Forumsee logo
Forumsee » Cars » Brands and models » Read message
RSS: Subscribe to RSS

DIY: Zionsville dual speed fan wiring 2001 740i

1995 - 2001 (E38) - Bimmerforums - The Ultimate BMW Forum -
I could not find any detailed write ups on this so I decided to make my own. This write up does not cover the actual radiator install as it is very straight forward. It drops right in with all the same hoses going into it. The only physical part that is different is the temperature multi sensor wiring that is covered here. To complete the job, you will need some wire connectors, soldering iron with soldering gear (to connect the ballast resistor on the slow circuit), and basic tools. All connectors and tools are illustrated in my guide.

Step 1 - Identify the placement for the two fan relays (slow and fast), identify always hot and ignition switched power. I placed both relays into the electric power distribution (fuse) box located on the passenger side under the hood. It actually has two ideally sized spaces for the two relays. Illustration 1 shows these spots marked 1 and 2 in red. The yellow circle shows the 'Always Hot' terminal and the green arrow shows the white stripe red wire with switched power. More pictures for these down below.

Illustration 1

Step 2 - Before securing the relays inside the box, connect the fuses provided in the radiator kit to the big yellow wires using round connectors which will bolt under the 'always hot' terminal wire.

Make the fuse wires look like on the Illustration 2 below.

Illustration 2

Step 3 - Connect the fuse to the yellow wire on both relays. The yellow wire only needs a couple of inches of length as it is very close to the 'always hot' terminal in the fuse box.

Illustration 3

It would be a good idea to mark one of the relays so you could differentiate them. I marked the slow speed relay with some BMW stickers and placed it into the #1 slot (Illustration 1).

Step 4 - Secure both relays in the fuse box. I used zip ties for this job and it worked out well. Each relay has a little side slot which perfectly accommodates a zip tie which can be looped around the plastic rack wall of the fuse box. See illustration 4

Illustration 4

...and tighten up the zip ties so that both relays are secured inside the fuse box. The following illustration shows the 'always hot' terminal with its plastic cap removed and the nut taken off, ready for connection.

Connect both fuse connectors using the existing nut on the 'always hot' terminal and tuck the fuses inside the fuse box like so:

Step 5 - Splice orange wires on both relays together and connect them to the switched power source (red wire with white stripe).

I used a regular splice connector for the orange wires. The orange wires will not require any connectors at the end as we will insert them into the relay socket directly and snap the relay back on.

The following illustration shows the splice connector:

Illustration 5

Here is the red with white stripe wire we will be connecting the spliced orange wire to:

Illustration 6

To connect it, disconnect the relay (I believe it is a wiper relay) to expose the socket like so and insert the orange wire in there:

With the wire fit inside and the relay back in its socket:

It does not look very professional, but the wire is very secure in there.

At this point, we have finished the main power wiring and can close the fuse box. I used smaller zip ties to organize wiring. Before doing this, I marked the slow speed wires with a marker so I could differentiate them from high speed wires. We should now have two pairs of wires coming out (Red and Grey) for each relay. Grey is the ground.

Step 6 - Prep the fan connector wire. With the two speed setup, we will have two main Red power wires coming into the fan connection and the original fan connector will not work as it is meant for a single speed setup. To address this, I replaced it with a round connector so we could connect the main fan connector with two reds coming from the fuse box using a single bolt.

Illustration 7 below shows the modified connector on the fan connection wire:

Illustration 7

Step 7 - Install the multi temperature sensor provided in the kit into the radiator. It has 3 wires coming out of it numbered 0,1,2. They are as follows:

0 - Chassis ground wire

1 - Switches to ground when coolant temp reaches 185 F

2 - Switches to ground when coolant temp reaches 195 F

Wires 1 and 2 will be connected to the gray wires coming from the fuse box, slow and fast relays respectively.

Illustration 8

Step 8 - Prep convenient chassis ground connection. It is up to you where you choose to do this, but I chose a screw located under the air filter box. It is situated very conveniently for the job. I removed the 10mm nut and removed the chassis paint under it for better connection. Once everything is assembled, this will need to be spray painted to avoid rust.

Illustration 9

We will connect two wires on here, the 0 wire coming from the temp sensor and the ground wire coming from the fan itself (black) like so:

Step 9 - Prep the ballast resistor for the slow fan circuit and install it. Soldering will be needed for this one. The ballast resistor will connect in line on the red hot wire of the slow fan circuit. On one side, we will need a piece of wire about 6-8 inches long that will connect to the fan. On the other side it will be connected to the red wire running from the fuse box. I used a little bit of wire from the relays. Before attaching the wire to the resistor, I put lots of solder on the little ear loop contact so it mates well.

Illustration 10

The other end (that connects to the fuse box wire) will need to be done when the resistor is installed on the car. I used the following location for it (illustration below). Instead of the bolts provided with the resistor, I used zip ties again. They worked out quite nicely. All 3 red wires should have round connectors on them like so:

Step 10 - Connect ground gray wires. We have two loose gray wires coming from the fuse box that need to be connected with the green wires on the temp sensor. Make sure you connect the slow circuit gray wire coming from the fuse box to wire #1. The other wire goes to green wire #2. I used basic in line wire connectors:

Step 11 - Final step. Once you routed the gray/green wire connections out of the way under the air filter box, connect the remaining 3 red wires with a bolt like so:

Isolate this connection with lots of electric tape to prevent shorting and route it out of the way. You are done.

Install the air box, put coolant into the new radiator and give it a test. Make sure you follow the coolant bleeding procedure before testing it. Not enough coolant and/or too much air in the system may cause the passenger side of the radiator not fill up quick enough and the temp sensor will remain cold while the car will be at full working temp. This should not happen.

This wiring worked out nicely for me, I did not have to troubleshoot anything as it started working as it should.

Final assembly looking good:

Read responses in


Last videos:

My Supercharged C7 Corvette
My Supercharged C7 Corvette
Long Beach Gran Prix - 3 Part Vlog Series
Long Beach Gran Prix - 3 Part Vlog Series
Steering Joint upgrade
Steering Joint upgrade
Whores Round Here vs THE WORLD
Whores Round Here vs THE WORLD
Repainting Brake Calipers
Repainting Brake Calipers
What if u could turn 14 RPM
What if u could turn 14 RPM
CR-Z on YouTube
CR-Z on YouTube
Test Driving C3 Corvettes
Test Driving C3 Corvettes
Cars · 
Mobility · 
Hardware · 
Motorbikes · 
Travel · 
Boating · 
Aviation · 
About Forumsee · Contact · Privacy policy · Request content removal
We use cookies for navigation analysis and personalized advertisement. By using our site, you agree to our use of cookies.
Accept  ·  Learn more