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How To Fix Mode/Blend Door Actuator Problems

'98 GMC 1500 Mode Door repair with detailed technical info
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How To Fix Mode/Blend Door Actuator Problems

Postby Gawain Tomlinson [OP] » Apr 15 2013, 2:25am

’98 GMC 1500
Mode Door Actuator Repair

By Gawain Tomlinson
San Diego, CA

My ’98 GMC 1500 was directing air only to the floor, regardless of where the Mode knob was set. The Mode door is located on the left side of the heater core box, which is located under the center of the dashboard.

The Blend Door and the Mode Door actuator are identical parts. I also believe that the Resirc Door actuator is similar, if not the same. The Blend Door actuator is located in the center of the core box, up under the dash. The Mode Door actuator is on the left, and the Resirc Door actuator is at the top of the dash near the windshield.

There is a replacement Mode or Blend actuator available at most auto parts stores, Dorman Part# 604-107, price $39.99.

If you look under the dash from the driver’s side you will see the Mode actuator. Turn on the ignition, set the fan to low speed, and rotate the mode knob through its range. There is a visible slot in the shaft of the actuator. You should see it rotate about 90 degrees as the knob is turned.

In my case, I could see it move only a few degrees. I found that someone had screwed a wood block onto the bottom of the heater core box using 2 inch drywall screws. It was obvious that at some point something had been attached to the wood block. I removed the block, but the symptoms did not change.

I turned off the ignition, and removed the actuator by removing two screws at the front and back of the actuator. I lowered it and disconnected the electrical plug at the top.

Do not try to operate the actuator while it is removed from the core box. It may lose its index position, and you may not be able to fix it.

GM Blend Door Actuator

Unplug the actuator. Turn on the ignition, and turn up the fan speed. Reach up with your fingers, and rotate the mode door linkage shaft to verify that it moves freely, and seems to work ok. If the door is working properly, air will come out through each of the outlet positions as it is rotated, floor, vent and defrost. You may feel some slight binding of the linkage. If you can rotate it easily with two fingers it is ok. The actuator is geared way down so it has plenty of power.

I opened up the actuator by removing the four screws on the front.

Actuator Opened

When inspecting for problems, look for missing gear teeth, stripped, or broken gears. Make sure that the drive gear is firmly attached to the output shaft. Also look and smell for burned electrical components. I understand that bad gears are the most common problem.

If you remove gears, make sure that you mark their positions, and put them back in the same position. Do not attempt to remove the position sensing gear. It is riveted to the potentiometer. If you get the position sensor out of sync with the output gear, go to the section “Zeroing the Actuator”, later in this article.

In my case the problem was obvious. The main drive gear was cracked all the way through to the output shaft.

You can test the actuator inputs by unplugging it, turning on the ignition, and checking the wire harness from pin 7 to 10 (outside wires) for +12V. Also check from pin 7 to 8 (middle wires) for a continuously varying voltage while rotating the Mode Selector.

(Note: broken drive gear)

Installing the actuator

1) Move the mounting plate from the old actuator to the new one.


2) Turn off the ignition switch and the HVAC control switch (if applicable).
3) Plug in the actuator.
4) Rotate the door linkage until it is in approximately the center of its travel.
5) Slide the actuator onto the linkage shaft. Rotate the linkage not the actuator gear.
6) Rotate the actuator body until the upper elongated hole aligns, and push it onto the guide pin at the top.
7) Install the two screws at the front and the back of the mounting plate.
8) Turn on the ignition and rotate the Mode Selector to test.

Circuit Description

I reverse engineered the GM Part #16177412. The schematic is shown below:

The actuator is a self contained servo system. Its inputs are supply voltage and a position command voltage.

The servo is attached to an internal position sensing potentiometer (variable resistor). The servo command voltage is equal to and opposite the position sensor voltage. The servo rotates in the correct direction until they are equal and opposite. The reference voltage is (+12V / 2).

The command voltage and the position voltage are summed through a resistor network. In the schematic it is R1, R2, R10, R11, and PBR. The command voltage comes from a ~10K potentiometer attached to the Mode Selector knob. If there is an open control input at P1-8, the actuator should drive to the center of its range and stop. If the position sensing pot, PBR goes bad, the results are unpredictable.

The summed voltage is compared to a voltage ½ way between ground and +12V. In the schematic that is R4 and R5. The +12V may vary but that does not matter, because everything is referenced to the +12V level (whatever that happens to be, normally 12V to 14.5V).

The servo is controlled by a Motorola 0372DP1 Dual Power Operational Amplifier integrated circuit. The two amplifiers are connected 180 degrees out of phase on opposite sides of a DC motor. This configuration is known as an H Bridge, and it allows a motor to be driven in either direction from a single supply voltage. The inputs to the amplifiers are the servo sum voltage and the comparator voltage.

