2017 ram 3500 6.7 diesel low heat output and is stuck on defrost.

[Jeff6917]

I can hear the blower fan running high but low output at defrost and I do not have control to change to vent or floor. I have no idea as to what can be wrong.

 

[Fast69Mopar]

I can hear the blower fan running high but low output at defrost and I do not have control to change to vent or floor. I have no idea as to what can be wrong.

If you are unable to change modes from defrost to vent to floor then there is either an issue with the HVAC control head or with the mode door actuator.

Low airflow output can be caused by a restricted evaporator which is very common. The amount of dirt and hair and grime that builds up on the airflow inlet side of the evaporator can easily cause a restriction with a resulting decrease in airflow.

Another thing that can cause low airflow output is a broken mode door or a mode door that is being improperly commanded by the control head.

You need to determine if the HVAC control head is sending commands to the mode door actuator and also determine if the mode door actuator is receiving the commands from the control head.

First things first, a scan is is necessary to diagnose this. A cheap unit won't do the trick. You will need a scan tool with updated software and bi-directional controls so you can actuate the mode door actuator.

My suggestion would be to find a shop that you can use to perform the diag.

 

[Jeff6917]

Thanks for the reply I have Alfaodb would that suffice for diagnostics?

 

[RamDiver]

Thanks for the reply I have Alfaodb would that suffice for diagnostics?

I have no experience using AlfaOBD for this purpose, but AFAIK, it is very capable for this type of diagnostics.











Fast69Mopar would have the practical experience to guide you further on troubleshooting.

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[Fast69Mopar]

I have no experience using AlfaOBD for this purpose, but AFAIK, it is very capable for this type of diagnostics.


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Fast69Mopar would have the practical experience to guide you further on troubleshooting.

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I have not used AlfaOBD in any capacity. I have a couple of scan tools that I own to be able to perform diagnostics. If you have Alfa along with an OBD dongle then I'd say give it a shot. Do not perform the actuator calibration until you record the data for the doors. If you calibrate first then you won't know what the doors were doing before the calibration.

First thing I would do is go to the data stream for the HVAC control head and look at the data for the mode door and blend door and recirc. door. Using the knob on the control head, turn the knob to full cold and record the data for the doors. You should see a voltage for each door and a percentage or a number represented in degrees for the sweep of each door.

Then, rotate the knob to full hot and record the data again.

Next, perform the actuator calibration. Clear any DTC's that were set during the process and cycle the key afterwards.

Then go back to the HVAC control head data stream and record the data again at the full cold position and the full hot position.

Now you can compare the data to see if something changed during the calibration process.

If you have a door that sweeps further than the full cold or hot position you can end up with broken doors and bushings. When you remove the actuators you can see what they attach to to make the doors move. Hopefully this will get you started and on a path to determine the problem.