HVAC Control Panel Circuit Board

Hi gang,

After resurfacing my A/C panel graphics layer, I noticed two resistors burned out on the circuit board. They are R3 and its twin brother hiding under that big blue Capacitor.

Anyone know what these resistors do? Bias for the light bulbs? Should I bother messing with them when my HVAC works fine?

Thanks in advance!



Yes, I've also broken a light bulb; that will be fixed soon.

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I am never one to look for trouble, but since those are easily replaced and you are right there, I would replace them.

Glad to see you are getting your hands dirty.

 

Here are some photoss of my HVAC panel refresh.

Pic 1: The panel as removed from my car.

Pic 2: The old HVAC graphics layer is cut open and peeled off. I began cutting from the side where it wraps around the edge. Eventually it opened enough for me to peel off in several large pieces. A layer of very nasty glue and some of the graphics remain on the panel. The original graphics layer says the supplier name Serigrafia Pertoldi of Udine province, in northeastern Italy. Someone on Club Scuderia (UK forum) actually sourced a brand new spare from them for a perfectly original look.

Pic 3: It's time to separate the shell containing the control panel circuit board. 4 screws out and the two halves come apart. Take all the care you can when separating the two halves, bit by bit. Even so, I found one of the two flimsy strut-supported light bulbs broken off. It was the "STOP" button backlight bulb. Yes, it's not an LED. It's a seriously old-school filament bulb wrapped in a tiny green rubber boot.

Pic 4: The surface glue is cleaned off with some warm soapy water and some scrubbing--OK, I lied, a LOT of scrubbing. I cheated a little and used some fine sandpaper that I had lying around. The result is a translucent surface, ready for the new sticker to be applied.

Pic 5: Applying the new sticker, be clever and use some soap and water between the sticker and the panel. It helps you slide it around and position it perfectly. I heated with a blow dryer for a minute. Fearing early separation, I sat a jug of water on top of it overnight.
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So......anyone have an idea of what those two burnt resistors do, and what their values are? I spent a few minutes searching for circuit schematics but didn't find anything for the F355 HVAC panel.

 

33 ohms, 5% tolerance. I don't know their function but resistors don't burn out unless there is a problem somewhere else. Something is pulling excessive current through them.

 

While we are on topic anyone know the bulb voltage for the two bulbs under the stop and recirc buttons. I replaced with 12 volt but it seems dimmer.

 

I'm in the process of refurbishing my HVAC panel, as the knobs were sticky and the overlay was the slightly naff original Hill item and will be replaced with the Sticky RX part.

I have tested the traces and components on the board. All traces are good but I have one diode (D2) which has failed, showing a voltage drop of 0.1v in both directions when tested using a multimeter. I can't imagine that it's the cause of my air direction issue as it's on the wrong side of the board, but I live in hope. One of the two green bulbs also departed the board when I dismantled the unit.

So, I will replace the faulty diode, which appears to be a 1N4148 although I'm happy to be corrected, and will also replace the two green bulbs with green 3mm LEDs with built-in limiting resistors, which are designed to be used across 12V power rails and would seem to be a suitable, and more durable, replacement. I will post pics of the results so that taz355 can decide whether it's worth swapping out the dim bulbs.

It's years since I've done any soldering, so this will be a bit of an adventure...

 

Be sure to test them at night. I replaced my hazard button backlighting LED with an orange LED instead of a red one, hoping that the lens colour would take care of the brightness and colouring, but it turned out the orange one was brighter. It looks conspicuous on the panel, but the upside is that the button is easier to find in an emergency )))

Those LEDs with the built-in limiting resistors sound like a great idea. Some of those Ferrari pusbutton switches have a real rats' nest of components inside them.

 

Except for very early 95's, the hazard button is laser etched and the lens as you speak is white plastic with transparent red paint. We refinish these exactly like the OEM. Some very early cars had in mold icons.

And yep - the amount of parts within these switches is pretty intense

 

As promised, here's the result of swapping out the STOP and recirculation button bulbs with LEDs. They seem a little brighter than the bulbs were. Should look pretty good when I install the new overlay panel.
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