No power at HVAC control panel?

Tiny
ALTON04
  • MEMBER
  • 2017 RAM 1500
  • 5.7L
  • V8
  • 4WD
  • AUTOMATIC
  • 29,009 MILES
I have no A/C, heat, defroster, vent fan or anything. I've replaced the humidity sensor and the HVAC control panel. I have two codes; one says lost communication with humidity sensor and the other says I lost communication with the HVAC control module.

The truck listed above is the Bighorn model.
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Tuesday, August 2nd, 2022 AT 4:16 PM

25 Replies

Tiny
AL514
  • EXPERT
Hello, can you give us the exact codes? This way we can look up the code setting criteria and other information. But looking at the wiring diagram, the HVAC control module and the Humidity Sensor get powered up on the same feed wire coming from Fuse 99 10amp. So, see if that fuse has power and take it out and see if it looks like it got hot, or if it is blown you may have had one of those two modules short out. If it has power with the key on, next check at the back of the HVAC module on pin 2 and pin 3 of the Humidity sensor for power. If that fuse is blown, that's the cause of your lost comms. If the fuse blows again, then there is a short to ground somewhere on that pink/green wire.
That dotted line highlighted is a connector there (1310), and since the pink/green wire is pin 31 on that connector, its most likely a bulk connector with a lot of wires running through it. We can use that as a testing point if there happens to be a short to ground on the circuit.
I think you're going to find you are just missing power to those two modules, if there was an actual communication issue, you would have codes for the other modules on the LIN bus that the Humidity sensor communicates on as well as other CAN bus codes because that's the network that the HVAC communicates on.

https://www.2carpros.com/articles/how-to-check-a-car-fuse

https://www.2carpros.com/articles/how-to-use-a-voltmeter

https://www.2carpros.com/articles/how-to-use-a-test-light-circuit-tester
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Tuesday, August 2nd, 2022 AT 6:56 PM
Tiny
ALTON04
  • MEMBER
U0164 and U1190.
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Tuesday, August 2nd, 2022 AT 8:47 PM
Tiny
ALTON04
  • MEMBER
And I have checked all the fuses and I took the TIPM out and checked all the connectors and pins to be sure there was no corrosion or blown fuses. I actually checked those things before I replaced the HVAC control panel and the humidity sensor.
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Wednesday, August 3rd, 2022 AT 7:02 AM
Tiny
AL514
  • EXPERT
Did you check for power on pin 2 of the HVAC control (pink/green wire) for 12 volts? Its also pin 2 on the Humidity sensor, should be 12 volts as well. You have to verify it with a multimeter and make sure its a full 12 volts. If theres voltage drop on that wire and its only getting 9 volts for example, it wont be enough for the module to communicate. The LIN bus from the Humidity sensor is a 0 to 12 volt signal.

