The Definitive E36 Heater Box Disassembly and Reassembly How-To Guide

[MR E30]

Alright, after an exhaustive search of the Google machine I was able to determine that while the internet has plenty of excellent threads and guides on how to remove the heater box from the chassis, I could not find a single thread that documented the disassembly, cleaning, and reassembly of the heater box once you had set it free.

That doesn’t necessarily mean that some excellent documenter of their build hasn’t done it, but it wasn’t posted in a manner that lets you easily find it with a search engine. That's what prompted the creation of the following guide.

This refresh uses the unit out of a 1997 E36 M3. This car has the digital climate control features. I am not privy to all of the differences between analog/digital, or early/late models of the E36 and how this may differ, so take this guide with a few grains of salt. Also, this guide is ‘definitive’ to me. There very well may be a better writeup, or better ways to do specific aspects of the refresh, but this is as comprehensive as I can possibly make it for what I wanted to accomplish. Which was to open the box up enough to give it a thorough cleaning of all major components while also replacing the heater core. So, there’s that.

This guide will assume that your heater box has been removed from your chassis and is sitting on a workspace of some sort. If you need a guide on how to remove your heater box that internet is full of them, though suffice to say that it isn’t the easiest thing to do and requires removal of your dashboard and metal structural pieces behind the dash as well as some work in the engine bay at the firewall.

The purpose of this guide is to give those who want to refresh their heater boxes all of the information necessary to completely refurbish their stock heater boxes. It will include torque specs (where applicable), cleaning and lubrication guidelines, proper connection removal and reinstallation methods, what tools you will need, and lots and lots of pictures.

All mentions of driver/left or passenger/right are from the perspective of the driver in the driver’s seat with the heater box mounted in the car. For example, the cabin air filter is accessed from the right side of the heater box on the engine side. The location of the cabin filter access in the picture showing the ‘door’, however the box may be situated on my workspace, is not applicable. All mentions of horizontal/vertical, clockwise/counterclockwise, and cabin/engine side follow the same convention.

‘Gentle’ force is just that. A small child could pull or push on something to get it to come free. More aggressive levels of force are categorized with a numbering system of 1 to 5. So, Gentle is a 1. Squeezing your thumb into the side of your index finger as hard as you can is a 5. So, if it says something can be removed gently, and you find yourself pulling/pushing hard, go back and make sure everything is as it should be before something becomes damaged.

Also, this car was a central Arizona car since day one and has very little rust or corrosion anywhere. If you are from a more rust prone location your levels of force for piece removal may be different.

So, with all of that out of the way, let’s get to diving in.

Tools Required:

- T20 Torx bit on a screwdriver
- 8mm socket with extension
- Needle nose pliers
- Wire cutters
- Small flat bladed screwdrivers
- Small pick set

We start with this, the heater box as it is immediately after it was removed from the car (I did wipe down the blower motor cover before deciding to document this refresh). We are looking at the top, engine, left side of the heater box in the following photo.

01 - Heater Box by Michael Rickerd, on Flickr

You can see that the engine bay side is much dirtier than the interior side, which is intuitive.

We will start with the removal of the cabin air filter. We will be replacing this at the end of the refresh, before putting the box back into the car.

02 - Cabin Filter Cover by Michael Rickerd, on Flickr

This photo shows the engine bay, right side of the heater box. My index finger is pointing towards the release knob while my middle finger is pointing towards the base of the cover itself. This part is simply removed by rotating the release knob counterclockwise from the 2 o’clock position to the 11 o’clock position. Once you feel the release knob come free of the heater box gently pull the cover straight away from the heater box.

Here it is removed from the box.

03 - Cabin Filter Cover Removed by Michael Rickerd, on Flickr

Wipe this down and put it on the shelf for later. I have a large bucket of Simple Green and water for dunking and soaking plastic parts, but this door has a gasket on it that I didn’t want to tarnish, so I cleaned up by hand using a rag and Simple Green and a small brush.

You remove the cabin filter by grabbing its tab and pushing the filter towards the engine side of the box. Once that is done slide it straight out, wiggling it gently as you pull if it catches. Discard this as it will be replaced with new.

The space the filter occupied will be dirty, but we will clean that area out at a later step, so leave it be for now.

Moving on, we will switch to the blower motor on the engine bay side of the heater box.

The central cover (previously wiped down, so it’s much cleaner than it was) is removed in four steps.

The first step is popping off the three metal clamps that hold it in place. These come off with your fingers by placing them behind the bent tab and pulling away from the heater box. Level 2 force is required. Once these three have been removed give them a good sanding to bring out the shine.

