SOLVED! Intermittent HVAC

[Blckshdw]

Started off on Saturday, went to use the AC during a drive and had no fan speeds on the front system. The rear worked fine. Couldn't do anything about it at the time, as I was on the way out. 5 hours later, on the way home, tried it again for kicks, system worked just fine. First assumption was maybe damaged wires on the resistor pack, to the point of intermittent contact?

Didn't use it for a few days until today. Went to turn it on for the morning commute, and nothing. But on the way home, worked just fine. I had looked up the resistor packs just to see what they looked like, and see the price ranges. Not bad. Get home, and get the sucker out and it looked to be in good shape. Put it back in, decided to test if the fan didn't fire, I would check and see if there was voltage at the pigtail or not. Naturally the blower kicked on at all speeds. For the first few minutes, had some issues with the air output. Air seemed to be blowing out of all vents regardless of the setting. Turned off the fan, and cycled through each vent setting. Could hear each actuator move, sounded smooth, no clicking. Tried the fan again, and the output followed the selections.

So now I'm thinking flaky HVAC control module... Tomorrow if it doesn't blow out of the front again, I'll try setting the rear control to read from the front. If the rear stops working on that setting, I'll be more convinced that it's the control module. Can anyone think of anything else I might be overlooking, or that I could test in the meantime?

 

[TJBaker57]

Grounds??

 

[Blckshdw]

Grounds??

I checked the one at the bottom of the radio stack, since it was right there. That one was tight, hadn't checked any of the others though.

 

[Mooseman]

Check for burnt wires and connectors to/from the resistors and the blower motor. I had a similar issue to yours on my old TB. I would kick under the dash and it would start again. Turned out to be the motor. I guessed it depended on where it stopped and/or the brushes were worn. Also had a burnt pin on my blower module (auto-HVAC)

 

[Blckshdw]

All the wires seem to be in good condition, no visible damage or burns. Back at it today. Worked fine on the way to work, nothing on the way home. Got the meter out, and tested the output from the resistor pack. With the key on, blower set to 5, I got 6V. With the engine running, blower set to 5, I got 9V. That didn't seem right.

Decided to try to cram the meter probes in the back of the pigtail, have it plugged into the motor, so when it did play nice, I could get the voltage readings for comparison. The leads were a little too long, I could get the positive pin wedged in place, but the ground side had the dash bracing in the way. As I'm fumbling around with things, I bump the dangling resistor pack, and the fan fires up full blast. Touch the ground probe to the splice pack bolt, and get 11V. That's closer to what I was expecting. Engine on, was getting over 12V.

To get some comparison data, I decided to prop up my meter, go through each fan speed setting, and grab a pic of the voltage. That way I could compare the resistor pack output when it fails again. Got through speeds 1 and 2, and then the fan cuts off. Voltage reads 0, unlike when I started out Gonna go ahead and order a new resistor pack

 

[JerryIrons]

Sometimes a blower motor wearing out causes the resistor pack to go bad, in case you find yourself replacing another resistor pack like I had to. Something about higher resistance in the motor causing higher temperatures to deal with I think.

 

[Blckshdw]

Turns out the resistor pack isn't the problem. New one arrived today, popped it in, and nothing. Have continuity on the ground side of the output, 12V coming into the pack, but nothing coming out on any blower setting. Figured, gotta be something in the signaling coming from the control module right? Couldn't see any damaged wires on the connectors, or anything in eyesight.

Checked the wiring diagram to see which color wires were for which signal. Tested them and got ~1.8V on each line with the key in run position. That can't be right, if anything I'd expect at least a 5V reference. Traced that back to the brown wire coming into the module, which goes back to fuse 44 (30A) in the rear block.




Tested both tops of the fuse for power, got 0V on one side, and ~1V on the other. Pulled the fuse and tested good for continuity. So now I'm really confused WTF is going on here. Something is squirrelly upstream. So far the other wiring diagrams I've found don't seem to list the routing on the upstream side of that fuse. Since it says "hot in run" I know I'm looking at the ignition switch at some point, but everything else that needs to be on, is working fine so it's gotta be something between those points.

Went back a moment ago to try testing fuse 39 since it's also connected to that run circuit, and it showed power. Tested 44 again, and it showed full power as well, so of course the blower came on. The hunt continues later

 

[Blckshdw]

Combing through the power distribution diagrams, looks like the ignition switch IS in play after all, as there's a secondary wire connected to the run circuit. Much more likely culprit than a single wire having a problem. I think I've got a spare switch somewhere, will need to track that down.

 

[c good]

This says it was solved. What was the final verdict? Key switch?

 

[Blckshdw]

This says it was solved. What was the final verdict? Key switch?

Yep it was the ignition switch.

