Having blown fifty bucks on control heads that aren't helping me any, I decide to investigate the vehicle wiring. NOW I'm on the right track. I trace Circuit 2 which starts at the electrical power center under the plastic cover by the right hood hinge/firewall area. Circuit 2 has a fusible link at the power stud. Fusible link is good. There's power all the way through that wire to Position 12, but it's not “good” power, it's marginal power. If there's a load on the circuit, the voltage plummets like a paralyzed falcon.
There's a 3-wire connector (C323) behind the glovebox. [Edit: Service manual says it's "below" the HVAC fan motor. No. That's where the wire bundle comes through the firewall. This connector lives near the ECM, behind the glovebox.] I pull it apart. BINGO! Two of the three wires seem OK, but the red one—circuit 2, my problem child—has melted the connector, and burnt the wire ends. (Green goes to the A/C clutch relay, and black/white stripe is a ground.)
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The other half of the connector:
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I clipped the wire ends off of Circuit 2 at that connector, then plugged that connector back together as the other two wire connections are OK. Circuit 2 gets re-routed through a brand-new Weatherpack connector. There's no excess of wire, so I can't pull this connector out farther to show more detail.
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I now have proper power at the control head, and two of my four control heads work except for the tiny light bulbs for night operation. The too-new unit with MAX button works, and has night-lights, and one is genuinely defective, I guess.
After examining the original control head, I did touch-up the solder at several points where the one circuit board plugs into the other. This will be the third time I've re-flowed the solder there. I expect it to last the usual two years before the display goes intermittent again.
The other working control head got the solder re-flowed at two connections, again, where the one board plugs into the other.
At some point real soon, I'm going to replace all the electrolytic capacitors on the pull-out circuit boards. Four each on the older boards, only one on the newer board. Electrolytic capacitors have a typical service life of about 20—25 years. They may leak, and corrode the circuit board traces. They may “vent” (industry term—my word for it is “explode”) where the housing is scored exactly for that purpose. And they may die silently—drifting capacitance value, or holding the correct capacitance but dramatically increasing the internal resistance. Point is, they have AT LEAST four failure modes, a recognized service life...and these caps are WAY beyond their service life expectancy. The largest capacitor on my original control board is leaking onto the circuit board, it's gotta go--and as long as I'm in there, the others are gonna vanish along with the failing one. I'm saving one more reply panel for that work.
[Later Edit]
Got brave, plugged-in the later ('92, I think) control head with the "MAX" button.
Works perfectly, I like being able to control the recirculation actuator manually, and even has front-panel lights that actually light up. Until I get the original control head re-capped and the three unbelievably-small light bulbs replaced, this is what's in the truck now. (...maybe forever.) A potential down-side is that the recirculation door activated by the A/C compressor pressure is kind-of a safety feature, I don't know if this control head will defeat that, or if the pressure switch on the compressor will still move the recirculation actuator.
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[/Later Edit]
