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Discussion Starter #1 (Edited)
Please point out inaccuracies and suggestions for improving this guide. This guide is meant to consolidate the information from several threads and several BB members on repair of the stepper motor assembly.

A download of this guide is available at the bottom of this post.

Introduction

The HVAC system actuators (stepper motors/gearboxes) in the 1991-93 12v 164's have a deserved reputation for poor reliability. There are two actuators, the one that drives the hot/cold air mixer door (hereafter call the "heat stepper,") and the actuator that rotates the air distribution drum which directs the blown air to the various ducts (called the "drum stepper").

The reason for this is the motors themselves over-torque the relatively delicate plastic gears in the individual gearboxes causing gear teeth to strip. When this happens, it can jam the gearbox, or just stop rotation since teeth are missing from a gear. The teeth can either be completely stripped, or may be damaged as shown in the pictures below. Even a slight scallop to a tooth is enough to cause intermittent jams. If you have an 94-95 LS, the steppers motors were de-torqued at the factory to to keep from damaging the gearboxes like the earlier cars. Hence, there are few problems with LS Steppers.

The usual symptoms of a bad stepper motor are:

1. "Tick tick tick" sounds from beneath the dashboard. This is the stepper motor trying to move the broken/jammed gears.

2. Unable to modulate heat.

3. Unable to change where the air blows out of the vents.

An adjunct to this is there could also be a broken air distribution drum where the drum stepper mates to it. When this breaks the air distribution drum usually rolls free of the stepper motor and comes to rest in the front windscreen defrost position. If you have a "tick tick tick" coming from the dashboard, are unable to control the vented air with the HVAC control panel, and the air will only come out of the defroster vents, you could have a failed stepper or a broken distribution drum, or both.
 

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Discussion Starter #2 (Edited)
This guide describes how to overhaul/refurbish the stepper motor assembly only. Removal and replacement of the stepper assembly and bracket is not covered here. The stepper motor assembly consists of the heat stepper and the drum stepper mounted on bracket that attaches to HVAC air box. Each stepper motor is attached to the gearbox via two small screws. The gearbox is held in place by a lug on one end and a snap fitting on the other. It is most likely that the problem with the stepper motor is a single bad nylon gear in the gearbox. While easy to repair once on the bench, getting the assembly off/on the car is what takes time.

New stepper motor replacement assemblies are very difficult to find and very expensive ($300+) even if you do. However, the problem is almost always the fault of one gear, so just changing out that gear puts the assembly back in operational order.

In your internet searches, you may find an old company called AlfaPro. For a time, they made brass gears as replacements. However, that is no longer the case, so our primary source of replacement gears are salvage gears out of early 90's BMW HVAC stepper motors. Specifically, the motors that we know have identical gears are:

Early 90's, 3,5,7,8 series E31, E32, E34, and E36 series cars. The stepper motors themselves should be labled Bosch 0 123 800 001 or 003. There may be others that work, but these are the ones I know are compatible for salvaging the gears.

Typically these salvage steppers sell for $15-$20 each, which means you can fix both steppers on your assembly for less than $50 in parts.

Any BMW specialist salvage yard should be able to source them easily. A couple that I've used are:

Blue Octane BMW (MI) -- "bmwpartshop" (eBay shop) They may not advertise these specific stepper motors on eBay. Send them an email inquiry via eBay.

Atlanta European Auto Parts (GA)
(770)-917-0690
 

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Discussion Starter #3 (Edited)
Disassembly and Inspection of the Stepper Motor Assembly

1. Remove the two screws from each stepper motor attaching it onto its respective gearbox. Lift the motor out of the gearbox. Don't worry, nothing is going to fall out. The motors will still be attached to the bracket via their wires. If desired, you can snip the wire ties holding the wires the bracket and snap the brown main connector out of the bracket. Just be sure to mark which is the heat motor and which is the drum motor, although it should be obvious by the wire length and way the they're bent.

2. Unsnap the white plastic cap off the drum stepper.

3. Unsnap the gearboxes from the bracket. Note that the heater gearbox has a long arm and the drum stepper a keyed shaft. If the heater stepper is in the extreme position, you can very gently move the arm slightly so it'll clear the bracket. Be gentle because you are moving the inside gears faster than the arm is moving.