04 - Blower Motor Clip Removed by Michael Rickerd, on Flickr

05 - Polished Clip by Michael Rickerd, on Flickr

The next step involves removing the central locking piece located where the end of my trim puller is. This is removed by rotating 90 degrees from horizontal to vertical in the counterclockwise direction. Once rotated and freed you remove it by pulling it straight away from the box. Clean this and stick it in a cleverly named bag with the three metal clips from the previous step.

06 - Blower Motor Retainer by Michael Rickerd, on Flickr

The next step involves removing the cable with sheath that ties the two doors of the blower motor together. This is done in three steps. The first involves the right side and is simple. Just use your fingers to pull straight down on the curled portion of wire to free it from the nub there. The next step is to pop the cable out of the central blower motor cover in two places. This is achieved with gentle pressure.

07 - Door Connector Cable by Michael Rickerd, on Flickr

Note the plastic features on the blower motor cover that hold the sheathing in place. The sheathing stays stationary while the metal cable inside slides back and forth.

08 - Connector Cable Unsnapped by Michael Rickerd, on Flickr

The final step is a bit trickier and involves using two small screwdrivers to free the clip from the flat piece of plastic. First, take a small flat bladed screwdriver (red handle) and work it in between the metal clip, located now of the left side of the blower, and the flat piece of plastic. You will notice a small plastic protrusion in this gap (it’s not viewable in the photo and I apologize about that, camera wouldn’t focus). Ensure your screwdriver is wedged in far enough to free this interaction. Next, take another small screwdriver and place it in the curved portion of the metal clip (green handle). With both screwdrivers in place apply low levels of pressure to the second screwdriver to pry the metal clip off of the flat piece of plastic. You may have to grab the left side blower motor door to steady the assembly as you gently pry on it.

09 - Connector Cable Clip by Michael Rickerd, on Flickr

Once removed clean this cable as required. Also lubricate it with some silicone spray if it takes anything other than gentle (anything greater than level 1) force to slide it back in forth in the casing. Mine was a bit tight, and after a clean and lube it came out sliding much, much easier.

Set this aside as we move onto step four of the blower motor cover removal.

Place your fingertips under the bottom side of the cover, as shown below. Gentle pressure pops the bottom free, but it will only swing a few degrees so be careful!

10 - Blower Motor Cover Removal by Michael Rickerd, on Flickr

The top has two retainers and a rounded edge that bites it all together that you can see in the top portion of the above photo. This part takes a decent amount of pressure. Use your left hand to steady the box and grab the side of the cover near the top with your right hand. Pull straight down steadily until the cover gives way. I’d say this is a level 3 force to remove.

Drop this in your cleaning bucket to scrub later.

There is a curved piece of plastic the goes between the cover and the blower motor. It is removed by gently pulling it straight out of the hole. Drop this in your bucket too.

11 - Inside Plastic Piece by Michael Rickerd, on Flickr

Now we will pull the blower motor.

Cut the small zip tie holding the wiring to the motor in the very center of the assembly. Discard.

Use a pair of angled needle nose pliers to pull the two metal wire connectors off of the blower motor. These may take more than gentle pressure to remove, level 2 if I had to say. Note the (2) different connector styles. One is removed by pulling 90 degrees to the wire (Green/yellow wire shown in photo) while the other is removed by pulling in line with the wire.

12 - Removing Blower Motor Leads by Michael Rickerd, on Flickr

Finally, there are (4) T20 Torx bolts hold the blower motor in. All are located in the center of the blower motor in the white holes seen in the photo above( I had previously removed them but then found out that the wiring is easier to manipulate with the blower motor securely in place). Turn those bolts counterclockwise to free them. Put them in a properly labeled bag and stash them on your shelf.

The blower motor wiggles straight out now. The wiring you cut free earlier runs through a plastic slit that holds it in place. Pull the motor out while being cognizant of this arrangement. When it is out far enough you can slide the wiring out.

13 - Blower Motor Wiring by Michael Rickerd, on Flickr

The blower motor is easy enough to get to, so I will be cleaning this unit and reinstalling it back into the box once we are done. My unit has no damage or broken blades. Your motive may vary. Replacements are available.

14 - Blower Motor Removed by Michael Rickerd, on Flickr

With all of this out of the way we will free the temperature sensor from the box. The white temp sensor is located on the right, top, engine bay side. It is freed by placing your thumb underneath its bottom end and your fingers on the box itself, as seen in the photo below. Push up on the sensor with level 3 force. It will pop straight up and out. Pull the wiring out of the plastic protrusions that hold it in place. Swing it out of the way for now.

