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RJAuto11, ASE Certified Technician
Category: Dodge
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Experience:  ASE Master Tech working on Chryslers 14+ Years
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1999 dodge: 4wd..v8..Engine temp normal, fan works

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1999 dodge ram 4wd 360 v8. Heat temperature control inoperative. Engine temp normal, fan works normal, duct settings normal. What do I check and how do I access the components for checking.

That year ram has a blend door actuator, that controls the door for blending hot and cold air in the HVAC box, the servo motor must be accessed by removal of the heater-a/c housing.


1.Disconnect the battery negative cable.

2. Remove the instrument panel from the vehicle, I have found that this step, you can tie up the passengers side to the grab handle and place fender covers over the 4x4 shifter and not have to remove the entire dash.

3. remove the heater-a/c housing from the truck.

4.unplug the wire harness from the servo motor.

and remover its 2 mounting screws that secure it to the housing.

and remove the actuator.

Customer: replied 6 years ago.
I don't quite understand the instructions. What do I have to tie up? Is there a way to test the component before removing the dash? Could it be a vacuum problem? It has been very cold here and the truck is outdoors. Can I temporarily block the cold air intake until better weather? If there is a way to block the air intake and that resolves the problem temporarily then that would tend to support the blend door malfunction too.
I am sorry I was not able to help you.
Customer: replied 6 years ago.
Relist: Answer quality.
What is the best way to proceed in diagnosing the problem. There are several possibilities. I would like to take the least destructive first. I will attempt to establish coolant flow through the heater. If it is good I will need to check for vacuum at the heater conrol panel and at the servo. Lastly and most difficult is to check the servo visually. I expected a thorough answer but the expert jumped to extracting the servo. The procedure he prescribed was a bit ambiguous too.

This is the diagram for your trucks HVAC system heater control.

  • graphic

This is the diagnostic tree from the service repair book for your Dodge ram.




This is the duct flow air chart for your HVAC box that is used in your ram truck.





You have told me that all of your mode doors work ok , so that leads me to think that the operation of your blend air door is the root cause of your heater malfunction. Use the wiring chart I have provided to test for voltages and grounds, If its not in your electrical wiring , and your switch is working correctly, the chances are , either your blend air servo motor is binding up or is defective, or your door in your HVAC box may be bound up or broken. Possibly some foreign matter has gotten into your air box and bound it up or the door hinge point may have snapped. --------------------



This is where your truck taps into the engine to supply your HVAC mode switch with the needed engine vacuum to operate the other none electrical servos in your HVAC box.




Under your windshield cowling, is the location of your vacuum reservoir, to supply vacuum even when your engine is under heavy load and producing low manifold vacuum. But your mode doors are working fine, so I think this part of your system is not the cause of your problem.



The Possible causes listed in the factory manual for heater outlet air temperature being unadjustable are as follows.

1. A faulty heater-A/C control.

2. A faulty blend-air door servo.

3. A faulty, obstructed or improperly installed blend door.

4. An obstructed cowl air intake.

5. The engine cooling system.


This is the exact info I have available to me on your Dodge ram , I hope this helps you to track down the source of your problem.

Please let me know if I can help you in any way to further diagnose this problem with your vehicle.


RJAuto11 and 2 other Dodge Specialists are ready to help you
Customer: replied 6 years ago.
I've started working the problem and I noticed something new. The a/c compressor comes on every 6-7 seconds and runs for 1-2 seconds. The climate control settings are defrost, fan low, temp = max hot. Is this an electrical or sensor malfunction that is causing my no heat problem?
The compressor is kicking on to remove moisture and is normal.
Customer: replied 6 years ago.
The problem is fixed. I flushed the heater and it worked for a few hours and then quit. I consulted a local Dodge mechanic and he spoke of a propensity for this system to develop an air lock. We added antifreeze through the radiator and expansion bottle. All is well. Thanks.
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