https://www.2carpros.com/articles/how-to-use-a-voltmeter
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Wednesday, August 3rd, 2022 AT 11:04 AM
Tiny
ALTON04
  • MEMBER
I have not. I only checked the power and ground wires on both. I will check them this evening after work and give an update. Thanks
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Wednesday, August 3rd, 2022 AT 11:06 AM
Tiny
AL514
  • EXPERT
Which power and ground wires did you check? The red wire on the HVAC? The pink/green wire is the one you need to go after; it powers both those modules from fuse 99 on the same wire. It's and Ignition switch feed.
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Wednesday, August 3rd, 2022 AT 11:12 AM
Tiny
ALTON04
  • MEMBER
That's correct.
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Wednesday, August 3rd, 2022 AT 11:13 AM
Tiny
ALTON04
  • MEMBER
I checked the power at the humidity sensor, the HVAC control module and the HVAC control panel. All showed power. But I was only using a test light.
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Wednesday, August 3rd, 2022 AT 11:14 AM
Tiny
AL514
  • EXPERT
Okay, go after the pink/green wire next with the Key On.
You haven't lost comms with any other modules and that one power wire ties the 2 modules together,
Do you have an oscilloscope?
The 2 modules run on different networks, the HVAC module has lost its comms on the Interior CAN bus network and the Humidity sensor communicates on the LIN bus, they are completely different signals, that's why I think there is a loss of power on that Ignition feed wire, if they are both missing power we'll need to trace it back to fuse 99.
It also goes to this in-car temperature sensor. It might be that the temperature sensor is shorted out and pulling the 12-volt feed down. So you might have to unplug that at some point. I'll try to find its location.
The last 4 diagrams are the in-car temperature sensor. Should have the same Pink/Green wire running to it with 12 volts.
Okay, the temperature sensor is not on any network, its hard wired in, so its just like a thermistor that changes resistance with temperature. Just like a Coolant Temp sensor, so it could pull down the 12 volts without potentially blowing the fuse.
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Wednesday, August 3rd, 2022 AT 11:16 AM
Tiny
ALTON04
  • MEMBER
So, I have 12 volts at the red wire on the humidity sensor and the HVAC control module. I have 12 volts at the green/pink wire on the HVAC control module and only 9 volts at the green/pink on the humidity sensor.
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Wednesday, August 3rd, 2022 AT 4:37 PM
Tiny
ALTON04
  • MEMBER
On a side note, rechecked all the fuses. Not sure if it should have power or not, but the fuse for the PCM 10 amp is grounded on both sides.
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Wednesday, August 3rd, 2022 AT 5:08 PM
Tiny
ALTON04
  • MEMBER
I noticed today on the way home from work, my auto headlights came on in the garage and never went off in the sunlight.
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Wednesday, August 3rd, 2022 AT 5:53 PM
Tiny
ALTON04
  • MEMBER
And this morning headed to work, remote start will now only run a few seconds and disables.
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Thursday, August 4th, 2022 AT 3:54 AM
Tiny
AL514
  • EXPERT
Okay, so are you meaning the pink wire with a green stripe at the humidity sensor only has 9 volts? The red wire is a battery feed that should be hot at all times, Its probably to keep memory in the Hvac and other modules. I will look into the PCM fuse today. When you say grounded, you can hook a test light to battery positive and it will light the test light, ? Or how are you testing and determined its grounded? Also do you have Start/Stop enabled? And is your remote start an aftermarket installation?
The Sun Sensor runs off the Hvac module, that is why your headlights are now malfunctioning. Whatever is happening is progressively getting worse.
Does the PCM fuse have a number too, I don't see one labelled PCM, I'll keep looking, but there might be a number on the fuse box label as well.
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Thursday, August 4th, 2022 AT 9:52 AM
Tiny
ALTON04
  • MEMBER
F72 is the fuse number. And I checked it with the test light. And it shows grounded on both sides when I hook to the positive battery terminal. I was just checking fuses for the third time and using a test light this time. And yes, the pink wire with green stripe has 12.46 volts at the hvac control module and that same wire has 9 volts at the humidity sensor. And I'm not sure about the start/stop being enabled. I haven't changed anything though. Where would I find the place to check that?
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Thursday, August 4th, 2022 AT 10:26 AM
Tiny
AL514
  • EXPERT
Do you notice it when you come to a stop? If you have the A/C off, the engine will turn off and start back up when you let your foot off the brake pedal. You may not have it or be noticing it. I think it happens less with the A/C on. It's a feature to save on gas, etc. I was just curious about that. I'll check on that fuse and get back to you. It's good to test with a test light because it puts somewhat of a load on the circuit. But if you only have 9 volts on that pink/green wire, that's a problem, there's a corroded connector or bad pin fitment issue going on. You can check the pink/green wire at the in-car temperature sensor too. It's under the panel right under the steering wheel, see if you have 9 volts there too. If so, try unplugging it and check the connector with it unplugged for voltage. You'll notice in the 3rd diagram, right next to the temperature sensor is the Sun Sensor and pin number 1 is your auto headlights signal wire. That's why there's an issue with the headlights acting up.
Okay, it appears fuse 72 will only be hot while cranking, and it goes to the Starter motor, so that's why you're reading ground on both sides of the fuse. I think it would light the test light if you cranked the engine over. It looks like you're getting ground through the starter motor, so we won't worry about that right now, we need to concentrate on these 9 volts and trace that down.
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Thursday, August 4th, 2022 AT 11:23 AM
Tiny
ALTON04
  • MEMBER
Oh no. I don't have that for sure then. I'll check the voltage at that sensor voltage as soon as I get off this ambulance and get back home this evening. Thanks!
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Thursday, August 4th, 2022 AT 11:32 AM
Tiny
AL514
  • EXPERT
Ok so your hvac is not working though correct? You have power to it but not functional, I think we have a bad Ground here. Ill have some more things for you to test when you get home.
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Thursday, August 4th, 2022 AT 11:35 AM
Tiny
ALTON04
  • MEMBER
That is correct. I have power to it and it's got 12 volts. But I can't turn on the heat, ac, defroster or the vent fan. I can even hit the climate control on the Unconnect and it pulls up the heated seat and heated steering wheel but doesn't give me any of the ac options. And if I hit the ON button on the screen and still nothing.
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Thursday, August 4th, 2022 AT 11:39 AM
Tiny
AL514
  • EXPERT
Oh okay, so you have some kind of smart display(Vehicle System Interface Module) to control different systems in the truck, and you have a scan tool I assume, what kind of scanner do you have? Can it do full system scans of every module?

And before you replaced the Hvac and humidity modules what was happening? Your initial post says "No power at Hvac control panel".
Also did you do any type of verification test on the new hvac module, or any programming?
There is a verification test listed for new modules, Im also going to have you check the actual CAN bus network with a multimeter to see if there is activity coming out of the module, separate from the rest of the network.

You can answer all this after work, Im just going to post as I go here.
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Thursday, August 4th, 2022 AT 12:56 PM

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