15 - Temp Sensor Removal by Michael Rickerd, on Flickr

Next, we will remove the servo arm that opens and closes the blower motor doors. It is located on the right, top, engine bay side of the box near the white temperature sensor. Put one finger on the black piece of plastic and use your other hand to grab the rod. Simply lift it straight out with level 1 force. Leave it where it stays after removal. You can also see the freed temp sensor wiring in this photo.

16 - Door Actuator Removal by Michael Rickerd, on Flickr

The next step will open up the engine bay side of the box to allow for a more thorough cleaning of the heater box. The two plastic covers with the blower motor doors in them are removable and we will pop them off.

There are (6) small metal spring clips around the sides and top of the blower motor area. There are none on the bottom. I removed these with a pair of needle nose pliers, grabbing the end of the tab and gently pulling away. You can also place a small screwdriver or pick in between the clip and the plastic and pop it away that way. Put these in a small bag for later.

17 - Small Connector Clip Removal by Michael Rickerd, on Flickr

Quick note: I ‘stack’ all of my parts bags in the order that I create them, so I have an easier time reassembling the box when I am done. When reassembly time comes, I simply grab the first bag (the top bag in my organizational system) and work backwards.

With those (6) clips removed, the blower motor surround comes off in (2) pieces. The left side piece (already removed in photo) is smaller than the right-side piece (what I am holding in the photo). Gentle force straight away from the box pops these off, with the resistance coming from the bottom. You should not have to twist or rotate these pieces to get them removed as you may break the locating tabs/nubs at the bottom.

18 - Blower Motor Surround Removal by Michael Rickerd, on Flickr

With that disassembled we can quickly remove the (2) metal heater box to chassis connectors on either side of the box. They simply slide out of the plastic to either side with level 2 force.

19 - Chassis Connector Removal by Michael Rickerd, on Flickr

With the engine bay side as disassembled as I am going to take it, lets switch to the interior side of the box and remove the main heater core cover.

(7) T20 Torx screws hold the cover on, with 3 sets of (2) being on the top and sides and a single being located at the bottom center. Remove these by turning counterclockwise and stash in a labeled bag for reassembly. Mine were clean enough so I won’t be tossing them in the ultrasonic cleaner like many of the other connectors.

20 - T20 Heater Core Surround by Michael Rickerd, on Flickr

There is wiring running down the center of the main cover and towards the left side. A small plastic strap holds it in place. This strap is removed by gently pulling straight down on the plastic lever located at the top connection (the separated connection in the photo) and gently pulling the strap away form the side of the box. Don’t pry too hard as it can’t rotate very far.

21 - Wiring Retainer by Michael Rickerd, on Flickr

The wiring at the center of the main cover in the photo below cannot be removed from the two white sensors you can also see in the above photo. You must remove the cover by gently pulling away from the box. Then you can remove the two white sensors from the inside of the cover by gently squeezing the plastic and pushing it through the hole (blurry in the photo below). Be careful, as on each of my sensors I broke (1) of the (4) pieces of plastic that clip them in. They still hold fine, thankfully, but they can easily be destroyed and become unusable.

22 - Internal Temperature Sensor by Michael Rickerd, on Flickr

With the cover out of the way and the wiring freed from in between the sensors, the heater core becomes exposed. This is the interior side of the box. Pay attention to where the heater core transitions from the fins to the yellowed tank portion. The heater box to either side of that is where two halves of the heater box (top and bottom) are connected. This will be important later.

23 - Exposed Heater Core by Michael Rickerd, on Flickr

We will next disconnect the two flaps attached through the metal bar that cuts across the center of the heater core height wise.

First remove the linkage on the right side of the box by squeezing the two nubs together and pushing it through the hole in the flap. This took level 2 force.

24 - Channel Door Removal by Michael Rickerd, on Flickr

Next, start on either side and use level 3 force to spread the side of the box away from the circular nub at the outside edge of the flap. Slide the flap free and work it out across to the other side of the box. In hindsight, this step could be done after separating the top and bottom halves of the box with level 2 force. It’s up to you. I will reinstall the flaps and bar into the bottom half of the box before putting the two halves back together.

With that bar out of the way it’s time to disconnect the aluminum feed lines from the heater core. This is the left interior side of the box. (3) 8 mm bolts hold the two together. Remove these by rotating them counterclockwise and put them in a labeled bag. Leave the hoses as they are for now.

