Heater modes not switching from defrost to heat to vent I think it may be a vacuum hose problem and I have not found a vacuum hose routing diagram. 1987 chevy S10 Wrangler ext. cab
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If the air vent mode doors are vacuum operated, (though some are cable operated), they could have disconnected a vacuum line from engine. A vacuum diagram sticker should be located under the hood. The a/c hose will be listed as (HVAC hose).
If the heater door controls are vacuum operated, you will have a hose from the engine enter the firewall and go to the back of the heater controls, where it enters a connector with several hoses going to places on the heater case where they work the doors by actuators-diaphragms that retract with vacuum applied causing doors to open or close. Begin with ensuring the vacuum hose from the engine has vacuum at the back of the heater controls with engine running-just pull off the right hose and check for vacuum-the hoses may be color coded to help you. If you have vacuum there, reconnect that hose and move to the actuators on the heater case, check if vacuum signal is routed there. Check the actuators for proper operation-sometimes they fail. And sometimes things that fall into vents get caught in the doors and jam up. Good luck, hope you find the problem.
If air not blowing out any other vents in any other modes. If it continues blowing out of the defrost vent , suspect a broken vacuum hose between the engine and firewall where it goes to the heater controls. Most systems that use vacuum motors to move the mode doors around are spring-loaded to go to defrost mode when there is a failure. That insures that even though your feet might freeze in winter, you will have a clear windshield. There is only one vacuum hose and check valve so there is nothing to mix up under the hood.
Besides that vacuum supply, it is also possible to have a defective vacuum motor under the dash. The clue to finding that is that some functions will change but some things will work improperly. It's when nothing changes at all when you switch modes that the vacuum supply hose should be suspect.----------------------Your vent controller is not working. Is the air coming out the defrost vents? If yes, this is the default when there is no vacuum to the switch. In this case, check the small plastic vacuum line that is connected to the engine intake manifold and going into the firewall. Make sure this line is connected to the manifold and the hose is not cracked. If this is good, go inside the car and see if you can trace that line to the control switch. If the line is good to the switch, see if you can pull the vacuum connector from the switch for troubleshooting. with the engine on, check for vacuum on the line from the engine with your finger. If the vacuum is there, your switch is probably leaking and needs to be replace.------------This will help.Thanks.Helpmech.
I didn't initially answer your question, or even read it but I know what you are talking about. The source vacuum to the HVAC system inside the van. It attaches to a vacuum port on the left side of the engine just above valve cover attached to a bracket. They are very brittle plastic and break easily. I found it better to replace the vacuum line with small rubber line, such as washer fluid hose. Its easier to route and can be installed without bending plastic. Feed it from the left side vacuum "T" above the left valve cover, over to the heater valve on the right side in heater hose split. Its the source vacuum to inside, and if lost it defaults to the defrost mode and won't change modes.
Your going to have to replace the switch. The switch it self may have gone bad, since it works on a vacuum. It's common for the diaphragm in the switch assembly to go bad, if it's not the switch. Then it's going to be a bad actuator door and will need to be freed up. I have found it to be the switch it self get dirty and the vacuum needed to activate the actuator door is not there.
Good luck and keep me posted. When you turn the switch from defrost to heater, do you hear a air leak??? If you do then it's a good chance that it's the switch it self.
It has vacuum activated doors to close and open when you move the switch. Your have to check your switch for a disconnected hose a broken hose that is leaking the vacuum.
I tried to give you a vaccum diagram but the manufacture will not release that info to outsiders. you need to find where the hose goes to.if you loose vac to the controls it goes to defrost mode.
Defrost is the default mode so if the system fais, you alays have defrost. The mode doors are controlled by vacuum motors. I would suspect a vacuum leak in the engine compartment on the hose going from the engine through the firewall and into the heater controls. Some systems have a vacuum storage canister that can also leak. The canister stores vacuum when vacuum drops going up a hill.
Or, you have a bad heater control head that is not passing vacuum to the vacuum motors on the mode doors.
The switch that does this is the mode control, You will need to pull the heater control out and check that it is sending a vacuum signal to the defrost air diversion door vacuum motor, if it is then the hose is either restricted or the vacuum motor is bad or more likely something has fallen down into the dash and jammed the defrost door, this is the most common problem I saw as a Ford dealer tech for 20 years. The usual item found is a pen or pencil.
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