal things that stick out below are the pins or terminals. In this case there are only 4 pins, 30,85,86,87. Why did they name them like this? I dunno.

This is how 4 pin relays are wired.
Power should always be available through a fuse on pin 30.
Pin 85 is connected to ground, and pin 87 is connected to what you want to drive, but only provides that power when you want it.
Pin 86 is the one you control, should you choose to turn on whatever is connected to pin 87, either a fan or horn or headlight or so on, you provide power to ping 86, and that power just goes through 85 to ground.
However what that also does, is create a magnetic field through a copper coil inside the relay.
And that magnetic field pulls a tab that is directly connected to pin 30, and slaps that tab onto another contact in the relay that comes out to be pin 87, then your fan or headlight or whatever is on.

That's why you here that audible click that a relay is on. Like when you use your turn signal, that audible clicking is the turn signal relay in the cabin.

Now what I mean by checking if you have power at terminal or pin 86 is, the relay has bunch of male pins sticking out the bottom, and they go into the female pins or terminals on wherever this is getting installed, in this case your car's fuse box.
So, you have to figure out which relay we are talking about here, on BMW E30's, the fuse box cover has the locations printed on it.

This is that cover. From the wiring diagrams posted above we know we are looking for the relay that is in the K1 position.
So you take out the relay, due to the arrangement of the terminals, it can only go in one way so it's fine.
Look at the bottom of the relay, the terminal numbers are printed in the plastic, next to each terminal there are numbers 30,85,86,87.
On the fuse box, you have to check for power where pin 86 of the relay goes into, not the relay itself.

And that's how you find terminal 86.

[snowfox]

hozzziii I pulled the relay and did as you said and its showing I have power. I used the diode testing on the multi meter and it says the diode is good. I am at a loss....maybe it is the female connections.

[reelizmpro]

The AC switch bypasses the dual temp switch so the connector is not the problem. The relay works since it kicks on when you jump the temp switch. Bypass the diode and see what happens. If the fan kicks on then you know for sure it's a bad diode. If not, then check the connections to the diode.