87 5KCSTQ: Solved intermittent AC overpressure

[Paul Meyers]

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With summer approaching, I began to use the AC in earnest, with the
symptomatic flashing "outside temperature" light blinking, signaling an
error condition. Since the AC head unit stores and reports fault conditions,
I retrieved fault code 14, meaning an overpressure had occurred. The fault
happened when I was driving in traffic, but almost never on the freeway. I
suspected a clogged orifice or bad sensor or other nasty problem. I called
my local wrench, who said, and I quote, "There's no such thing as an
intermittent clog. It can't happen. Check the fan operation. You're likely
not getting the first fan speed to work due to a faulty resistor pack."

He was almost exactly right. As you remember, these TQs have 3 fan speeds,
the lowest of which is only used when the AC compressor is engaged. This
prevents sudden heat buildup and overpressure. The resistor pack checked out
OK, but the connection between the first stage relay and the resistor pack
had become high resistance (and it doesn't take very much when the resistor
pack resistance for the first speed is only 0.4 ohms). I removed the pack,
chased the threads on the pack's screws with a die and cleaned up the very
corroded nuts with a tap, applied conductive grease to the terminal and all
was again well in AC land. With the fan working, the overpressure was no
longer evident.

Paul Meyers
'88 5KCSTQ Pearl, 128K Mi, 2pc EM, Fuchs, K24, 1.8 bar, Bilsteins
'87 5KCSTQ Gray, 154K Mi, Fuchs, SJM chip, 1.8 bar, Boges
'87 5KCSTQA White, 208K Mi, Replica TT wheels, Adjustable WG, 1.8 bar,
Bilsteins, no roof rack

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