I am helping a neighbor with the air conditioner on his '99 3500, Cummins. I have charged the system. Pressures look good and I am quite sure that the evaporator is getting COLD.

The system, however, is pouring heat into the heater core and mixing it with the cold air. I need some help with this part since I have no "familiarity with the particular breed."

How does this particular system work? Does it modulate the flow of coolant to the heater core or does it mix the air with a door in the air handler? The temp control does not seem to be mechanical, so I assume that it sends a signal to a controller that in turn regulates the temp by controlling coolant flow to the heater core or moves an air flap.

Is there a common failure item involved here?

Thanks for any information that you might offer.

Have a great day,

 

Only On Max A/C coolant flow is shut off from the heator core
Blend air doors and Mode doors regulate temp and flow of air
What exactly is the problem????
What are your pressures at idle, recirc A/C, fan one notch down from Hi???

 

JCswm,

Thanks a bunch for your reply. I don't know pressures under all those conditions, but I generally got right at 200 high side and about 25 low side, with system running max with highest fan speed. Under these conditions I could only get 80 degree vent temp. I felt a box below the passengers feet that appears to be a heater core box. It was HOT with the a/c in max position.

The fact that the heater valve should be off in the max position tells me alot. Is that heater valve buried in the dash just opposite to where the heater hoses go through the firewall or is it more accessible? Does it get a modulated signal from the controller? Is it normally open or normally closed?

Thanks again,

 

First off you are low on R-134a!!!
Your high side should reach 250psi at Idle low side should drop to around 35psi
Again AC pressures are taken with the vehicle at normal operating temp
Manual systems;
blower fan one speed down from high
temp at full cold
max AC with recirc ON
I need to know what type of HVAC system your working on to look up valves/doors
Do you set a temp like 72 or is there a red to blue **** for adjusting temp???

 

Okay, but the book I have shows 190 to 220 high side. But it's not much of a book. I still feel that the refrigeration system was making cold air but it was being negated by the heater core. The box where the heater core apparently resides was flaming hot even in Max position.

This is not an automatic climate control. It simply has a cold/hot temp dial, a red/blue **** as you call it. I will add some charge to the system, but I'm really convinced that there is an additional problem.

Thanks VERY MUCH for your help,

 

I guess it also depends on outside temp
If it's over 70F you should see 250psi
I'll sign on to chrysler right now & look it up since your online
BRB

 

I was wrong.... sorry, Hot coolant always flows through the heator core, a mode door bypasses air flow around the core