HVAC issue

mikee30 · #1

Hi all, looking for ideas on where to start...
My climate control unit is not operating properly. The fan speed works but i can't get air to go towards the windshield or floor. The flaps do not move. Even when using the full defrost switch, it will engage but air only comes out towards the cabin (nothing towards windshield).
I removed the unit from the car and clearly someone was here before me since the unit had some cracked clips etc...
Since buying the car, i installed the white clip under the driver footwell that is commonly broken and the PO bypassed the heater control valve so I put that back to stock with a new unit. Not sure if this is related but upon startup i will sometimes hear "water flowing" from behind the dash....something like coolant circulating.
Any ideas where I should begin?

Tom · #2

I'm guessing you have a bum control unit, but you can run the tests in the link below to rule out other components.

viewtopic.php?t=357

If the climate control unit is malfunctioning, most just get a replacement on eBay and hope it works. If you are an electronics type, there's a lot of info about the control unit here:

viewtopic.php?t=1007

mikee30 · #3

Thanks again Tom. I agree the unit is likely dead.
Curious to know what you and/or others normally use as a power supply for 12v testing?

Tom · #4

I have an adjustable DC voltage supply on my bench.

They're cheap and super useful - I use mine all the time!

https://www.amazon.com/Adjustable-Encod ... 123&sr=8-9

Black944 turbo · #5

Tom is more than likely correct, the head is probably dead. I have also had issues with my stepper motors that drive the functions. You can clean them, but that was only a temporary fix in my car.

Black944 turbo · #6

You can also try to disconnect the rod and see if the flaps manually move. There is a slight possibility that they are seized:

stitch2k1 · #7

Have you verified the "Heater" fuse is not blown? That fuse controls everything in the HVAC, minus fan speed (which is constant 12v and the knob controls a resistor to slow it down).

mikee30 · #8

Hi all, I ended up removing the HVAC unit and it definitely has some issues. I quickly noticed someone tried reflowing some solder joints and also noticed one of the yellow capacitors is completely blown. I can replace the capacitor but would be curious what caused it to blow. My plan was to try repairing since I can't seem to find a decently priced unit where the seller will ship to Canada. Does anyone know how I can unsolder the different boards that make up the HVAC unit so I can work on each of them individually? They seem to be soldered together at the corners?

Tom · #9

mikee30 wrote: Sat Dec 16, 2023 6:03 pm Hi all, I ended up removing the HVAC unit and it definitely has some issues. I quickly noticed someone tried reflowing some solder joints and also noticed one of the yellow capacitors is completely blown. I can replace the capacitor but would be curious what caused it to blow. My plan was to try repairing since I can't seem to find a decently priced unit where the seller will ship to Canada. Does anyone know how I can unsolder the different boards that make up the HVAC unit so I can work on each of them individually? They seem to be soldered together at the corners?

The boards are connected via a series of skinny solid right-angle wires. Those wires tend to corrode and become one of the common sources of problems with those control units. It's easiest to clip them in the middle to separate the boards, then remove the stubs with a vacuum-desoldering gun (or braided wire and lots of de-soldering patience). To put them back together, I just find the largest solid wire that will fit in the holes, form them into little right angles, and solder them all back in place. The whole process defines the term tedious, but I've fixed a couple of units by doing nothing but that....

mikee30 · #10

Tom wrote: Sat Dec 16, 2023 8:19 pm
mikee30 wrote: Sat Dec 16, 2023 6:03 pm Hi all, I ended up removing the HVAC unit and it definitely has some issues. I quickly noticed someone tried reflowing some solder joints and also noticed one of the yellow capacitors is completely blown. I can replace the capacitor but would be curious what caused it to blow. My plan was to try repairing since I can't seem to find a decently priced unit where the seller will ship to Canada. Does anyone know how I can unsolder the different boards that make up the HVAC unit so I can work on each of them individually? They seem to be soldered together at the corners?
The boards are connected via a series of skinny solid right-angle wires. Those wires tend to corrode and become one of the common sources of problems with those control units. It's easiest to clip them in the middle to separate the boards, then remove the stubs with a vacuum-desoldering gun (or braided wire and lots of de-soldering patience). To put them back together, I just find the largest solid wire that will fit in the holes, form them into little right angles, and solder them all back in place. The whole process defines the term tedious, but I've fixed a couple of units by doing nothing but that....

Thanks Tom - I was going to try this, but then figured I would test for continuity at across the boards at each pin....this test showed that there is continuity but given I am likely going to have to open it up, I'll be resoldering these pins anyways. I do plan on changing the yellow capacitor (since it looks like the one in this thread: viewtopic.php?t=1007), but I doubt that will solve the problem. Per that post, it states the capacitor is a 25V 47uF .... is that correct? Do these install in a particular direction (writing on my capacitor is not visible). Also, I am getting conflicting information on which microchip controls are activating the flaps? Is it the 4201A or LM2902N? What is the full part number of the 4201A?
Thanks again!
Mike

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HVAC issue