In July I repaired my wife's digital Heating, Ventilation, A/C control panel.
After 2 months of daily use, it still works perfectly, so I now feel confident
sharing it here:
There is probably a website or archive on this repair somewhere, but since
this was trial and error job (and can save you over $400), I thought this
proven method might be added to what you may already have. Most people
would probably rather just fork over the $400, but the average person with
PCB soldering experience can expect to complete the entire repair in about
4 hours time. Actual soldering time is less that 1 hour, with removing
the coating taking the most time. I recommend reading through the entire
procedure before beginning.
Symptoms:
HVAC was intermittent at first, then no lights or controls worked. Wiggling
the main blue connector on the back would cause it to operate temporarily.
Occasionally, the A/C light might be on, but no buttons worked at all. From
what I understand, this is a fairly common problem, as many of the dealers
and web sites selling this part (Back in July, 2002) were sold out, and the
part was on back order for 5 - 9 months. Dealer retail for these are between
$290 and $351 depending upon the exact one you may have, PLUS another $90 for
the "usual and customary" (read MANDATORY) programming required by the
dealer's special computer.
Root Problem:
Some of the PCB (Printed Circuit Board) solder pads and feed throughs heat up
too much from current flow during usage, causing components to 'de-solder'
themselves from the pads, or in the case of feed throughs, open up entirely.
This is actually a design problem, more than a build quality problem.
Skills Needed for Professional Repair (Substitute / eliminate as you see fit):
- Experience soldering and handling delicate PCB's with surface mount components.
- Knowledge of grounding oneself during handling of PCB's.
Tools and Materials Needed for Professional Repair (Substitute / eliminate
as you own skills dictate):
- 1 set small Torx drivers
- 2 1/8" flat blade screwdrivers
- 1 stereo zoom microscope, or large suspended magnifying glass or magnifying glasses and an illuminating lamp
- 1 small vice, with soft jaws (or wrap several layers of electrical tape on a hard jaw vice)
- 1 very small soldering iron
- 1 damp sponge to wipe soldering iron tip on
- 5 feet of very thin (24 AWG) rosin core solder
- 1 new #11 Exacto blade in an Exacto handle
- 1 stiff plastic stick with a fine point
- 4 ounces of 99% wood or ethyl alcohol (Warning: Methanol and Rubbing alcohol contain too much water and are NOT recommended). 198 Proof (99%) liquor WILL work!
- 1 old toothbrush (cleaned and dried)
- 1 can of compressed air (computer duster)
- 2 feet of #30 AWG wire wrap or tinned hook up wire
- 1 spool of de-soldering wick.
Repair Methodology:
Using 2 small flat blade screwdrivers, one on the left side and one on the
right side, pry the HVAC control unit from dashboard. Some minor temporary
compression of the soft dash material will occur, and it will re-form back
to normal over time.
Disconnect the 2 or 3 connectors from the unit.
Using small Torx driver, remove front panel screws, and 2 fan screws from
the control unit.
Un-snap front panel, being careful not to kink or sever the delicate flex
strip connecting front panel to main PCB. It's actually tougher than it feels,
but hey, let's not take any chances. Feel free to lay the front panel any
orientation you choose, as the buttons are secured and will not
fall out.
Remove the fan.
Carefully remove main PCB by pulling it forward, as evenly as possible.
What this means, is that if you pull one side out more than the other side,
it will bind (kind of like an antique dresser drawer). It is also helpful to
have someone squeeze the housing from top to bottom, on both the right and
left sides, which will elongate the housing's length, which will in turn ease
the pressure on the edges of the PCB. As the PCB comes out, so will the fan
duct. Notice the orientation of the fan duct to facilitate easy re-assembly.
Set aside the fan duct to keep it out of your way.
Once the PCB is out, carefully clamp it in the vice so you can see the
bottom side (the side with the least amount of components on it). My vice was
small enough to clamp it on the main blue 26 pin connector. You could also
clamp it on the edges of the PCB, as long as you aren't pinching any
components.
Wet the toothbrush with alcohol and soak / scrub the area of the PCB within
2 inches of the main blue 26 pin connector. This step will soften the brown
parylene coating / varnish on the PCB.
While looking through the microscope or magnification device using plenty
of light, take the plastic stick and / or Exacto knife and CAREFULLY remove
the parylene coating from ALL the feed throughs (little round holes that
traces use to 'feed through' to the other side of the board) and solder pads
connecting to components, within about 1.5" of the main blue connector. Hold
the Exacto in such a way as not to cut through any of the traces or damage
any of the components on the PCB. Both are actually tougher than they look,
but hey, let's not take any chances. It is not necessary to remove coating
from the circuit traces, only from the areas you wish to re-flow (re-solder).
Leaving any coating on the areas to re-flowed, will contaminate the solder
joint, so work thoroughly.
Turn Board over and repeat steps 8 & 9 on component side of board. Extra
care must be used again to avoid damage to the many components here Again,
only feed throughs and pads within 1.5" of the main 26 pin blue connector
need be cleaned.
Scrub the area again with alcohol and dry with compressed air.
Flip the board back over to the bottom and repeat step 11, then goto step
13.
While looking through the microscope or magnification device using plenty
of light, take the soldering iron and solder, and re-flow each of the
connections you have cleaned the coating off of. Wiping the soldering iron
tip on the damp sponge after EACH connection will eliminate the black
oxidation from contaminating the joints.
On my PCB, I noticed 2 areas that were failing:
Connections to the small black rectangular transistors (the tiny ones with
3 leads). On 2 or 3 of these devices, the lead had gotten so hot, that not
only had it de-soldered itself, it had had 'sunken' the pad into the PCB.
Feed throughs. 1 or 2 of these took too much solder, indicating "opens".
Use the #30 wire to tie one side of the board to the other side if you find
any of these.
Soldering TIPS:
A good solder joint will have a nice shiny appearance to it, and will not
have any dull or bubbly areas.
The proper amount of solder will give a
slightly concave flow, and will not become a huge blob of solder. Use the
de-soldering wick if you need to start over.
Be careful not to heat up the components too much. Blow them gently with
compressed air if you think they got too hot. Hint: If the component starts
to 'sweat', it's too hot!
Flip the board back over to the bottom and repeat step 13.
15) Scrub both sides of the board with alcohol to remove excess rosin flux
and dry with compressed air.
16) If you feel confident in your abilities to work around live circuits
without shorting anything out (no danger of shock), you may choose to test
the unit in the car without actually re-assembling it first. The fan is not
needed, but you MUST be sure you don't touch the circuits while they are
live, as the conductivity of your fingers causes temporary malfunctions.
I did this numerous times through my trials and errors without any damage.
17) Re-assemble unit in reverse order of disassembly (steps 6,5,4,3,2,1,
except no screwdrivers are needed in step 1)