The gain of the comparator is set by R3 and R7. The 3.6M Ohm resistors provide circuit stability at near target position, while keeping the gain high so the circuit functions as an on/off comparator.


Connector Function
P1-5 Motor V- (Test/Diag)
P1-6 Motor V+ (Test/Diag)
P1-7 Ground
P1-8 Position Cmd Input (Var 0-12V)
P1-9 Pstn Output (Var 0-12V) (Test/Diag)
P1-10 +12V
GM Part #16177412

The IC is powered by an overvoltage protector, consisting of the resistor/diode chain across the power input pins, P1-8 to P1-10. The overvoltage protection is provided by the Zener diode CR1. The diode had “9727” printed on it. I was unable to find a datasheet for that part number. However a bench test showed that the diode did not reach its regulating range with supply voltages between 12V and 14.4V. My guess is that it is a 30V regulator, which would make it comparable to a 1N972 Zener diode.

I unsoldered the Position sensing potentiometer, and got a reading of 9.45K Ohm. This is virtually identical to the sum of the comparator voltage divider resistances, R4 plus R5.

Zeroing the Actuator

You should use this procedure only if the actuator has rotated beyond its normal operating range.

There are numerous cautions in the blogs, and included with the Dorman replacement Part #604-107.

Installation and Warning Notice

I have not tested this, but I believe it should work.

1) Remove the cover from the actuator, by removing the four Phillips head screws.
2) Remove the idler gear that meshes with the output gear.
3) Rotate the output gear until the indicator slot on the output shaft is vertical (aligns with the center mark on the cover).
4) The position sensor gear’s indicator dot should align with the point where it meshes with the output gear. See diagram in the upper right of the schematic, as well as the picture below.
5) If they do not align properly, you can remove the output gear and manually rotate the gears to the proper orientation. The position gear rotates quickly in response to output gear motion. It would be easy to get it mis-positioned by a tooth or two.
6) Reinstall the idler gear and put the cover back on.

(Observe the output shaft and the position sensor gear)

Further Troubleshooting

If you have a good actuator, and it still fails to work, the problem is probably with the HVAC control unit. It is the panel with the switches and knobs in the dashboard. Such as it is, it is the brain of the HVAC system.

Here are some useful links I found while troubleshooting:

Troubleshooting Document
http://www.justanswer.com/car/0u7km-199 ... ables.html

HVAC Control Unit GMC 1500 Troubleshooting & Repair Instructions
http://www.flashoffroad.com/Maintenance ... oblems.htm
http://www.flashoffroad.com/Maintenance ... rlhead.pdf
http://www.sparkys-answers.com/2011/08/ ... -from.html
http://www.sparkys-answers.com/2011/08/ ... edure.html

If you are a mad scientist, and end up with a bad actuator, the circuit board could be useful for all kinds of applications, such as low power motor drivers or amplifiers. Do a Google search on “0372DP1 Datasheet”.
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Gawain Tomlinson
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Re: How To Fix Mode/Blend Door Actuator Problems

Postby backer » Jan 02 2014, 2:15am

:D Thanks a ton Gawain. This was an accurate and informative post. I had the same problem with air only going to the floor and was able to fix my problem by just re-zeroing the accuator. So I verified your procedure and it does work. Just one added note. The acuator, mode door linkage AND the mode selector on the dash all have to be in the center position to for the install to work correctly. Thanks again.
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Re: How To Fix Mode/Blend Door Actuator Problems

Postby Welcher774 » Jan 02 2014, 2:59am

Very informative thanks for the detailed how to and sharing.
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Re: How To Fix Mode/Blend Door Actuator Problems

Postby renaissanceman » Mar 16 2015, 10:42pm

Just an FYI, these gears are now available on eBay, so there's no need to buy a new actuator:

http://www.ebay.com/itm/GM-Blend-Door-A ... RK:MESE:IT
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Re: How To Fix Mode/Blend Door Actuator Problems

Postby jerezac88 » Apr 30 2015, 2:02am

I need help. I had exactly the same problem but when I was trying to remove the actuator it was so stuck to the shaft and the linkage shaft broke in half. I have no idea how am going to replace that shaft in order to put the new actuator back.
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Re: How To Fix Mode/Blend Door Actuator Problems

Postby cmm6798 » May 17 2015, 8:26pm

When I opened up my actuator, all the gears were in tact, but the gears are over greased and there was grease on the contact strips causing. The grease was preventing the contact strips with making contact. I cleaned off the extra grease on the contact strips with a Q-tip, and it works the way it should for now.
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