Inconsistent Fan Control -Works, Doesn't work

[Badunit]

I am having trouble with the HVAC fan control on my dual-zone HVAC. Sometimes it works correctly, sometimes the fan stays on low even though I turn the knob and the fan speed indicator indicates the fan spee I set. It seems to be about every other time I start my truck it either works or doesn't work. When it does not work, a diagnostic tool can control the fan speed, even when the knob on the dash (and the controls in the uconnect) will not.

I've had the battery disconnected a few times recently. It was disconnected while I was getting my PCM unlocked. A week or so later I disconnected it again to do an aux battery delete (the aux negative has been disconnected for a long time, I just finally got around to taking the battery out). I've also installed new PCM and TCM programming. I don't know when the fan started acting up or if it is even related to any of this. First time I noticed it was a few days ago, which was days after the last time I touched anything. But the weather has been good recently and I haven't been using it so might not have noticed.

One other oddity came when I got the idea to disconnect the battery to "reset" the system (disconnected for 5 minutes or so). Immediately after doing that I had fan control but my temp adjust buttons (and the same controls in the uconnect) would only do Lo and Hi, nothing between. This cleared up after starting the truck a second time but then the fan control quit working again.

I unplugged my Tazer, no difference.

Any ideas other than the dealer?

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[Badunit]

Bringing this up again. still hoping for assistance on this intermittent blower motor problem where about half the time when I start my truck it is stuck on fan speed 1 and the knob has no effect on it.

I have an Icon T10 and have done the following diagnostics:

1. Read codes. There are none.
2. Watched live data of the blower control knob, blower speed, and blower PWM as I turned the fan knob. The knob position is recognized regardless of whether the blower reacts to it or not. The knob position is also displayed on the UConnect and it, too, recognizes it. The blower speed and PWM remain static when it is not working correctly. They go up and down when it is working correctly.
3. Controlled the knob though the T10. The blower speed changes if I control the knob position in the T10. After cancelling the control, the actual knob still does not work.
4. Controlled the fan speed though the T10. The blower speed changes correctly when I control it through the T10. After canceling the control, it goes back to fan speed 1 and the actual knob still does not work.

It seems that each individual part is working correctly. The knob works and is communicating its position. The heater module controls the blower and obeys fan knob commands from the T10 (but not from the actual knob). What the heck would cause the heater control to not hear (or not obey) commands from the fan knob?

 

[DylanM]

Overall it sounds to me like the HVAC module is likely bad.

The only other thing that comes to mind would be that the Auto button may be somehow sticking, thus keeping the HVAC module in Auto mode and not allowing manual control of the blower speed. I'd recommend next time you replicate the problem, take note of whether the light in the Auto mode button stays lit despite your manual blower speed control input, and/or if you can see via your T10 the status of the button and whether the HVAC module is operating in automatic or manual mode.

 

[Badunit]

The auto light is not lit and I can change which vents the air comes out of, which can only be done in manual mode. I recall some value switching to "manual" in the T10 when I turned the knob the first time. I didn't follow up on that.

I think I read somewhere (wish I could remember where) that a misconfiguration of the HVAC system would render the fan control inoperative but I checked it with JSCAN and it reports as auto dual zone. I tried setting it to the other configurations (including manual dual zone) while I was there but it still didn't work.

I can't be 100% sure right now but I don't recall the fan speed going up in Auto either. I'll check that next time just to be sure. I did try the heated seats and wheel and both heat up even when the fan knob does not work.

Right now I am leaning toward the HVAC module being the culprit but I don't understand how the T10 can get the module to change the fan speed but the knob does not.

 

[DylanM]

The HVAC module controls the blower speed via PWM to the blower's linear regulator. It does this either in response to your manual inputs of the blower speed knob/screen inputs or in auto mode in response to the set temperature vs detected cabin temperature (plus humidity level and solar sensor inputs). It'll normally use a low fan speed when the cabin temp is at or fairly near the set temperature, with higher blower speeds only used when there's a significant difference between those temps. To test the HVAC module's ability to use high blower speeds in Auto, you'll generally need to set the target temp at full hot or full cold.

The fact that in the T10 you can see the blower speed knob input track accordingly and even the screen recognizes that input yet the HVAC module doesn't respond correctly points toward an error occurring inside the HVAC module. To me the T10 being able to control the fan speed verifies the blower isn't faulty since it does respond correctly to the PWM output from the HVAC module. Essentially the T10 speed input isn't or can't be ignored by the HVAC module, whereas the speed inputs from the knob or screen are.

Your heated seats and steering wheel are controlled by a different module altogether than the HVAC.

 

[J Jackson]

This guy, () addressed and solved a problem that I had with my 2014 JRU hvac system with my blend door activating every 15 seconds even when the jeep was shut off. I paid $250 for a shop that fixed it with a diag tool. If I had seen this guys vid's, it would have been free to fix. Watch and see if it might help.

 

[Badunit]

With so much communication going over the bus instead of direct wired like in the old days, it is hard to tell where the breakdown is without being able to read the bus communication (which I can't). I wonder if user input on the integrated center stack (the knob and buttons) is listened to by the other modules or sent to only the UConnect, and the UConnect is the device that sends the commands to the other modules. Or maybe it is the other way around. Or maybe they both send commands to the various other modules (and also to each other to stay synced). I am wondering this because, while I can see that the fan knob is registering my input, that doesn't necessarily mean the HVAC module is being sent a command to adjust the blower speed.

Regardless, the cannon has started firing. I have an HVAC module on order.

 

[DylanM]

Best I can find combing through the service manual and wiring diagrams is the HVAC control panel (and obviously the screen inputs) are integral to the center stack. The stack communicates via the CAN C Bus to the HVAC module which then has direct controls to the various actuators, blower motor, etc. in order to control cabin conditioning.

Another thing that popped to mind when going through all that is the possibility there could be a problem with the bus Star connector, though typically problems with the Star connector manifest themselves with a multitude of fault lamps lighting up the instrument cluster. The main thing that makes me doubt that as the cause though is it doesn't make sense why you only have problems with blower motor speed inputs over the bus from the center stack, the rest of the manual HVAC control buttons seeming to work normally. Maybe it's a software glitch with the center stack, but then why does the knob position track reliably and the signal problem to the HVAC module intermittent?

 

[Bandit’s Lair]

Sounds like you had a Bad Unit………………. Somebody had to say it.

 

[Badunit]

Latest update: I installed a new HVAC control module. No difference. I noticed recently that when it is not working correctly, whatever the fan speed was when I last shut off my truck has become the new maximum. So if it had been on 4, the settings 1-4 work but 5-7 are still "4". If it was on 7, I have full control. Also, it never works correctly if I immediately start my truck, I have to go to Run with my foot off the brake, then start it. I don't want to take it to the dealer because I've done the Aux battery delete and they'll probably blame it on that. Or the engine and transmission tune. Or that a Tazer was installed. Or some other BS excuse to charge me to fix it instead of warranty.

 

[Bandit’s Lair]

That’s a really weird bug to have. Do you have a Jeep shops in the area that aren’t dealer related? If you were in SoCal I could help with directing you but unsure where you’re at. If in SoCal PM me and I can make a shop suggestion for this kinda thing.

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