 

[mrrsm]

THAT is some Pretty Damned Good "Circuit Sleuthing" @Blckshdw ...and just to cover the Question you raised at the bottom of Post# 7... @MAY03LT mentions One More Ground Point that can also affect the HVAC Module Circuit and many others at 2:25 into his Video; specifially about How to ensure that G-201 and SP-201 for the Class 2 Network are "Well Grounded":

 

[Blckshdw]

So apparently the ignition switch WASN'T the problem after all, but just a coincidence that the few times I needed the AC before temps cooled off for the winter and this spring, it decided to function. On the ride to work this morning, went to turn the AC on, and nothing...

Hopefully I'll be able to recreate the issue while it's in the garage at some point this weekend, so I can probe the orange wire coming out of the ignition switch, and where it enters the rear fuse block. This would suggest there's gotta be a pinch point or a break in the wire that's intermittently going open. I don't think there's anything I tapped into the run circuit for, for a mod. The radio uses a PAC module, so no splicing.

The gremlin hunt continues...

 

[Blckshdw]

So between last night and this afternoon, I was able to do some tracing. All HVAC fuses had power with the key on, so that rules out the ignition switch like before. Pulled the glove box and foot panel to test the resistor pack, no juice at the pack on any wires. Pulled the radio face to loosen the dash cover. Maneuvered the HVAC control module out. Found power on the 2 brown lines going into the module, but nothing on the wires coming out to the resistor pack.

Hop online to look at new control modules, and didn't expect them to run quite that much. Also seems like Dorman is the only option, not sure I'd trust a junk yard unit at this age. The reviews I've been reading pretty much echo what we already know. Many of them developed issues in under a year's time.

 

[Mooseman]

I think I have one if you're interested or I could probably find one at my PnP yard. I'd have to go through my pile 'o parts. Shipping from here ain't cheap though.

Between Dorman and a used one, I'd go with the used one. Cheaper and likely more reliable.

 

[Blckshdw]

I think I have one if you're interested or I could probably find one at my PnP yard. I'd have to go through my pile 'o parts. Shipping from here ain't cheap though.

Between Dorman and a used one, I'd go with the used one. Cheaper and likely more reliable.

I appreciate the offer, but I'm gonna roll the dice and hope I get an outlier. It'll be here today so we'll see how long it plays nice.

 

[budwich]

you might consider going into the guts once your get a replacement and check for any cracks internally nearby where you see the outputs for the motor. All you might need to do is touch up a couple solder joints. I suspect this is your problem. I haven't pulled one apart so I don't know how easy it is but it should have a degree of "inspection capability".

 

[Mooseman]

Oh, that's right. You have the manual HVAC. Mine are auto so that won't work.

Was your modded with LEDs? You gonna mod this one?

 

[Blckshdw]

Well, the control module isn't it. Same problem, zero voltage on any of the output pins. Looked at the pin assignments and went through any of the others that had power or ground. What does it mean when the 5V reference pin shows 11V?

For kicks and giggles, jumped a wire from one of the cigarette lighter connectors to the orange one on the controller output and the blower kicked on high speed.

 

[Mooseman]

Did you check the voltages going in? IIRC, the blower has its own fuse or breaker.

I know it's a manual but does it need programming? It does have some controls for the actuators.

 

[Blckshdw]

It also helps if you have a proper understanding of what connectors do what. I thought the blue one was strictly input to the module, while the black one was output to the blower motor. Which is incorrect. The brown wire on the black connector is power from fuse 44, and while the fuse has power, the brown wire at the control module does not.



So I guess this means I get to take a whole bunch of stuff apart and look for a broken wire someplace.

 

[Blckshdw]

I love my truck, I love my truck, I love my truck. Have to keep telling myself that.

So I took apart the radio stack, figuring the biggest likelihood of an issue with wiring would be here, considering how much stuff I have crammed in there for the HU. Turned the key on, grabbed, squeezed and shook the wire harness for as far as I could trace into the dash. No joy.


Then had an idea, what if the problem was with one of the traces on the fuse block, kinda how they have failed on the front ones for folks and caused connectivity issues. So wrestled with the back seat to get that out of the way, taped off the power supply from the front fuse block, and got to taking apart the rear block. For the life of me, I couldn't figure out how to get the 2 halves apart to access the traces, so took a detour. Since I had the connector off, try checking for continuity between the fuse block end, and the HVAC controller end.



Couldn't get a reading, but wasn't 100% sure if that could have been from me trying to daisy chain wires together to reach that far. Then I counted 11 thick brown wires coming from that connector, all in row 8. Meaning tracing the wire through the bundle to the radio stack would not only mean taking much of the interior out of the truck, I'd have to untape the whole bundle to be able to keep the correct wire pulled free for testing. Immediately lost all interest.