25 - Heater Core Piping Disconnect by Michael Rickerd, on Flickr

Transition to the right side of the box and use your T20 Torx to remove (4) screws that hold a side cover plate on. This plate would prevent the two halves from being separated and must be disconnected to continue. This time I simply screwed the screws back into their holes. Sometimes I do this when I don’t want to label a bag.

26 - Side Cover Removal by Michael Rickerd, on Flickr

Still on the right side of the box, but to the left of the cover you just removed, disconnect two of the servos that open and shut flaps by removing the (3) T20 Torx that hold them in place. There is a linkage (shown in the photo below with a white dot) that needs to be disconnected. Work a small flat bladed screwdriver with level 2 force in between the two pieces of plastic and pull down on the linkage arm to set it free.

27 - Servo Removal by Michael Rickerd, on Flickr

Disconnect the two wiring connectors by gently pulling them straight out of their connector. There are no clips or retainers that need to be pushed/manipulated. Just pull them straight out. Carefully set these aside for reassembly.

Still on the left side of the box, cut the small white zip tie that holds the wiring to the box. This tie will also prevent the two halves from separating and has to be removed. This frees most all of the wiring and will allow you to clean it and rewrap it with Tesa Fabric Tape for that oh-so clean look.

28 - Bundle of Wiring by Michael Rickerd, on Flickr

Next, we need to remove the evaporator temperature sensor that probes into the evaporator. It is located on the left bottom of the box and is removed by using level 2 force to pull it straight out of the rubber grommeted hole.

29 - Temperauter Sensor Removal by Michael Rickerd, on Flickr

Finally, there are (6) small metal spring clips, exactly like on the blower motor surround, that need to be removed to free the top and bottom halves of the box. I didn’t get a photo of this step, but they are removed just as before, though it was harder to get needle nose in to reach them, so I ended up popping them off with a pick instead. Put these in a labeled bag for later. The two halves slip apart easiest by flipping the heater box upside down and gently pulling the bottom half of the box upward.

With the two halves separated we can finally free the heater core. Since we disconnected the (3) 8mm bolts earlier we can simply gently wiggle the heater core back and forth and up and down to slide it off of the three tubes. Be careful not to damage these tubes in any way as you want a satisfactory seal when installing the new heater core.

30 - Heater Core Removal by Michael Rickerd, on Flickr

You can see the (3) black O-rings on the ends of the heater core feed tubes in the picture above. These need to be removed and replaced with the fresh O-rings that will come with your new heater core. BE CAREFUL! Marring the aluminum will probably result in a leaky heater core. I used my fingernails to remove them. It was difficult, and they were pressed into the aluminum from thousands of heats cycles, but I was able to break a small portion of the hose free, and then work my nail around the O-ring until it slid off. I would not recommend the use of a screwdriver, or any tool, regardless of its composition, that you can get a lot of muscle into. Trust me here.

Once those are off, set them with your old heater core. I never throw old things away (learned my lesson too many times in the past) until after the new thing has arrived and has been successfully installed. Sometimes mistakes are made and it’s good to have your old stuff on hand for comparison purposes.

Alright, moving on, we are almost there.

One of the last major portion of the box left to be removed is the ac evaporator. With the box disassembled it is only loosely held in by the tubing connection that is in the engine bay. The white/yellow clip shown in the photo below pops off of the metal piece that holds the two pipes in place. It has two small clips in the center of the longer sides that require level 2 force to pop off of the piece of metal.

31 - AC Line Surround by Michael Rickerd, on Flickr

Once that is removed, carefully work the rubber gasket off of the plastic of the heater box and push it into the hole (into the image above). You may gently pull on the ac tubing to keep the rubber from snapping back onto the plastic. Once the rubber is freed from the hole simply lift the whole assembly out of the box and put it on your shelf. I won’t be doing any modifications to this component of the box, so I will just clean it (gently vacuum the fins and wipe down the exterior) and pop it back in once the time comes.

With both the evaporator and heater core out of the way there is only one more step to take to get the box disassembled. Looking at the top of the box, towards the engine bay side, you will see (4) T20 screws connecting a plastic flange to the box. You can see my screwdriver placed on the last of the (4) screws for removal (I forgot this picture earlier, so I reassembled the box a bit to get the photo for you guys, so that’s why the wiring is already wrapped). You will most likely have to cut a zip tie or two, as the wiring for the temperature sensor and blower motor run along this flange. Note their location for reassembly phase.

