HVAC temps in a good working TJ with a good HVAC system

mrblaine

mrblaine

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JMT said he was going to swap in a heater core that was touted to deliver 150 degree air. I said I had no idea where that put things but I'd find out, so I ordered in a Robinair instant read HVAC thermometer to find out. These are the temps from one with known good Mopar stuff in it.
Ambient with the sun heating up the black dash bits. No AC or defrost running.
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Very close to actual ambient.

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I started it up and while waiting for the engine to get up to temp and open the thermostat, I turned on the AC just to see what it was doing.

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It got up to temp, opened the thermostat and the heater started working. Center vents first.
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And then the fun one. I was trying to hold the thermometer in the floor vent but it kept roasting my fingers. I had to go get a clamp to do it. When I saw the temp, I understood. Scalding is 150. 164 is toasty.

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Oddly, the defrost is a bit less but I suspect that would go up as the ductwork warmed up, this was all cold when I started. I didn't leave it there long since the heat was driving me out of the rig even with the windows down. If it matters, I can do it again to see what happens when I don't mess with the AC first, but the floor vents tell the story.
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I'm still driving with mine bypassed, have not had the time to replace it yet... when I was breaking in the brakes this past fall, I was speeding back up, heard a pop and thought I hit something, then got a lot of steam, and coolant.

I'll be looking forward to the report, might need to get on and install it too...
 
Here's what the manual says for A/C performance:

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And for heater performance:

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That TJ seems to be slightly under-spec on A/C and considerably over-spec on heat. I've done my own A/C work for many years on my vehicles, and I've never had one performing to spec. Even on new-ish vehicles, the A/C always under-performs. I've never tested heat before, though, so I have nothing to compare to. Based on that, I'd say that TJ's HVAC systems are in fine shape!
 
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Thanks @Blaine, I was about to ask what your results turned out to be. I'll buy a Robinair and do the same tests when I get everything re-installed. My ambient air temps will be very different here, but it will at least give a base line for comparison. The heater core should arrive tomorrow and I hope to have the dash re-installed by the end of Thursday. Hopefully in the next couple of weeks I can get the Robinair and do a test.

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p.s. this is the heater core I ordered through RockAuto.

p.s.s. what do you know, it apparently was delivered a day early. Just got notice it's in the mailbox.
 
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mrblaine said:
Oddly, the defrost is a bit less but I suspect that would go up as the ductwork warmed up, this was all cold when I started. I didn't leave it there long since the heat was driving me out of the rig even with the windows down. If it matters, I can do it again to see what happens when I don't mess with the AC first, but the floor vents tell the story.
Click to expand...

The defrost is cooler because it turns on the AC to dry the air out.
 
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JMT said:
Thanks @Blaine, I was about to ask what your results turned out to be. I'll buy a Robinair and do the same tests when I get everything re-installed. My ambient air temps will be very different here, but it will at least give a base line for comparison. The heater core should arrive tomorrow and I hope to have the dash re-installed by the end of Thursday. Hopefully in the next couple of weeks I can get the Robinair and do a test.

View attachment 500072

p.s. this is the heater core I ordered through RockAuto.

p.s.s. what do you know, it apparently was delivered a day early. Just got notice it's in the mailbox.
Click to expand...

The delta between ambiant and your measured temp is the important thing (should be linear...so if your ambiant is 20 degrees cooler, your A/C should be the same)
 
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Well crap. The performance radiator AC evap that I was going to install beside the heater core has a manufacturing defect. Doesn’t give me much hope. They’re shipping another one out. Won’t be here till Wednesday, so I can’t get started in the dash reinstall till Thursday.

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OEM
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Performance Radiator


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I'd be interested to see how much of a measurable difference in AC performance there is based on condenser airflow. While the bulk of the cooling is driven by evaporation, supercooling the condensate can make the low side temperature somewhat lower even at the same pump throughput. Increasing condenser airflow also has the effect of lowering high side pressure, generally increasing the efficiency of the system.

