In-dash HVAC control head non-functional (mostly fixed.)

[Schurkey]

1988 K1500, but this should apply (with some wiring changes) to any '88--94 C/K with C60 option code A/C-Heater. The HVAC system quit working. NONE of the buttons did anything useful. The display went dark. No illumination for the buttons when the headlights are turned on.

The display has been intermittently non-functional for years. I poke my thumb onto the controller in a certain place, and the display comes back to life (for awhile). Re-flowing some solder joints fixes this for a couple of years, and then the solder joints break again and the display becomes intermittent.

But the latest problem is TOTAL non-functioning of the HVAC system, not just an intermittent display.

I got several control heads from two different Treasure Yards, none of them worked. Which is when I started tracking-down the wiring of my vehicle instead of blaming the control head. The '88 control head has a black face, '89-up have grey faces. '91 or '92 to '94 have an additional "MAX" button that the earlier ones don't have.



As it turns out, my '88 service manual set was printed in '86, and is so full of errors that they published a "Service Manual Supplement" to correct (some) of the screwups. Even the Supplement has errors. The '88 service manual set is not to be trusted--always verify with the '89 manual if there's any question about accuracy.

I began to suspect that the control head was not getting power through Circuit 2, Position 12 (red wire) on the connector at the back of the control head. I was surprised when I tested Circuit 2 with the control head disconnected, that I got "12 volts". However, when I plugged the control head back in, that voltage dropped to about 3 volts. Not enough to run the control head.



It turns out that I had very high resistance in Circuit 2, which starts at the power center near the right hood hinge, under the plastic cover on the engine side of the firewall. Circuit 2 has a fusible link (tested good) and comes into the passenger compartment near the HVAC blower fan. The Circuit 2 wire goes "up" behind the glove-box where it's part of a 3-wire connector C323. I had to remove the glove box to access C323. Circuit 2 had melted that connector, and had crappy contact across the two halves of the plastic connector body. The other two wires in that 3-wire connector seemed fine. I clipped the Circuit 2 wire on either side of C323, and then plugged C323 back together.





Then I stripped the two "clipped" ends of Circuit 2, and reconnected them using a one-wire Weatherpack connector.



All but one of my control heads now function just fine except for having burnt-out "night lights" to illuminate the buttons when the headlights are on. There are three unbelievably tiny light-bulbs in the control head that need to be replaced; and I haven't found a source for them yet.

Each of the control heads has four electrolytic capacitors (older heads) or one electrolytic cap (control heads new enough to have the "MAX" button.) Electrolytic caps have a known "expected service life" and these are aged-out. I'll solder in new caps once I get the values off the old boards, and order replacements. The biggest electrolytic cap on the original circuit board for my vehicle is leaking acid onto the circuit board. Not good.

Older control head, without "MAX" button. Note four electrolytic caps, circled.


Verify all solder joints in the two rows, circled.


The two circuit boards pull apart via long contact pins. Two of the three round, black 1/4-turn sockets for the "night lights" are visible in this photo. Light bulbs themselves are about the size of a big whitehead zit.


Newer-style (probably more reliable) HVAC controller with "MAX" button, using only one electrolytic capacitor, and wired connections between the two circuit boards.


I installed the newer HVAC control head in my '88, the color doesn't match but it's nice having manual control over the "air inlet valve" (recirculation air-door) which is what the "MAX" button does. In stock form, the recirc air door is controlled by AC high-side pressure as a sort-of safety feature--if the A/C compressor is developing extra-high pressure, the recirc door closes in an effort to reduce high-side pressure. I don't know if this feature still works with the newer control head. Also, the '92 control head has night-lights that actually work.

[maxwedge]

Yikes, well done, a lot of patience and knowledge displayed here.

[Blue Bowtie]

That electrolyte is usually water-soluble, and can also be neutralized. Adding a little clear coat over the finished repair can be a preventive measure for corrosion in the future.

[ASJT3]

There are three unbelievably tiny light-bulbs in the control head that need to be replaced; and I haven't found a source for them yet.

Has anyone tried Neo Wedge bulbs? I bought the size T5/T4.7 bulbs but they turned out not to be the right fit. Anyone know if T3 or some other size works?

[Blue Bowtie]

If the lamps are a soldered base/axial lead type, as found in '80s-90s Delco radios and instruments, the 8099 lamp is a good solution.




If the lamps are axial lead without a base, soldered in, the 8111 lamp will work. Those are found in controls, switches, and similar applications.




If the lamps are a wedge base in removable sockets, the 74 lamp is likely the proper item.

[Schurkey]

I bought "neo wedge" 5mm bulbs, and haven't had the gumption to find out that they don't fit properly.

The base is reusable, if you can find the correct bulbs with LONG leads to wind around the base. The units I have came with new bases, though.

One of my internet searches said that 3mm bulbs worked but were too small compared to OEM. I looked all over for 4mm bulbs, thought I'd found them. Ordered 'em...and soon discovered that I'd put the wrong ones in my "cart" so I'd bought the 5mm by mistake instead.

In short...I've gone nowhere with this.

[j cAT]

electric capacitors 15 years and the take it out and the new caps ...
also the cleaning of the pins ,,
also the lamps