32 - Screw Removal by Michael Rickerd, on Flickr

On the left side, center of the box you will notice the T20 screw in the image below. It has to be removed, though the entire servo does not need to be removed if you don’t want to, to free the two pieces of box.

33 - One Servo Screw Removal by Michael Rickerd, on Flickr

The linkage arm from earlier that controls the blower motor doors will need to be disconnected, as it is connected to both portions of the box that we are trying to disassemble. I did not have luck removing the rubber grommet from the plastic, so I opted to pull the linkage out of the servo instead. The photo is blurry, but you simply need to pry the metal arm towards the foam on the right (in the ~2 o’clock direction in the photo below). This requires level 2 force for removal.

34 - Door Linkage to Servo by Michael Rickerd, on Flickr

With that disconnected the two piece will come apart. All of the wiring attaches to one part of the box, so fish it around the heater pipes and you are all set. You have now disassembled all of the main components of your heater box. If you have made it this far without an explosion and you want to remove the servos and continue to break the box down, I would go for it. But not me.

This is as stripped as I will take my heater box. I now have access to all of the components and can clean everything as thoroughly as I want. There is a bit more that can be taken apart, but I personally have gone far enough.

All plastic gets wiped down and cleaned as I remove it, and then it gets checked when reassembly time comes, and I will touch up anything that isn’t clean enough for me. Reassembly time is the time for the watchful eye, and the little bit extra time required to get things just right (for you). During reassembly, I noticed my right blower motor door was destroyed and unusable. The left side of the image below shows the broken section where the linkage from the servo comes in to swing the doors open and closed. So I will be tracking down another one of these as this one is no longer usable.

35 - Broken Right Door by Michael Rickerd, on Flickr

The wiring had all of the old cloth tape removed, then each wire was wiped down with Goof Off to remove any residue and then I fully retaped all of the wiring in its entirety, being careful to leave any connectors out of the wrap at the appropriate location as required.

36 - Wrapped Wiring by Michael Rickerd, on Flickr

As these guides usually go, assembly is the opposite of disassembly. Basically, read the guide backwards, look at the photos in the reverse order, and pull from your cleverly marked bags one by one as you organized/stacked them. I will add some tips below for those who are interested:

- Wiggle the heater core back onto the tubes in the same manner that you did for disassembly. You can use the 8mm bolts to pull the core back onto the O-rings. You can use a small amount of silicone grease to make this go easier. The torque specs for the (3) 8mm bolts is 27 in*lb, i.e. not very much!

- The T20 screws are threading into plastic, so go easy. No ratchets for reassembly. Go with a screwdriver to tighten the screws. You will feel the screws spin easily at first, then they will get slightly more difficult at the end for ~3/4 of a turn and then the screw will abruptly stop. This is where you want to be. No more rotation after this point, just nice and snug against that hard stop.

- Reconnecting the small metal spring clips is easy. Place the sharp, non-rounded edge into the plastic, and then push inwards on the center of the clip to snap it onto the other side of the plastic.

- Review the guide at each step so you don’t forget a component or screw. Nothing quite like back tracking, so might as well avoid it if you can.

- Clean things. This is your ‘one time’ opportunity to get this right.

- Pivot the servo linkage arm that controls the doors so that the doors are closed against the heater box. This will make the box fit back into the chassis much easier. Do not attempt to pivot the door to get it closed! Instead, grab the servo arm itself and pull/push it (depending on your orientation) to get the door closed. This requires level 3 force to get done.

- Don’t forget to zip tie the two wires that power the blower motor to the central body of the blower motor. Don’t want those flopping around when you set the heater on high on day.

And with that, we are finished. Please let me know if you have any questions. Thanks for reading.

- MR E30

Approximate Hours to Complete: 8 hrs

[Ernlis921]

This is one of the best instructional posts I've ever seen on here. My problem is definitely associated but I haven't been able to isolate it. My heater heats, my AC cools, and the blower blows but no air flows out of the vents. It's not a clogged cabin air filter (there's no filter in the housing). One clue is that when I start the car, I can hear a [clunk] in the upper part of the dash board above the radio. It sounds like one of the blend doors that's supposed to open tries but fails to do what it's supposed to do. Based on your expertise, what should I be looking at?

[jpwrx2002]

My 1997 318is had a leaking heater core and while replacing the core I broke the arm at the top right of the heater core that's connected to one of the stepper motors.This arm operates all the flaps. The part number on the arm is 90.704.41.497 Does anyone know how I can get one? or even how to disassemble this to that I can replace it. Really struggling to find any information on this.