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Bob, Auto Tech
Category: Chevy
Satisfied Customers: 4087
Experience:  Plus 40 years GM, over 30 certified with Chevrolet, ASE certified Master Tech. (Expired, Retired)
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1995 suburban: fan bottom but other vents

Customer Question

1995 suburban heater and fan works blowing out fo bottom but other vents do not work
Submitted: 8 years ago.
Category: Chevy
Expert:  Bob replied 8 years ago.
Hello, need to know if the back side of your control panel had only wiring or if it also had a vacuum harness connected to it. That will determine which system you have and how we go about checking it out. Thanks
Customer: replied 8 years ago.
how can you tell
Expert:  Bob replied 8 years ago.
When you changed the control did you have to unhook any vacuum hoses made of rubber or was it just wiring connectors?
Customer: replied 8 years ago.
only wires
Expert:  Bob replied 8 years ago.

Good deal, thanks for the added info. This means that the reason the sir flow does not change is because one of the mode doors on the case is not opening up to allow the air flow to change. These door are controlled bu electric actuator motors that are connected to them and they in turn are controlled by the control panel that you have already replaced. Sometimes the actuators fail and sometimes the linkage that connects them to the door will fail or break. Once in a great while the problem will be inside the case with a door itself. You will need to look under the dash at the electric actuators and watch to see if they move when you change the control setting. If they do, then you know the problem is with the linkage or the door itself. If they do not move., then you will need to use a volt meter to see if the voltage going to them changes when you change the setting on the control panel. If the voltage changes, then the actuator itself is bad and will need to be replaced. You should only run into problems if the door itself has problems because then it is necessary to remove the complete HVAC case assembly to access and repair the doors. Below is a picture of what a mode door actuator looks like. This will give you an idea of what you are looking for. It is located just about in the center of the floor area on the HVAC case, under the dash. You may need to remove the plastic close out covers to access them. Thanks

Object Number: 258020  Size: SH

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Customer: replied 8 years ago.
Relist: I still need help.
the electric actuators has no movement. using the volt meter 3 wire connector wich wire do you test there is voltage with test light voltage remains the same on imput wire does resistance? actvater has been replaceed &blend door is moving manually by hand.
Expert:  Bob replied 8 years ago.

If the actuator has been replaced and the control panel also, there has to be a problem in the wiring between the panel and the actuator. One of those wires should change resistance or voltage reading as the control is changed to open the door with the actuator. I thought there was a factory bulletin on a wiring problem behind the dash on that year but have had no luck finding it. I will need to check with some other experts to see if they have any info on it. It looks from the diagram like the white wire is the one that should control the changing of the actuator.Thanks

Customer: replied 8 years ago.
we bought the truck used we did not get any factory bulletin since we bought it. will you let me know if you find out anything. my e-mail [email protected]
Expert:  Bob replied 8 years ago.
They did not issue a recall. I was trying to find a technical bulletin on it. And yes, I sure will let you know. I have requests out now and just waiting to hear from some of the other guys. Thanks
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Expert:  Bob replied 8 years ago.

Hello again, Our first comment was to check the link on the side of the actuator to see if it is broken, but I'm thinking you already did that. Then there was another comment from one fellow but again I think that is how I had you test it before, but will post it just in case. Thanks

The mode doors and the temperature doors are positioned in proportion to the variable voltage signal received from the HVAC control module. The temperature dial at the HVAC control module adjusts a rheostat which provides a voltage signal proportional to the dial position. CKT 733 (LT BLU) carries the voltage to the front temperature door motor. The front temperature door motor responds to the voltage signal. The mode dial at the HVAC control module adjusts a rheostat. The mode dial provides a voltage signal proportional to the dial position. CKT 454 (WHT) carries voltage to the front mode door motor. The front mode door motor responds to the voltage signal.

Have him backprobe white wire and turn mode knob and see if voltage changes it won't change unless plugged in (voltage drop)

Brown should have source and black 0 volts