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acstechgm 4/1/2010
2002 Dodge Ram 2500 Base 8Cyl5.9L - Body & Interior
Fan Switches Vents Randomly
Air randomly switches back and forth from dash vents to defrost vents.
1 Answer
  • HouseCallAuto
    HouseCallAuto 4/1/2010
    Here is some real good data. You can see for yourself what the cause is>>

    Driving with the A/C on under light or heavy acceleration, the airflow will stop blowing out of the vent ducts and switch to blowing out of the defrost vents.
    Tests/Procedures: 1. Check the vacuum check valve at the right side of the intake manifold and check the second check valve in the line to the left of the receiver drier.

    2. If they are good, check for a leaking line or defective and leaking vacuum motor in the Heater Ventilation Air Conditioning (HVAC) system.
    Potential Causes: Plastic Line
    Vacuum Check Valve
    Defective Vacuum Motor

    Tech Tips: When vacuum is lost to the system as in acceleration, the HVAC system will switch to the defrost position as a safety default.
    Diagnostic Codes: None
    Author: Brainard Janicki
    Average Reported Mileage: 124159



    Confirmed Fix Summary

    Confirmed Fix 15 - Vacuum Check Valve
    2 - HVAC Vacuum Hose
    1 - Check Valve
    1 - Vacuum Hose



    Confirmed Fix Details

    Tech Reported Fix Details Confirmed Fix
    Brad Edwards
    May 02, 2007 2000 Dodge Ram 1500 Pickup, 5.9L, Vin Z, Eng Cfg V8 *
    Vacuum Check Valve
    - I was having trouble finding out why the HVAC was shifting, after trying several different aftermarket vacumn check valves I finally got one from Dodge and that fixed the problem. I dont know why but the aftermarket valves would not hold vacumn while driving. Vacuum Check Valve
    David Lee
    November 26, 2007 2002 Dodge Ram 1500 Pickup, 4.7L, Vin N, Eng Cfg V8 *
    Vacuum Check Valve
    - Replaced vacuum check valve at firewall, very common on these models, did nor fix problem. After consulting IDENTIFIX checked for and found another check valve at manifold which had failed. This check valve is not present on most models. Good info. Vacuum Check Valve
    Stephen Webb
    May 22, 2009 1999 Dodge Ram 1500 Van, 5.9L, Vin Z, Eng Cfg V8 *
    Replaced Vacuum Check Valve
    - changed vacuum ck valves. like brad edwards I found that oe vacuum checks work better than aftermarket. Vacuum Check Valve
    Jay Fortino
    February 21, 2008 1999 Dodge Ram 2500 Pickup, 5.9L, GAS, Vin Z, Eng Cfg V8, USA/Canada *
    Vacuum Check Valve
    - installed vac check valve near fuel rail on pass. side of engine. not holding vac. Vacuum Check Valve
    SCHMIDTSHAW1
    March 05, 2009 2000 Dodge Ram 2500 Pickup, 5.9L, GAS, Vin Z, Eng Cfg V8 *
    Replaced Vacuum Check Valve
    - found grease in the inlet vavle/line blew out and all is working OK. Vacuum Check Valve
    Terry D. Wren
    April 30, 2008 1996 Dodge Ram 1500, 5.9L, GAS, Vin Z, Eng Cfg V8 *
    Vacuum Check Valve
    - FOUND VACCUM CHECK VALVE BAD AT INTAKE MANIFOLD/CRACKED/LEAKING Vacuum Check Valve
    Larry Throndson
    October 21, 2009 1996 Dodge Ram 1500, 5.9L, GAS, Vin Z, Eng Cfg V8 *
    Replaced Vacuum Check Valve
    - replaced both check valves in right vaccum hoses Vacuum Check Valve
    Denny Mandeville
    September 19, 2007 1998 Dodge Ram 1500, 5.2L, Vin Y, Eng Cfg V8 *
    Vacuum Check Valve
    - Replaced second check valve by receiver Vacuum Check Valve
    Bob Parra
    September 06, 2007 1999 Dodge Ram 1500 Pickup, 5.9L, Vin Z, Eng Cfg V8, USA/Canada *
    Vacuum Check Valve
    - INSTALLED UPDATED VAC CHECK VALVE Vacuum Check Valve
    Tommy Gregory
    July 06, 2009 1999 Dodge Durango, 5.9L, Vin Z, Eng Cfg V8 *
    Replaced Vacuum Check Valve
    - bad check valve Vacuum Check Valve
    Bill Taylor
    June 09, 2006 2000 Dodge Ram 1500 Pickup, 5.9L, Vin Z, Eng Cfg V8 *
    Vacuum Check Valve
    - MISSING CHECK VALVE Vacuum Check Valve
    Bob Parra
    September 06, 2007 1999 Dodge Ram 1500 Pickup, 5.9L, Vin Z, Eng Cfg V8, USA/Canada *
    Vacuum Check Valve
    - UPDATED CHECK VALVE Vacuum Check Valve
    Aamco Center 43050
    June 22, 2009 2001 Dodge Ram 1500 Pickup, 5.9L, Vin Z, Eng Cfg V8 *
    Replaced Vacuum Check Valve
    - check valve Vacuum Check Valve
    Jim Cruse
    August 07, 2009 1996 Dodge Ram 1500, 5.9L, GAS, Vin Z, Eng Cfg V8 *
    Replaced Vacuum Check Valve
    - check valve Vacuum Check Valve
    Eric Anderson
    September 11, 2007 1997 Dodge Ram 1500, 5.9L, GAS, Vin Z, Eng Cfg V8 *
    Vacuum Check Valve
    - vac valves Vacuum Check Valve
    Rick Eakins
    November 24, 2009 1998 Dodge Durango, 5.9L, Vin Z, Eng Cfg V8 *
    Repositioned HVAC Vacuum Hose
    - vacuum line at vacuum t was pushed too far into check valve not allowing enough vacuum to go to switch HVAC Vacuum Hose
    Nick Berry
    January 25, 2010 1998 Dodge Ram 1500, 5.9L, Vin Z, Eng Cfg V8 *
    Repaired HVAC Vacuum Hose — Found a broken HVAC vacuum line under the dash board. HVAC Vacuum Hose
    Bob Pierson
    September 15, 2008 1998 Dodge Ram 2500, Quad Cab 5.9L, Vin 6 *
    Check Valve
    - bad vacumn check valve on intake manifold Check Valve
    Sabrina Luedke
    July 01, 2006 1998 Dodge Ram 1500, 5.2L, Vin Y, Eng Cfg V8 *
    Vacuum Hose
    - THE VACUM LINE FROM MANIFOLD TO VACUM SWITCH UNDER WIPER COWL Vacuum Hose


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    HouseCallAuto
    JCalvin70 4/29/2010
    This is a very detailed answer. Take a shot at my question with the same eye for detail. It probably is easy for you to find a way to add a light to my Dodge diesel truck. Thanks for giving it a go.
    HouseCallAuto
    Momosapien 7/24/2010
    Wonderful answer HouseCall. I am going to print this and try to trace my blower fan issue with your tips. I have spent several hunderd dollars at a few 5 star dealerships only to be told that I must have a short somewhere and that it will be several hundred, to over a thousand dollars more to trace it down and repair it. One dealership actually tried to charge me $120 bucks to replace the HVAC blower fuse. Not once has anyone made any reference to a possible vaccum problem. Thank you.
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