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FlatTwin

Trying to understand A/C and heating

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Guys, having a tough time wrapping my thick head around the heating and A/C systems on the Ram 2500. Have worked on cars for most of my life and never seen something like this.First off had a diverter door go out. Took it into a good A/C shop here in town and they fixed it, pulling the entire dash out and replacing the door assy. Drove off thinking that finally after owning the truck for 18 months that the little knob called the "temp control" might now work! Well dammit, it does not!Put the A/C in max A/C and no change but believe that this is normal. Move the mode controller to the second position, normal A/C and once again the temp control knob might as well not even be in the truck. Move it to full hot and its still pumping out cold air. Now lets throw a wrench in the works. If I stop the truck, go inside and then return, if the temp control knob is in the hot or medium position I now get hot air out of the system! Anyone have an idea why it functions after switching the power off? Have talked with the shop and they have agreed to fix it for no cost but I want to make sure that we know what is going on here. First was a diverter door but any suggestions on this? One of my main concerns is that if I start the truck and the temp control is in the cold position then drive a long distance, is my compressor switching on and off maintaining temp or is it on all the time? Do not want to burn out the compressor and I need to make a 400 mile drive next week. Thanks,Joe

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Normally charged A/C system will not cycle on and off if charged properly. Like mine runs solid never cycles and pumps out 45*F air from the center vents on a 95*F day... :whistle2: The only time you might see the cycling on and off is in the dead of winter and using DEF then the A/C is cycled because the freon reaches freezing point rather easy so it kicked off. The allows the system to condense moisture on the evaporator and dump it outside the vehicle this why DEF in the newer vehicles work better than old school...

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Mike,Good to hear that I am ok to drive the truck on long trips. You say that a normal A/C system will not cycle on and off. That may be true on the Dodge system but sure not with most other A/C systems. I have a couple of other cars that I fuss with and they all cycle on and off depending on the temp in the interior, sending a signal to the A/C compressor clutch that goes on and off as needed. Any ideas what's going on with my temp control not working unless I shut the power off? When its in the "normal position" on the mode switch the temp switch is worthless. You can turn it from right to left and makes no difference in the outlet temps. Thx,Joe

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The Heat AC "system" is crazy. The output vent: floor, dash, defrost is controled by vacume. It this goes bad the default mode is defrost. A diesel does not make enough vacume so a pump is mounted to the engine. There are threads on this. I think the dash has to come out to service these controls, but I'm not there yet.The Blend door is under the dash just to the right of the hump & is electrical. There are threads on this here as well. While recommended to remove the dash to service it's possible to do so with enough tricks & persistance without. There is also the normal Heater & AC parts.

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Russ,Thanks but believe that there is still some black magic involved here. As to why the temp control does not work until being reset by turning the power off is a good question. I asked the shop about that and they did not know either.

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Mike,So the temp control feeds info to the blend door and it moderates the airflow and controls the temp? Please do not tell me that the A/C runs all the time the mode switch is in either of the two A/C positions (unless shut off by the sensor switches) and the blend door mixes A/C cooled and air heated from the heater matrix? Boy, the person who designed this system needs to be in a small room with padded walls IMHO...Next question, can you get to the blend door wiring without pulling the dash? Am wondering whats going on in my case, that the door is not responding to the input signals from the temp control? The shop has said that they will do it for no charge (already paid them once!) but wondering if this is something that is accessable from underneath or not? Thanks much!Joe

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Mike,

So the temp control feeds info to the blend door and it moderates the airflow and controls the temp?

Yes..

Please do not tell me that the A/C runs all the time the mode switch is in either of the two A/C positions (unless shut off by the sensor switches) and the blend door mixes A/C cooled and air heated from the heater matrix?

Actually it run continueous in MAX A/C, A/C, Bi-level, Floor-DEF, and DEF...

Boy, the person who designed this system needs to be in a small room with padded walls IMHO...

Well actually its rather simple compare to the dual zone climate and newer systems...

Next question, can you get to the blend door wiring without pulling the dash?

Yes... The blend door motor is on the passenger side floor near the transmission hump.

Am wondering whats going on in my case, that the door is not responding to the input signals from the temp control? The shop has said that they will do it for no charge (already paid them once!) but wondering if this is something that is accessable from underneath or not?

Thanks much!

Joe

Comments in blue... Here is a quick vid of the blend door...

http-~~-//www.youtube.com/watch?v=-h6ufg1-Dag

And here is the wiring...

post-2-138698166532_thumb.jpg

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Hmmm, thanks much. Reason I ask is that the shop is 20 miles from my house and while just dropping the truck off and picking it up is the best way, getting a ride there and back is not very easy and a cab is too expensive. If they can do this while I wait for an hour or two thats fine but not going to stay there for 6 hours. At least now we know what we are working with! Learning more every day!Appreciate the help!Joe

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My compressor stays on constantly too. When I am idling the rpm drops maybe 25-50 and I can sit there forever and it never jumps back up. Not sure why it's not like other vehicles that cycle them. Maybe the compressor clutch lasts longer leaving it engaged the entire duration of using the A/C :shrug:

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The blend door can be done with the dash in place. You have to peel back the rug & cut away a bit of rubber insulation. I had to buy a tiny right angle driver. I had to do it mostly one handed (my disability again) but I got 'er done. Read more at: http://www.heatertreater.net/ It is a multi position control. Pay very close attention when you open ther gear case & MARK the fr*ggen motor as it's reverseable & no way to guess the orientation. I bought the kit but mine was not broke, turns out mine is the vacume controled side like I thought at first. Now that I know how & have cut the rubber, I could remove / replace in 1/2 hour (one hand). Russ

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You have cold air so the AC is working.You have cold air coming out of the dash. If the vacume control fails, it would go to the default mode which is defrost vents on the windshield. You don't indicate this happening.The only other thing is the blend door. As I said, it's on the passenger side of the hump. The little plastic connector could have failed as they are prone to do. Or the door could be just stuck. When the ignition is turned on, it goes through an orientation cycle which must be induced after it's been apart. Russ

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My a/c cycles quite a bit. Blows cold as can be thou, was 104* on the road here today and nice an cool in the truck.

Now after doing the injectors yesterday I notice my fan was unlocked and free spinning. Then I hear the idle fall and raise. Huh? The A/C was kicking in and out. I didn't bother to put gauges on it but found out the high side switch was kicking it out... The condenser out front was roasting hot (high side pressures must of been high). But once I got rolling and got wind going through it it stopped cycling. Also notice that during my test run the fan stayed locked up and A/C never cycled after that point. Oh yeah... It was only 104*F degrees yesterday... :stuned:

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