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Bob's Blog


Bob's corner Blast From The Past. This is an old web page (2009) I created when my GMC Jimmy would roast you out in the summer even with the air conditioning on. I have helped countless folks in solving this problem and if you are lucky to still own one of these old relics, maybe this will help you and save you time, frustration and a lot of Money $$$$$.

  Blend Motor Gear Box Assy.

Click to enlarge any photo.








GMC Blend Motor Gear Assy. 
GM official name “mixer actuator”


GM OEM Part # 89018365 5 pin Actuator.  There is also a 4 pin so make sure you give your GM Dealer the VIN to your vehicle.  The dealer is after the last 8 number (letter) sequence of your VIN.  If I said this unit looked like anything I would say it looks like a miniature grand piano.


Welcome back to Bob’s Corner.   


Let me guess. No one can answer your question why your blazer or jimmy is hot inside even though your air conditioning works.  And the heat coming out of the vent is not just warm it’s really HOT. Right.  Or your unit is blowing hot, cold, hot, cold.  Hopefully this will answer some of your questions and save you some time and money.


I know you have already checked all of the vacuum units and they all work so what’s the problem.


1.       Symptoms:  Hot, really hot air blows out the vents.  No cool air at all even though you know the air conditioning works and the receiver drier is cold and wet.


2.       Hot then cold, hot then cold.  The unit is confused and can’t find its center point.  May be a simple calibration or a stripped gear. 


3.       Rattling noise behind the glove box.  The unit gear is stripped inside and trying to find its center point.  The noise you are hearing is the broken nylon gear spinning on the splined blend motor shaft.  Therefore the “brrrrbbrrb” noise.  OK, I’m not good at noise writings.


Yes, there is hope for those trying to find out why their air conditioning works, but yet nothing but HOT air blows out the vents.  This problem was diagnosed on a 2000 GMC Jimmy but the unit is the same on earlier models.


It appears that the main gear in the Blend Motor gear box breaks or cracks.  The blend motor is located right behind the glove box.  The complete glove box must be removed.   After removing the glove box, unplug the vacuum line from the vacuum unit.   Lift up on the vacuum unit to remove it from the vent box.  You do not need to remove the lever from the nylon (white) lever on top.  Just swing the vacuum unit out of the way. 


The blend motor gear box is located right behind the vacuum on top of the vent box.  You will need to remove both 7/32” screws from the blend motor to remove it.  Don’t worry about the wire harness hooked to it.  You can remove the wiring harness after you remove the blend box very easily.  Use a boxed end wrench 7/32” instead of an open end wrench.  It takes a little time removing the two screws once you get your left arm in position but they will come out completely.   Once the screws are out you will need to lift up the blend box to remove it off the blend shaft.  I used a long screw driver to pry up slightly and it came right off.


Once you remove the blend box you will need to remove the 4 fillip screws.  Lift up on the center idler gear and you will see if  your gear is cracked, broken in half or missing teeth.  You can replace the whole unit or just the gear.


Important Note:

Before you install the new or repaired unit, turn the door shaft that the unit mounts on.  Make sure nothing is caught in the door and it moves freely. Otherwise when you install the repaired or new unit you may break it immediately.  If you find it binding you will most likely find yourself cutting a hole in the side of the blower housing to get the object out.  I can’t imagine how long it would take for someone to pull that whole unit out and apart.  If it were me I would cut a 4”x4” hole in the side of the blower housing and seal patch it back up with a piece of aluminum, black RTV and rivet it back on.  The housing is quite soft and I did cut a 1” inspection hole to see if the top vacuum door was working and you can see that door open and close.  


When re-installing the new or repaired unit, remove the RDO 15 amp, fuse in your main fuse holder to the left of your steering wheel before you touch the ignition switch.  You must open the driver’s door to open your fuse door.  Not the fuse box under the hood.   Install your repaired or new unit and re-install the RDO fuse.  Turn your ignition switch in the “ON” position for at least  1 minute. Do not start the car at this time and DO NOT TOUCH any buttons in the car while the key is in the “ON” position?  That is very, very important.  This allows the unit to calibrate itself.  Also you will see that there are timing marks on the black cover and you must align the marks with the notch on the new drive lug.  Follow the instructions that come with the new unit.  The list price on the little unit is about $265.00 so find someone who can get a discount to cut the cost.  Or go to a car junk yard and try to find a good used one.

  You may hear the unit center itself while the key is in the “On” position.  Do not start the car at this time.   Turn the key off.  Now start the car and turn your temperature control all the way hot then after a moment or so, turn it all the way cold.   Shut off the car and restart it.  The unit should now be re-calibrated.    Do not unhook the battery during anytime of installation as this only confuses the unit as you are touching the cable to the battery.  From what I understand this gear breaks when the power is jolted such as removing and installing the battery if the key is on or a sudden voltage drop or increase.  The unit tends to “Overreach” its intended beginning and ending cycle causing the gear to break. 

 Be sure to re-hook the vacuum assembly back up prior to installing the glove box and the vacuum line to it.

 This is not a perfect service repair for everyone and it doesn’t cover everyone’s circumstances.  However, it is to help you to determine if the blend motor is the problem with your unit.

 Like I said before in some of my repair stories “you can be a parts replacer or you can save yourself time and money by finding out what is really wrong”

 Some causes of this problem are the following.

 1.       Recent battery disconnected and re-hooked while the key is on. Or a bad battery connection.  Especially occurs in the “Auto” mode fan units. 

2.       Not letting the unit reset itself after installing a new battery. Needs one minute in the key “On” position before starting the car.

3.       Dead Battery and new battery installed.  Make sure your key is in the off position. Better yet, pull the keys before installing a new battery.

4.       Object(s) blocking the Blend door in the air duct. Such as pens, pencils, crayons, other objects that can interfere with the blend door and jam and break the blend gears.

 I put the new assembly in my vehicle and it works GREAT! No problems.  Heat works, Air conditioning works and I’m a happy man.

I hope you enjoy the new items on this site and come back often to see what’s new.

 Thank you for visiting.


Bob Baumgras









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