AC performance is also going to be heavily dependent upon engine RPMs. As RPMs increase, so does compressor output volume, meaning a higher high side temperature/pressure (meaning increased heat rejection at a given airflow), and typically a lower low side temperature/pressure due to lower effective heat gain in the evaporator combined with the increased "suction" of the compressor. At a certain point, the compressor will begin to cycle, and above this RPM, additional engine RPM will have no bearing on AC performance.
 
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Steel City 06 said:
I'd be interested to see how much of a measurable difference in AC performance there is based on condenser airflow. While the bulk of the cooling is driven by evaporation, supercooling the condensate can make the low side temperature somewhat lower even at the same pump throughput. Increasing condenser airflow also has the effect of lowering high side pressure, generally increasing the efficiency of the system.

AC performance is also going to be heavily dependent upon engine RPMs. As RPMs increase, so does compressor output volume, meaning a higher high side temperature/pressure (meaning increased heat rejection at a given airflow), and typically a lower low side temperature/pressure due to lower effective heat gain in the evaporator combined with the increased "suction" of the compressor. At a certain point, the compressor will begin to cycle, and above this RPM, additional engine RPM will have no bearing on AC performance.
Click to expand...

Start a thread. This is about heater core swaps and what we might be looking for there to ascertain which ones are junk and which ones might work.
 
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mrblaine said:
Just looks unbent, bend it back.
Click to expand...

Won’t hurt to try. They said scrap it. I need to bend it about halfway along the tube. Could I heat it a little with propane torch? When I tried to just bend it earlier it just wanted to bend where the neck meets the body.
 
Steel City 06 said:
I'd be interested to see how much of a measurable difference in AC performance there is based on condenser airflow. While the bulk of the cooling is driven by evaporation, supercooling the condensate can make the low side temperature somewhat lower even at the same pump throughput. Increasing condenser airflow also has the effect of lowering high side pressure, generally increasing the efficiency of the system.

AC performance is also going to be heavily dependent upon engine RPMs. As RPMs increase, so does compressor output volume, meaning a higher high side temperature/pressure (meaning increased heat rejection at a given airflow), and typically a lower low side temperature/pressure due to lower effective heat gain in the evaporator combined with the increased "suction" of the compressor. At a certain point, the compressor will begin to cycle, and above this RPM, additional engine RPM will have no bearing on AC performance.
Click to expand...

A good fan clutch will help ac performance at idle.
 
JMT said:
Won’t hurt to try. They said scrap it. I need to bend it about halfway along the tube. Could I heat it a little with propane torch? When I tried to just bend it earlier it just wanted to bend where the neck meets the body.
Click to expand...

Stick a wood down in the end that fits tightly. Use that to lift up while you push down where the bend needs to happen.
 
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JKP said:
The defrost is cooler because it turns on the AC to dry the air out.
Click to expand...

Also, the different flow paths (defrost, dash, floor) each likely vary in their relative restriction. Reduced airflow will increase the air temperature gain across the heater core, so the temperature should always be measured at the floor outlet for an apples to apples comparison.
 
New MOPAR heater core in Feb 2018. New MOPAR radiator, water pump, thermostat and all new hoses and water pump pipe in Nov 2019. Flushed the system with Prestone before replacing everything in 2019, no thermocure, just Prestone. No idea on the temp gauge brand. Likely a cheap one that came with an old A/C recharge kit.

62° and sunny.

Dash vents after thermostat has opened.
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Floor vent.
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A/C on recirculate. A/C evaporator core replaced in Feb 2018 at the time I replaced the heater core. I had one of the valve cores leak some last year so I don’t know that it’s operating at its best.
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In need a new heater core as well... @JMT I know you do a ton of research on stuff - what did you determine was the best available heater core to get?

Do you have a rock auto link? thanks!
 
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