Put everything back together, and turned the key to make sure everything went right with reassembling the rear fuse block and guess what? The blower motor kicked on, and worked on all speeds. We'll see how long this lasts. I would hope that would mean that whatever problem with that wire is right there near the fuse block, but given how thick and secure that bundle is, I have doubts. At any rate, thank you Amazon Prime free returns, the new module will be going back.

 

[Blckshdw]

Almost forgot I had a thread on this already. With much of the interior dismantled (again) I got to testing the circuit, and it seems like I was on the right track before...

Then had an idea, what if the problem was with one of the traces on the fuse block, kinda how they have failed on the front ones for folks and caused connectivity issues.

For my initial test of the circuit tester, I used the wire coming out of HVAC fuse 39. Got decent readings all along the floor (even through the carpet), up into the radio stack, and at the control module. So at least then I knew what to look (listen) for.

Since Fuse 44 had power when the key was on, I put the circuit tester in line from there, and tried testing the wire bundle in the middle of the floor. Nothing. Worked my way back towards the fuse block, and eventually got a really weak signal right near the block, but a good amount of interference too.

Decided to make the following video, and then also move the tester up near the radio stack, and send the signal backwards. Was able to get a good strong signal in the wire bundle going back to the fuse block connector.

So next step is to get the block taken apart, and closely examine the trace coming off the fuse.

 

[Blckshdw]

Got the fuse block taken apart, and found something unexpected. Top side of the block looked fine....



Pulled the board from the base to check out the bottom, and found some melted plastic on the pin exiting the HVAC blower fuse. Something tells me, that might be a problem worth fixing. Grabbed my Xacto knife, and scraped away as much of the melted plastic as I could.



Reassembled the block, connected all the wiring, put the dash and floor consoles back together and ran a test. Blower worked on all speeds. Put more of the interior back together, and tested it a few more times before wrapping up. True test will be the evening commutes in the middle of the week.

 

[Blckshdw]

I think the EXT was feeling neglected, since I spend all of my free time on house projects, and not tinkering with it much anymore. This issue reared it's head again on Thursday evening. Started to head home from work, and turned on the AC. Blower motor came on like normal, then 10 seconds later, turned off... Then back on, and off... One more cycle, then dead in the water

In the previous week I caught whiffs of what smelled like something was warmer than it should be, but couldn't quite place it. Sometimes it smelled like hot metal (wire) other times, melted plastic, other times, burning leaves, and other times, something else. But it wasn't constant, or consistent, so I figured it was something I was driving past.

So this morning I pull out my tools, and instead of checking the usual HVAC blower suspects, I went right back to where I found the fix before. Instead of removing the back seat completely, I had a light bulb moment, and just moved it to the cargo area so it was out of the way. My back was thankful. Got the fuse block disassembled pretty quickly since it had been separated before.




Checked the output from the fuse that had gotten hot before, and melted some of the plastic, but that was fine and looked the same as it did when I cleaned it up, so that's not the culprit this time. Worked my way up the chain, and went to the control module next, so opened up the radio stack to slide the HVAC module out. Probed the brown wire, and used the cigarette lighter plug for my ground, and turned the key on...




OK, so the module is getting power, so moved on to the motor, figuring maybe that finally failed? Pulled the output connector of the resistor pack, probed it, and turned things on... So motor is probably fine, time to focus on the resistor pack.




Got the lower kick panel pulled, and removed the glove box. Forgot how much contortion is necessary to get up under there, definitely don't have many years left in me to be able to do that lol Got the 5mm screws out, pulled the resistor pack, and the smells that I had caught before, were evident here.

Pulled the control module connector so I could get a better look, and the ground pin had melted plastic at the base. The connector itself showed some signed of melting too. Culprit confirmed









Since I was already in there, and had everything apart, decided to test all of the pins in the connector, just because. Everything had voltage when it should have, which was reassuring.




Have an aftermarket resistor pack, with replacement control module pigtail in route from the blue smile site. Going to try and de-pin the connector, and see what condition that ground wire pin is in. If all it needs is melted plastic scraped off, I'll just repurpose the housing from the replacement, and not have to splice all those wires. Worst case, I just have to splice the ground one by itself. Kind of annoys me that all the wires are black and white, instead of color coded, but I suppose these might have been used in multiple GM models

 

[Blckshdw]

New resistor pack arrived today. Got the old harness de-pinned, and the ground wire pin was looking pretty bad, so snipped that off and spliced the new one in.




Quick test, and blower worked on all speeds as expected. In hindsight, the resistor pack was probably the problem all along, it just caused the issue at the rear fuse block, before showing signs of its own. Lesson learned either way