1998 Blazer Heater Control Problem

Hi, I have a 1998 Bazer, 4.3 Engine, Heater Control is on A/C. Won't change when I turn the knob, no matter what I turn it to, the heat or cold air comes out the dash vents. Temperature and blower seem to be fine. I read an earlier post about the same type of issue. I have A/C but it is not climate control.
Question, could this be the control mode actuator (Black Hose) or a combination of the other hoses.
Question, where in the $^%* is the actuator. Have all of the lower dash panels off and the black hose disappears behind the heater box to the right of the gas peddle.
Is this as easy as starting at the controller and testing the vacuum back through the system, again is it the black hose that I should be testing?
Help........

There are 4 vacuum operated actuators in the manual HVAC system. When the engine is at idle, do you have vacuum on the black vacuum line that connects to the mode switch?

I won't know until tomorrow. Took everything apart today to do a visual, hoped to find something unplugged or an obvious leak. I have all of the under dash panels, and the center console removed. I have arranged to borrow a vacuum tester tomorrow. When you say the mode switch are you talking about the actual dash controller. So far visually I have been able to find all of the actuators under the dash, just to the right of the gas peddle except the one the black hose goes to. It seems to disappear behind the heater box at the upper forward corner. I have also checked the lines under the hood. Everything looks good. I will start with the black hose at the controller plug in tomorrow. If there is no vacuum to me the next likely spot is the plug that is right behind the glove box. From there unfortunately the next will be the actuator that I can't seem to find.
Am I right that the cause would seem to be from the black hose, I am under the impression that this is the place to start?

The black line goes through the firewall to engine vacuum. It connects to the controller, (mode switch) in the dash, (the dial you turn to make the air flow change). There are 6 ports on the back of the mode switch. The black line goes to port # 6 and must have vacuum if the engine is running. The other 5 ports go to the 4 different actuators. 2 of the actuators have 2 vacuum lines to them. When you have the vacuum tester, I can explain when each port should have vacuum.

Thanks for your help so far, I have the schematic showing the ports, and what each colored hose is for. This is why I origionally suspected it was the black mode hose as othing is switching. Now I have the tester. I am going to start with the black hose right behind the rotary switch. If there is no vacuum I will go to the connector behind the glove box, and then back over to the gas peddle location. There is a connector between the other 2 actuators for the other lines. The black has what looks to be a corner connector. I will check there and then all the lines under the hood. I haven't been able to locate the mode actuator itself yet.

Can you tell me exactly where the mode actuator is mounted. As I mentioned the black hose disappears bhind the top fwd corner of the heater box assy. I thought it was supposed to be mounter right up at the top fwd corner of the heater box, just above the gas peddal?

The black vacuum line goes from the mode switch through the firewall and then to a check valve and reservoir. It does not connect to anything else in the passenger compartment. The mode valve actuator has two vacuum lines attached to it, brown and yellow. You can trace them back to the mode valve actuator. The brown line goes directly from the mode switch to the mode valve actuator. The yellow line also runs from the mode switch to the mode valve actuator, but there is a Tee with a grey line that goews to the slave valve actuator. Been a while since I had a 98 apart, but I think the mode valve actuator is mounted on top of the heater case, left side. Removing the radio might help to get at it. Make sure the door moves freely in both directions.

Thanks by the way for posting back so quickly, you have been a big help so far. I understand about the black hose now, thanks for clearing that up and it makes sence.
I tested the black hose. I disconnected the plug from the back of the heater control. At that location on the plug there is vacuum so the black checks out as okay.
I have been able to locate the other 2 actuators for the other hoses. Do I disconnect the lines one at a time at the actuators and test by setting the switch at a certain position, if so do you know what they are?
Or is there something else I should check first?

With all of the vacuum lines and the mode switch connected, the engine at idle, the mode switch in MAX position, the following lines should have vacuum: black, brown, blue, orange. The yellow line should have vacuum in the OFF and HEATER positions only.

If you have a vacuum pump with a guage on it, disconnect the main vacuum connector on the mode switch, apply 16" of vacuum to each line, (not the switch) and each one should hold vacuum.

I didn't have a lot of time last night, prior committment. I don't have a vacuum pump, only a guage. I will start with the first test you mentioned.
If I understand correctly the yellow is to be checked in all positions other than the air conditioner position.
Until I hear from you the only thing that confuses me is the "the mode switch in MAX position" by this will I test the lines in the "Max Air" position. This is the only "Max" on the position control (Mode Control). I will test them in both Air conditioning and Heater positions just to make wure that I have some answers ready as far as what is working with which position. The yellow is clear so I can check that okay.

The yellow vacuum line should have vacuum only when the mode switch is in the OFF and HEATER positions.

Will do, I'll post in a bit with results.
Thanks for the help