Question about 1987 Chevrolet S-10 Pickup

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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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Posted on Jan 02, 2017

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SOURCE: i need a 1987 chevy S10 vacuum hose routing diagram

Go to, become member, will give you repair manual,location of parts,diagrams.

Posted on Apr 09, 2009


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My ac air is

Without doing so testing it would be hard to say , would just be guessing . Where did you have it checked ? If it does change every once and a while , I would think it could be a sticking actuator or in the heater and A/C programmer module . The heater and A/C programmer will direct the blended air in one of several air delivery modes. The side window outlets receive the airflow during all of the modes of the operation. The heater and A/C programmer directs air to the other outlets by energizing specific vacuum valves. This feeds the appropriate combination of mode valve actuators.
  1. Turn the engine ON.
  2. Select HI BLOWER.
  3. Connect the Scan Tool .
  4. Use the Scan Tool in order to command recirculation door movement.
Does the recirculation door operate?
Go to Step 2
Go to Step 3

Using the
Scan Tool , select AIR DOWN.
Is most of the air flowing from the heater (floor) vents, with only a small bleed out of either the defrost vents or the A/C vents?
Go to Step 7
Go to Step 4


  1. Remove the programmer vacuum hose connector.
  2. Measure the vacuum on the orange hose.
Is the vacuum less than the specified value?
10 in Hg
Go to Step 14
Go to Step 13


Attach a hand held vacuum pump to the YEL CKT.
Does the vacuum pump hold vacuum?
Go to Step 5
Go to Step 6


  1. Inspect for a stuck up/down valve or actuator.
  2. Inspect for the proper hose routing. If the routing is OK, replace the heater and A/C programmer. Refer to .
Is the repair complete?
Go to Step 16


Repair the leaking hose or the actuator.
Is the repair complete?
Go to Step 16

Using the
Scan Tool , select A/C DEFROST.
Is most of the air flowing from the the A/C vents or the defrost vents, with only a small bleed from the heater vents?
YES Go to Step 8
NO Go to Step 9


Is most of the air out of the defroster vents?
YES Go to Step 10
NO Go to Step 11


Attach the hand held vacuum pump to the RED CKT.
Does the pump hold vacuum?
YES Go to Step 5
NO Go to Step 6

Using the
Scan Tool , select AIR UP.
Is most of the air flowing from the A/C vents with only a small bleed out of the defroster vents?
System OK
Go to Step 12


  1. Inspect for a stuck A/C-defrost valve (door) or actuator.
  2. Inspect for proper hose routing. If the hose routing is OK, replace the heater and A/C programmer. Refer to .
Is the repair complete?
Go to Step 16

  1. Repair the leaking hose or actuator.
  2. Inspect for a stuck A/C-defrost valve (door) or actuator.
  3. Inspect for proper hose routing.
Is the repair complete?
Go to Step 16


Attach the hand held vacuum pump to the ORN CKT.
Does the pump hold vacuum?
Go to Step 15
Go to Step 6


Repair the vacuum source.
Is the repair complete?
Go to Step 16


  1. Inspect for a stuck inlet valve (door) or actuator.
  2. Inspect for proper hose routing. If OK, replace the heater and A/C programmer. Refer to .
Is the repair complete?
Go to Step 16


  1. Clear the DTCs.
  2. Verify that the system is operating properly.
Did you clear the DTCs and verify the operation?
System OK

Good luck hope this helps ! Do you need directions on how to get the heater / A/C controller out .

Jun 17, 2016 | 1999 Cadillac DeVille

2 Answers

1986 Chevy Nova - the defroster/heater vents stopped working. Any ideas? Before they found the found the fuel filter, they messed with the air filter. Could that be the problem?

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).

Jan 15, 2014 | 1986 Chevrolet Nova

1 Answer

Vacuum hose heater line

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.

Dec 19, 2012 | 2004 Chevrolet S-10

1 Answer

Air conditioning will not switch from defrost vents

Vacuum control motors are used to position these airflow control doors.
SECTION 412-01: Air Distribution and Filtering 2000 Expedition/Navigator Workshop Manual DESCRIPTION AND OPERATION Procedure revision date: 06/17/1999 Air Distribution There are two sources of air available to the air distribution system:
  • outside air
  • recirculated air
Recirculated air is only used during MAX A/C, AUTOMATIC (if equipped with EATC) and OFF modes.
Air distribution within the vehicle is determined by the function selector switch position, if equipped with manual A/C, or is controlled by the electronic automatic temperature control system in the AUTOMATIC mode but can be overridden by the driver if desired. Airflow control doors are used to direct airflow within the heater air plenum chamber. Vacuum control motors are used to position these airflow control doors.
The air distribution system is designed to provide airflow from the defrost nozzle when no vacuum is applied to any of the vacuum control motors. This is done to prevent a situation where defrost cannot be obtained due to a system vacuum leak.
Air enters the passenger compartment from the:
  • instrument panel A/C registers.
  • heater outlet floor duct.
  • windshield defroster hose nozzle.
  • side window demisters.
  • rear seat airflow duct (if equipped).
Passenger compartment air is exhausted from the vehicle through open windows or cab air vents.

expedition air flow hvac system-xvcmtr1jbqa4kfrdszeh0lpe-3-0.gif Item Part Number Description 1 18A318 Vacuum control motor - panel/defrost door 2 18A478 Panel/defrost door (full vacuum position) 3 - Defrost airflow 4 - Side window demister airflow 5 18B545 Temperature blend door (full heat position) 6 19860 A/C evaporator core 7 19A813 Air inlet duct door (full vacuum position) 8 - Outside air inlet 9 18A318 Vacuum control motor - air inlet duct door 10 - Recirculated air inlet 11 19805 Blower motor 12 - Vacuum to emission control 13 - Vacuum from the engine intake manifold 14 19A563 A/C vacuum check valve 15 19A566 A/C vacuum reservoir tank and bracket 16 19B888 A/C-heater function selector switch 17 18476 Heater core 18 18A559 Floor/panel door (full vacuum position) 19 - Floor airflow 20 18A318 Vacuum control motor - floor/panel door 21 - Panel vent airflow
Check vacuum control to the panel defrost motor ( vac,diaphragm )

Nov 01, 2017 | 2001 Ford Expedition

1 Answer

My A\C stop blowing out the A/C vents but blows on high and low throw the defrost vent

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.

Jul 05, 2011 | 2003 Mercury Sable

1 Answer

I just read an answer post you had made in response to a question about a 2003 Chevy Astro van vacuum line keeping the vents from switching from defrost to floor, to dash vents, etc., you mentioned...

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.

Jan 29, 2011 | Chevrolet Astro Cars & Trucks

2 Answers

AC only blows out the defrost vents

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.

Aug 14, 2009 | 2007 Ford F-150 Harley Davidson

4 Answers

A/c comes out of front defroster. I also found a vacuum hose lose near the front firewall and I cant find out where it goes.

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.

May 27, 2009 | 1999 GMC Safari

1 Answer

Switch on dashboard does not change air direction to flow & vents

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.

Here's a diagram of the system for reference.


Feb 15, 2009 | 1999 Ford F150 Regular Cab

1 Answer

Defrost will not work

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.

Nov 21, 2008 | 1996 Ford Taurus

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