4. On a clean dry pad, split the gearbox halves of your salvage units by releasing the four small lock tabs holding them together. These can be a little difficult to separate, but keep at it with some small screwdrivers. I've had best luck be separating the fat end of the gearbox first and working forward. Also sometimes the very pointed end of the gearbox needs a bit of persuasion with a very small pry/screwdriver to snap apart. If you use too much force sometimes you can crack off a small part of the case's side, however, it shouldn't affect the operation of the gearbox.
 

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Discussion Starter #4 (Edited)
5. Spit the gearboxes halves and do the same thing as with the salvage gearboxes. Extract the two middle gears (in this example, green and pale amber, but could be black as well) Using a good magnifying glass, examine each tooth of all gears to make sure they are whole and not scalloped/damaged. Clean the old grease off very gently. Do not damage the teeth. Set the gears aside in safe place.

In addition to the picture below, see pictures on first page for examples of what to look for. In the picture below, I marked the bad teeth on the amber gear with magic marker. This gear is the usual one to fail.

6. Carefully clean out the caked/dried grease and re-lubricate with some normal wheel bearing grease. Use grease very sparingly, just like what was originally used.

7. Rotate the gears carefully by hand and feel for any hang-ups or binding. When satisfied, snap the box halves back together again making sure that the actuator arm is in a position that you can reinstall the gearbox back on the bracket.

8. Snap gearbox back onto the stepper bracket and reinstall the stepper motor to the gearbox. Make sure that the snap tab on the back of the stepper motor actually locks into the bracket and is secure.
 

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Discussion Starter #5 (Edited)
De-Torque/Resistor Modification

As mentioned before, the 91-93 models had stepper motors that produced too much torque and that often caused gearbox failure. A fix for that is to "de-torque" the stepper motors to the point they won't damage the delicate gears. This is accomplished by installing 33 ohm resistors stepper motor wires. Be sure to get 33 ohm - 1/2 watt resistors, NOT 33k ohm (33,000 ohm). Big difference. Easy to confuse.

There are two gray wires that go from the connector to each stepper motor. They are connected together at the connector. Therefor, you can install a single resistor for each stepper motor, or a separate resistor for each wire (2) to the stepper motors. If you use a single resistor for each stepper motor, you'll need to cut both gray wires and twist them together so that all current for both wires to the stepper motor goes through the single resistor. In the example below, I installed a separate resistor on each wire
(2 resistors/motor x 2 motors = 4 resistors).


Solder, shrink wrap, and tie the wires to the bracket. An alternative method is to install the resistors on the car's wiring harness just prior to the connector to the stepper assembly bracket.
 

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Discussion Starter #6 (Edited)
Testing the Stepper Assembly

Before installing the Stepper Motor Assembly, you should test it to ensure normal operation. With the assembly in your hand, simply plug into the car's wiring harness, then run through the controls on the HVAC panel. Start with the heat control. Go full hot and watch the stepper arm move to the full hot position, then go full cold and watch the arm to to full cold position. Do this several times to make sure you're not getting any hang-ups or hesitation during movement.



For the distribution drum stepper, press each button for changing the vent position. The drum stepper shaft should position itself as shown. Below are pictured the various positions, one oriented to the large tang without the white plastic cover and the other with the cover installed and oriented to the indexing ridge on the shaft itself. Run the stepper full range several times to ensure normal operation.

END
 

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while I dont have these to my knowledge as I have a very arly euro car, this write up is of interest. My compliments Mr John... ciao, chris
 

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I had the problem a year ago also, it's possible to access the stepper motor from the passengersite, you do have to remove the dashboard partly, it's a hell of a job.
To bad i had to remove the whole dashboard because of the complete heaterunit removal, inside flap for directing the air was broken....

The worst part is to remove the airco unit, behind that unit there are 8 bolts that needs to be removed, these bolts hold the metal frame where the dasboard is fastened at to the chassis.

at the moment i'm working with a collegue on a "project" to make new toothwheels out off brass. If this works ok i let you know, in case you need to replaces them it will be for 1 time.
 

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A model year '94' - same issue?

What a great write up. My car is a 1994 164Q. Would it be susceptible to these issues?
 

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Discussion Starter #11
What a great write up. My car is a 1994 164Q. Would it be susceptible to these issues?
It's my understanding that the 24v cars had the motors /modified de-torqued to correct the problem from the 91-93 12v cars.

Regardless, I would make it a habit to normally run the vent system in the center vent/lower vent setting to keep the motor from constantly driving the stepper motor to the limit position. Also turn off the HVAC system (OFF button) before shutting down the engine.

Maybe fear and superstition, but I do it as a matter of course.
 
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