Heater cuts in at idle when A/C is on

[george3405]

Hi All,
I have a 2001 Dodge Ram 1500 Quad cab 2 wheel drive. Lately, when I had the A/C on it blows cold, but when I come to an idle the heater kicks in. When I accelerate and am driving there is no problem. Could it be a vacuum leak? I'd like to fix it asap. Any help is really appreciated. Thanks, George3405

 

[Mopar1973Man]

Well... I don't think its a vacuum problem. Here is the HVAC box.


The blend door is a electric stepper motor beyond than all other doors are vacuum.

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So I'm going to bet your condenser in front of the radiator is plugged up or maybe the fan clutch is not functional. What is happening when you stop is the condenser can't dump heat any longer and freon pressure on the high side rises till the compressor shuts down. (Hide side cut out).

 

[george3405]

I know that it is the heater kicking in because I have a small coolant leak at the core which lets in the smell of antifreeze when the heater first kicks in. Otherwise the A/c is good and cold. By the way, when the heater kicks in it is not for long. Thanks, George3405

 

[Mopar1973Man]

The blend door traps the heater core back behind the door so there is no air flow around it unless the blend door is moving (which I kind of doubt). As for heater flow the heater get contant flow from there engine there is no valves that I know in the coolant lines (at least on the Cummins powered Ram's). So since heater core is constant hot and the blend door is mechanical stepper motor I can't see how heater can be activated.

You could test this for yourself too by just letting it idle and hooking up a set of A/C gauges and watch the high and low pressures. Typically it would be 35-40 PSI low side and about 250-275 High side.



As for the heater leak it might be like mine with a cracked tube... (Look closely at the pic)

 

[Bigtman07]

Not sure what year they changed to a stepper motor on the blend door. Mine uses a cable to rotate the blend door. So my thought is either the door isn't closing all the way or there is an issue in the ac system. Most of the time you can see the blend door actuator by taking the glove box off. But I will agree that it is most likely the ac system.

Sent from my liberty x!

 

[Mopar1973Man]

Not sure what year they changed to a stepper motor on the blend door.

Well Thinking about it... My 96 has the old school cable so I'm going to assume that is was at 99 when they changed over with the newer dash style.

 

[Bigtman07]

Very possible. Gotta love the little tweeks they do. Lol

Sent from my liberty x!