3-second answer: Warm air from an OBS A/C: 1) check the compressor clutch — engine running, A/C on, watch the clutch: no click = no power to the clutch (pressure switch, relay, or wiring) or no refrigerant. 2) Check pressure — low charge = leak (the hose o-rings and the evaporator are the common spots). 3) 92-93 trucks are R12 — you'll likely be converting to R134a (the conversion kit is legal and standard).
The checklist (in order)
- Compressor clutch engages? No click with A/C on = electrical or pressure-switch issue (or empty system — the low-pressure switch kills the clutch).
- Pressure — manifold gauges (or a cheap can with a gauge): low side low = leak. Find it with UV dye.
- Common leak points — the hose o-rings at the compressor/accumulator (the $5 fix), the evaporator, the condenser (rock hits).
- Orifice tube — clogged = cold evaporator but warm vents, or ice buildup. The $10 part is in the liquid line.
- Blend door — if it's cold at the vents on one side but not the other, see blend door.
R12 vs R134a
- 92-93: R12. 94-96: R134a from the factory.
- R12 systems converted to R134a need the retrofit kit (fittings + new o-rings) and usually run a touch warmer — acceptable for a 30-year-old truck.
- Never mix refrigerants. If it's been 'topped off' with who-knows-what, recover + evac + recharge properly.
Numbers
- R134a capacity (94-96): ~2 lb (24-28 oz) — check the sticker under the hood.
- Compressor: the FS10 is the common unit; a reman is ~$150-200.
- Orifice tube: ~$10.
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